energies
Review
Advantages and Limitations of Anaerobic Wastewater
Treatment—Technological Basics, Development Directions, and
Technological Innovations
Marcin Zieliński 1 , Joanna Kazimierowicz 2
1
2
*
Citation: Zieliński, M.;
Kazimierowicz, J.; D˛ebowski, M.
Advantages and Limitations of
Anaerobic Wastewater
Treatment—Technological Basics,
Development Directions, and
Technological Innovations. Energies
and Marcin D˛ebowski 1, *
Department of Environmental Engineering, Faculty of Geoengineering, University of Warmia and Mazury
in Olsztyn, 10-720 Olsztyn, Poland
Department of Water Supply and Sewage Systems, Faculty of Civil Engineering and Environmental Sciences,
Bialystok University of Technology, 15-351 Bialystok, Poland
Correspondence: marcin.debowski@uwm.edu.pl
Abstract: Anaerobic wastewater treatment is still a dynamically developing technology ensuring
the effective degradation of organic compounds and biogas production. As evidenced in the large
scale-up, this technological solution surpasses aerobic methods in many aspects. Its advantages
stem from the feasibility of operation at a high organic load rate, the smaller production of difficultto-manage sewage sludge, the smaller space and cubature required, and the high-methane biogas
ultimately produced. The exploitation of anaerobic reactors is in line with the assumption of a circular
economy, material recycling by reduced CO2 emissions and energy consumption, and the production
of renewable energy. Despite their unquestionable advantages, there is still a need to seek novel
approaches and improve the currently exploited installations. The key avenues of research entail
improvements in the stability of bioreactor operations and the enhancement of bioreactor adaptability
to changing and unfavorable process parameters. The versatility of such systems would also be
greatly improved by increasing nitrogen and phosphorus removal rates. Attempts have been made
to achieve these goals by setting up separate zones within bioreactors for the individual steps of
methane fermentation, incorporating active fillings to promote nutrient removal, and introducing
chemical and physical treatments. An interesting solution is also the use of microwave radiation
to stimulate temperature conditions and induce non-thermal phenomena, such as enhancing the
enzymatic activity of methanogenic microflora. Another prospective approach is to integrate digesters
into microalgal biomass production systems. The aim of this review paper is to present the thus-far
technological knowledge about anaerobic wastewater treatment, including standard solutions and
innovative ones, the effectiveness of which has been corroborated in pilot-scale installations.
2023, 16, 83. https://doi.org/
10.3390/en16010083
Academic Editor: Giorgio Vilardi
Keywords: wastewater; anaerobic reactors; methane fermentation; biodegradation; nutrient removal;
biogas; active fillings; physical factor
Received: 15 November 2022
Revised: 13 December 2022
Accepted: 14 December 2022
1. Introduction
Published: 21 December 2022
The development of wastewater treatment methods entails searching for novel, efficient, and technologically feasible solutions that could replace those currently in use [1].
The aim is to achieve not only the best possible performance in terms of pollutant removal,
but also expected technological effects driving down investment and operating costs [2].
These criteria have greatly increased interest in pursuing anaerobic technologies, rapidly
expanding the range of available technologies and deploying anaerobic reactors [3,4]. As
shown in Figure 1, in recent years, there has been growing interest in the studies focusing
on both anaerobic wastewater treatment and anaerobic biological reactors.
Copyright: © 2022 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Energies 2023, 16, 83. https://doi.org/10.3390/en16010083
https://www.mdpi.com/journal/energies
Energies
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83
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and(d)
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Direct
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theyears
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usingthe
thekeywords
keywords“anaerobic
“anaerobicwastewater
wastewatertreatment”
treatment”and
and
“anaerobic
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Thestatistical
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Digesters offer a wide range of applications for industrial and municipal sewage
Digesters
offer [5].
a wide
range of
applications
formethods
industrial
and
municipal
sewage
treatment
systems
Anaerobic
sewage
treatment
can
efficiently
biodegrade
treatment
systems
[5].keeping
Anaerobic
sewage and
treatment
methods
can efficiently
biodegrade
organic matter
while
investment
operating
costs relatively
low [6].
Anaerobic
organic
matter
while keeping
investment
low [6].
Anaerobic
plants and
accompanying
facilities
do notand
takeoperating
up muchcosts
spacerelatively
and produce
little
anaerobic
plants
and
accompanying
facilitiesthe
doattractiveness
not take up much
and produce
littledigestion
anaero‐
surplus
sludge,
further bolstering
of thespace
technology
[7]. Finally,
bic
surplus
sludge,
further
bolstering
the
attractiveness
of
the
technology
[7].
Finally,
di‐
technologies can also be used to produce and capture methane-rich biogas—a considerable
advantage
over the can
aerobic-activated
[8]. Anaerobic
methods
have con‐
been
gestion
technologies
also be used to sludge
produceprocess
and capture
methane‐rich
biogas—a
shown toadvantage
be particularly
to treating food-industry
is characterized
siderable
oversuited
the aerobic‐activated
sludge effluent
process [9],
[8].which
Anaerobic
methods
by high
that can only
be to
removed
aerobic methods
have
beenpollutant
shown toloads
be particularly
suited
treatingwith
food‐industry
effluent in
[9],large-scale
which is
plants, with considerable
energyloads
expenditures
of themethods
wastewater)
characterized
by high pollutant
that can (for
onlythe
beintensive
removedaeration
with aerobic
in
and
the
production
of
hard-to-manage
sludge
[10].
The
clear
advantages
of
anaerobic
large‐scale plants, with considerable energy expenditures (for the intensive aeration of the
wastewater treatment
have beenof
recognized
by manufacturing
evidencedof
by
wastewater)
and the production
hard‐to‐manage
sludge [10].operators,
The clearas
advantages
the
growing
number
of
new
full-scale
plants.
Furthermore,
existing
energy-consuming
anaerobic wastewater treatment have been recognized by manufacturing operators, as ev‐
aerobic systems
have been
upgraded
andfull‐scale
retrofittedplants.
with new
technologies
[11,12].
Table 1
idenced
by the growing
number
of new
Furthermore,
existing
energy‐
presents a comparison
between
and anaerobic
wastewaterwith
treatment
methods.
consuming
aerobic systems
haveaerobic
been upgraded
and retrofitted
new technologies
[11,12]. Table 1 presents a comparison between aerobic and anaerobic wastewater treat‐
ment methods.
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Table 1. General comparison between aerobic and anaerobic wastewater treatment methods.
Feature
Aerobic
Anaerobic
Reference
Table 1. General comparison between aerobic and anaerobic wastewater treatment methods.
Alkalinity demand
Low
High for certain industrial waste
[13–15]
Bioenergy and
nutrient recovery
No
Yes
[14,16,17]
Feature
Aerobic
Anaerobic
Reference
Energy
demand
High
Low
to
moderate
[14,16,17]
Alkalinity demand
Low
High for certain industrial waste
[13–15]
Mode
ofnutrient
treatment
Total
Essentially
[13,15,18]
Bioenergy
and
recovery
No
Yes pre‐treatment
[14,16,17]
Energy demand
High
Low to moderate
[14,16,17]
Nutrient
demand
High
Low
[13–15]
Mode of
treatment
Total
Essentially
pre-treatment
[13,15,18]
Odor
Less opportunity
for odors
Potential
odor problems
[13,15,18]
Nutrient demand
High
Low
[13–15]
Organic loading rate
Moderate
High
[13,15,18]
Odor
Less opportunity for odors
Potential odor problems
[13,15,18]
Organic
removal
High
[13–15]
Organic
loadingefficiency
rate
Moderate
HighHigh
[13,15,18]
Sludge
generation
High
[13–15]
Organic
removal
efficiency
High
HighLow
[13–15]
Sludge generation
High
LowHigh
[13–15]
Temperature
sensitivity
Low
[13,15,18]
Temperature
sensitivity
Low
High
[13,15,18]
Wastewater quality
Great
Moderate to poor
[13–15]
Wastewater quality
Great
Moderate to poor
[13–15]
Anaerobic reactors are quite often used to treat food‐industry effluent [19,20]. They
Anaerobic
reactors
areto
quite
oftenwaste
used from
to treat
[19,20].
They
have been
successfully
used
degrade
thefood-industry
production ofeffluent
dairy goods
[21,22],
have
been
successfully
used
to
degrade
waste
from
the
production
of
dairy
goods
[21,22],
fruit and vegetables [23], meat [24], beer [25], spirits [26], paper/paper‐pulp [27], chemicals
fruitand
andpharmaceuticals
vegetables [23],[29].
meatWhat
[24], makes
beer [25],
spirits
paper/paper-pulp
chem[28],
such
waste[26],
particularly
well‐suited[27],
to anaer‐
icalsprocessing
[28], and pharmaceuticals
Whatorganics,
makes such
waste
particularly
well-suited
obic
is its high content[29].
of labile
which
are converted
to biogas
underto
anaerobicconditions
processing[30].
is itsPre‐treatment
high content of
labile
organics,
whichcontaining
are converted
to biogas
anaerobic
can
be used
for waste
hard‐to‐de‐
underbiodegradation‐resistant
anaerobic conditions [30].
can besubstances
used for waste
hard-tograde,
or Pre-treatment
digestion‐inhibiting
[31]. containing
Pre‐treatment
op‐
degrade,
biodegradation-resistant
or
digestion-inhibiting
substances
[31].
Pre-treatment
tions include microwave radiation [32,33], ultrasound [34], high‐voltage disintegration
options
include
microwave radiation
ultrasound
[34],bases
high-voltage
disintegra[35],
thermal
depolymerization
[36], and[32,33],
treatment
with acids,
[37], or strong
oxi‐
tion
[35],
thermal
depolymerization
[36],
and
treatment
with
acids,
bases
[37],
or strong
dizing agents [38]. Industrial effluent has a consistent composition due to the uniform
oxidizing agents
[38].the
Industrial
effluent has a consistent
composition
to thethat
uniform
production
cycle and
use of retention/equalization
tanks
[39]. Thisdue
ensures
the
production
cycle
and
the
use
of
retention/equalization
tanks
[39].
This
ensures
that
the
ratios of carbohydrates, proteins, and lipids remain fairly stable in the long term [40]. In‐
ratios
of
carbohydrates,
proteins,
and
lipids
remain
fairly
stable
in
the
long
term
[40].
dustrial effluent tends to have a high and uniform temperature, which further facilitates
Industrial
effluent
tendstypes
to have
a high andreactors
uniform
temperature,
whichapplications
further facilitates
its
processing
in various
of anaerobic
[41].
The commercial
and
its processing in various types of anaerobic reactors [41]. The commercial applications and
market share of anaerobic technologies are presented in Figure 2.
market share of anaerobic technologies are presented in Figure 2.
(a)
(b)
Figure 2. Types (a) and applications (b) of anaerobic reactors.
Figure 2. Types (a) and applications (b) of anaerobic reactors.
The
Theuse
useofofanaerobic
anaerobicwastewater
wastewatertreatment
treatmentsystems
systemsisisininline
linewith
withthe
theassumptions
assumptions
ofofaacircular
economy
[42].
Biomethane
as
a
source
of
renewable
energy,
digested
circular economy [42]. Biomethane as a source of renewable energy, digestedsludge
sludge
used
usedasasa abiofertilizer,
biofertilizer,and
andtreated
treatedwastewater
wastewaterare
arethe
themain
mainpillars
pillarsofofthe
theresource
resourcerecovery
recovery
process
is is
a universal
energy
carrier
thatthat
cancan
be used
on site
or stored
[44].[44].
In
process[43].
[43].Biogas
Biogas
a universal
energy
carrier
be used
on site
or stored
addition,
its production
and implementation
technologies
have a have
significant
impact on
the
In addition,
its production
and implementation
technologies
a significant
impact
reduction
of gas emissions
from conventional
energy carriers
It should
be noted
on the reduction
of gas emissions
from conventional
energy[45].
carriers
[45]. also
It should
also
that
anaerobic
digestion processes
play a keyplay
roleainkey
wastewater
treatmenttreatment
processesproin
be noted
that anaerobic
digestion processes
role in wastewater
cesses in terms of ensuring their energetic neutrality [46]. Anaerobic reactors merge the
processes of wastewater treatment and water recovery [47], the production of renewable
Energies 2023, 16, 83
4 of 39
energy [48], and the production of biofertilizers and soil improvers that are widely used on
marginal, degraded, and low-quality soils [49]. Their use reduces greenhouse gas emissions
and directly affects the energy-efficient protection of surface waters, groundwaters, and
groundwater reservoirs [50].
The multifunctionality of anaerobic wastewater treatment is a great strength of these
technological solutions. Sustainable biogas systems include processes for treating wastewater and other organic waste to protect the natural environment; converting low-value
material into higher-value material; and producing electricity, heat, and/or advanced
gaseous biofuels, including hydrogen [51]. The feasibility of biogas production can contribute to the decentralization of energy security by balancing local energy supplies [52].
The methane fermentation of wastewater and the production of anaerobic digestate also
contributes to improving the efficiency of nutrient uptake in agriculture by replacing synthetic fertilizers based on fossil fuels with biofertilizers, as well as to economical water
management in agriculture and industry [53]. Following the circular economy assumptions,
anaerobic digestion contributes to the treatment of wastewater streams throughout the food
supply chain. Waste previously considered difficult to neutralize is being reintroduced into
the production cycle where water, organic material, and nutrients are returned to the soil to
replace chemical fertilizers [54].
The aim of this review paper is to identify the strengths and weaknesses of digestion
reactors, characterize the technological parameters that influence the course of anaerobic
wastewater treatment, present primary reactor design types and their technological performance, and indicate the potential avenues of development of anaerobic methods for
contaminant removal. A novel aspect of this review paper is the presentation of innovative
technological and design solutions that minimize or eliminate the weaknesses of anaerobic wastewater treatment systems. The focus is on installations with a high technology
readiness level (TRL), the effectiveness of which was confirmed in a pilot and fractional–
technical scale. The improvements made in these installations allow to bolster wastewater
treatment effectiveness, increase the efficiency of biogas and methane production, promote
the removal of biogenic compounds, and save energy.
2. Basic Principles of Anaerobic Wastewater Treatment
Anaerobic wastewater treatment is predominantly based on conversions that occur
during methanogenic fermentation [55]. Anaerobic digestion (AD) is a four-step process,
with each successive step conducted by different groups of microbes and yielding different end products [56]. The assorted steps of the process are mediated by facultative and
obligatory anaerobes [57]. The first step involves hydrolysis—the biological decomposition of macromolecular organic matter into simpler compounds more readily taken up by
anaerobic microbes [58]. During acidogenesis, the pre-hydrolyzed compounds are further
degraded into organic acids, which are, in turn, converted into acetic acid during acetogenesis. This acid serves as the primary food source for anaerobic, methanogenic bacteria [59],
which conduct the last step—methanogenesis—and produce gaseous metabolites [60]. A
step-by-step conversion flowchart for the anaerobic digestion (AD) of wastewater is given
in Figure 3 [61].
Hydrolysis entails the biochemical and accompanying physicochemical depolymerization of complex organic compounds into monomers or dimers that can be directly
consumed by bacteria [62]. This stage is crucial for the successful biodegradation of organic compounds in subsequent steps [63]. Proteins are hydrolyzed into amino acids, and
polysaccharides into simple sugars, whereas lipids are hydrolyzed into polyols and fatty
acids [64]. Hydrolysis is carried out by a wide spectrum of microbes, including facultative and obligatory anaerobes of the Aerobacter, Alcaligenes, Clostridium, Flavobacterium,
Lactobacillus, Micrococcus, Streptococcus, Pseudomonas, and Lactobacterium genera [65].
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43
Figure 3. Methanogenic fermentation pathways for anaerobic wastewater treatment.
Figure 3. Methanogenic fermentation pathways for anaerobic wastewater treatment.
The same groups of microbes carry out acidogenesis, by which the hydrolysis prodHydrolysis
the biochemical and accompanying
physicochemical
depolymeri‐
ucts are
degradedentails
into low-molecular-weight
organic compounds,
including mainly
fatty
zation
of
complex
organic
compounds
into
monomers
or
dimers
that
can
be
directly
con‐
acids—primarily acetic acid, followed by propionic acid and butyric acid, as well as smaller
sumed
by
bacteria
[62].
This
stage
is
crucial
for
the
successful
biodegradation
of
organic
quantities of isobutyric acid, valeric acid, isovaleric acid, caproic acid, and lactic acid [66].
compounds in
subsequent
steps
[63]. Proteins
hydrolyzed
into and
amino
acids,
and pol‐
Acidogenesis
can
also produce
alcohols,
such asare
ethanol,
methanol,
trace
amounts
of
ysaccharides
simple
whereasinclude
lipids are
hydrolyzed
into polyols
and[68].
fatty
ac‐
propanol
[67].into
Other
majorsugars,
end products
hydrogen
and carbon
dioxide
The
ids [64]. Hydrolysis
carried
out by
a wide spectrum
oftechnology
microbes, including
facultative
preferred
outcome of is
the
anaerobic
wastewater
treatment
is the direct
producand
obligatory
anaerobes
of
the
Aerobacter,
Alcaligenes,
Clostridium,
Flavobacterium,
Lacto‐
tion of acetate, carbon dioxide, and hydrogen. This fermentation pathway provides more
bacillus,
Micrococcus,
Streptococcus,
Pseudomonas,
and
Lactobacterium
genera
[65].
energy for the microbes, and its products can be directly taken up by methanogens [69].
same groups
of microbes
carryby
outobligatory
acidogenesis,
by which the hydrolysis
OtherThe
products
must first
be converted
hydrogen-producing
bacteriaprod‐
via
ucts are degraded
into low‐molecular‐weight
organic
including
mainly
fatty
acetogenesis
[70]. Acetates
are generated through
one compounds,
of two pathways.
The first,
which
acetic acid,
followed
by propionic
acid
and butyric
acid, assteps
well(in
as
isacids—primarily
usually critical, converts
organic
substances
generated
during
the preceding
smaller
quantities
of
isobutyric
acid,
valeric
acid,
isovaleric
acid,
caproic
acid,
and
lactic
particular, butyric acid, propionic acid, and long-chain fatty acids) to produce hydroacid[71].
[66].The
Acidogenesis
can occurs
also produce
alcohols, such
as ethanol,
methanol,toand
trace
gen
other variant
when homoacetate
bacteria
use hydrogen
reduce
amounts
of propanol
carbon
dioxide
[72]. [67]. Other major end products include hydrogen and carbon diox‐
ide [68].
The preferred
outcome
of the
anaerobic
wastewater
treatment
is the
Methanogenesis
is the
last step
of the
digestion
process, and
the onetechnology
where biogas
is
direct production
acetate,known
carbonspecies
dioxide,ofand
hydrogen. This fermentation
produced
[73]. The of
currently
methane-producing
microbes are pathway
divided
provides
more groups
energy for
the microbes,
its products
can be directly
taken up by meth‐
into
three main
according
to theirand
nutritional
requirements
[74]. Hydrogenotrophs
anogensH[69].
Other
products
must
first
be
converted
by
obligatory
hydrogen‐producing
oxidize
and
reduce
CO
into
CH
;
methylotrophs
use
methanol,
methylamines,
or
2
2
4
bacteria via
acetogenesis
[70]. CH
Acetates
generateduse
through
of two pathways.
dimethyl
sulfides
to synthesize
acetotrophs
acetateone
to produce
CH4 (TableThe
2).
4 ; andare
first, which is usually critical, converts organic substances generated during the preceding
steps (in particular, butyric acid, propionic acid, and long‐chain fatty acids) to produce
Energies 2023, 16, 83
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Table 2. Stoichiometric reactions for methane production via methanogenesis.
Process
Reaction
Bacteria
Phylum
Reference
[75–77]
Hydrogen reduction of
carbon dioxide
CO2 + 4H2 → CH4 + 2H2 O
hydrogenotrophs
Methanobacteriales,
Methanomicrobiales,
Methanopyrales,
Methanococcales,
Methanocellales
Conversion of
methanol to methane
and carbon dioxide
4CH3 OH → 3CH4 + CO2 + 2H2 O
methylotrophs
Methanosarcinales,
Methanobacteriales,
Methanomassiliicoccales
[75–77]
Conversion of acetate
to methane and
carbon dioxide
CH3 COOH → CH4 + CO2
acetotrophs
Methanosarcinales
[75–77]
The most energy-efficient of these is the CO2 reduction by H2 . Conversely, the
acetotroph-mediated acetoclastic reaction is the least efficient. Some methanogens do
not belong to any of these categories. For example, some are hydrogen-methylotrophs
which use hydrogen to reduce methanol and synthesize methane [78]. Methanogenesis
is the bottleneck for the entire process, since methanogenic microorganisms grow much
slower than acidogenic bacteria [79]. Therefore, the efficiency of the entire process is contingent on ensuring optimal conditions for methanogenesis [80,81]. All methane-producing
microorganisms are classified into a separate domain. Procaryota are grouped into two
fundamentally different domains of organisms: the domain Bacteria (most of the extant
strains of bacteria, no methanogens, few extremophilic species) and the domain Archaea
(all methanogens, many extremophilic species) [82]. Archaea are some of the oldest known
microbes, vastly different from the other Procaryota. These differences relate not only to their
metabolism, but also to some of their morphological features [83]. For example, Archaea
lack the typical peptidoglycan (murein) mesh. Methanococcus only has an outer protein
layer, whereas the cell wall of Methanosarcina is built of a polysaccharide constituting uronic
acids, neutral sugars, and amino acids [84]. Ribosomes of methanogens are similar in
size to those of Bacteria, but the sequences of ribosomal RNA (especially 16S rRNA) are
completely different. It is believed that approximately 70% of the methane produced in
anaerobic reactors is synthesized from acetic acid by Archaea, including Methanobacterium,
Methanothermobacter, Methanobrevibacter, Methanosarcina, and Methanosaeta. Most of the
remaining methane is generated by CO2 -reducing hydrogenotrophs, such as Methanobacterium formicicum, Methanobacterium thermoautotrophicum, Methanobacterium bryantii, and
Methanobrevibacter spp. [85]. An overview of the conversions occurring during the anaerobic
digestion of organic compounds is presented in Table 3.
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Table 3. Overview of AD steps.
Step
Substrates
Products
Hydrolysis
Protein,
fats,
carbohydrates
Amino acids,
fatty acids, polyols,
simple sugars
Hydrolysis products
Fatty acids: acetic acid,
propionic acid, butyric
acid, isobutyric acid,
valeric acid, isovaleric
acid, caproic acid,
lactic acid;
Alcohols: ethanol,
methanol, propanol;
H2 , CO2
Acidogenesis
Acetogenesis
Methanogenesis
Reference
[63–65]
Aerobacter, Alcaligenes,
Clostridium, Flavobacterium,
Lactobacillus, Micrococcus,
Streptococcus, Pseudomonas,
Lactobacterium
[66–68]
Acetics,
H2 , CO2
Syntrophobacter wolinii,
Sytrophomonos wolfei,
Clostridium spp., Peptococcus
anaerobius, Lactobacillus,
and Actinomyces
[70–72]
Biogas (CH4 , CO2 )
Methanobacterium,
Methanobacillus,
Methanococcus,
Methanosarcina. Methanosaeta
[73,74,85]
Earlier products
Acetogenesis products
Species
3. Process Parameters
3.1. Temperature
All biochemical processes of AD are subject to the laws of thermodynamics [86]. On the
one hand, elevated temperatures will accelerate the rate of biochemical reactions catalyzed
by enzymes (in accordance with the Arrhenius theory), and on the other, the constituent
proteins of enzymes can denature if the temperature is too high, drawing the process to a
halt [87]. Fermenting microbes can be divided into three groups according to their preferred
temperature range: psychrophilic (under 25 ◦ C), mesophilic (30–40 ◦ C), and thermophilic
(50–60 ◦ C). Numerous studies have examined the relationship between AD performance
and process temperature. According to Dhaked et al. (2010), the organic methanogenesis
cascade has been observed in nature within the very broad temperature range of 0 to 98 ◦ C.
Out of the entire complex of microorganisms involved in the AD process, methanogens
are the most sensitive to temperature changes, especially to excessive or severely unstable
temperatures [88].
There have been numerous reports of psychrophilic and psychrotolerant methaneproducing Archaea in cold natural environments [89,90], though their metabolic activity
and rate are relatively low [91]. Research on adapting these bacteria also does not bode well
for their practical applicability in contaminant degradation [92]. Temperature can change
the methanogenesis rate by affecting microbial activity and counts. Changes have also
been observed in the efficiency of available methanogenic pathways. At low temperatures,
most of the methane is produced from reduced acetates [93]. Importantly, methanogenesis,
which is the bottleneck for the entire anaerobic conversion chain, is also the step most
sensitive to sudden changes and spikes in temperature [94].
In practice, mesophilic processes are preferred for systems designed to ensure the high
efficiency of anaerobic organic matter degradation, though thermophilic conditions are
used as well in special cases [95]. This is primarily to offset the power expended for digester
heating with improved digestion performance. Thermophilic AD offers faster rates, more
complete organic matter degradation, and better elimination of pathogens [96]. However,
when compared with mesophilic systems, these benefits do not compensate for the considerably higher energy input [97]. There are also certain drawbacks to thermophilic digestion,
Energies 2023, 16, 83
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such as diminished stability and higher process sensitivity [98]. Thermophilic conditions
lead to higher levels of volatile fatty acids compared to mesophilic processes. This holds particularly true for propionic acid, since acetogenesis is inhibited above 55 ◦ C [99]. Anaerobic
wastewater treatment performance as a function of temperature is presented in Table 4.
Table 4. Digestion performance vs. temperature parameters.
Reactor Type
Wastewater
Temperature
(◦ C)
Initial COD
(mgO2 /dm3 )
COD Removal
(%)
CH4 Yield
(m3 CH4 /kg
COD)
Reference
Synthetic dairy
35
3–12
97.9–98.8
0.32–0.39
[100]
Whey
37
33
90
NA
[101]
Synthetic whey
40
1–10.2
85–93.8
0.158–0.35
[102]
Synthetic dairy
Room
temperature
NA
46–59
NA
[103]
Simulated milk
37
10–77.5
78
0.27
[104]
Fluidized bed reactor
Ice-cream
wastewater
35
5
70–90
0.157–0.193
[105]
Anaerobic filter
Biofilm-supported
CSTR
Hybrid UASB
Dairy
35
1.9–5.34
91–97
0.27–0.359
[106]
Hybrid reactor
Dairy
35–55
1–2
64–76
0.03–0.24
[107]
Anaerobic moving
biofilm reactor
Milk permeate
20–25
50–75
57.9–77
0.341
[108]
Whey
35
6–17
70–97
0.33
[109]
Whey
30
1–6
76–95
NA
[110]
Anaerobic sequencing
biofilm batch
reactor (ASBBR)
NA: not available.
3.2. pH and Acid/Base Balance
The pH level plays a major role in AD. As the pH changes, so does the solubility
and form of chemical compounds [111]. The bacteria responsible for different steps of
fermentation have different pH preferences for optimal growth [112]. For hydrolyzing
and acidogenic bacteria, the optimal pH ranges from 4.5 to 6.3 [113], whereas methaneproducing Archaea prefer neutral conditions between 6.8 and 7.5 pH [114]. Single-step
reactors generally require a neutral pH for nominal operation. The pH level itself is regulated by acid or alkaline metabolites produced in the course of anaerobic degradation [115].
The pH is therefore the net result of the feedstock pH and the acid-/base-forming activity
of microbes [116]. Reduced pH is an indicator of methanogenesis disruption and acidic
metabolite accumulation [117]. As such, digesters are fed with alkaline substances (e.g.,
sodium hydroxide) to ensure high methane production. This is particularly crucial for highload anaerobic wastewater treatment reactors. At loads approximating 20 kg COD/m3 /d,
continuous pH adjustment is usually required to maintain methane production [118].
A multi-step process design can be used to avoid the constraints related to the different
pH requirements of microbes across the various stages of waste degradation [119]. To that
end, separate hydrolyzers are operated at high organic load rates (OLRs), short hydraulic
retention times (HRTs), and acidic pH (4.5–6.3). The hydrolyzer discharge is fed into
a large digester with a low OLR conductive to high methanogen activity, in which the
pH is maintained at around 7 [120]. This design is typically used in agricultural biogas
plants [121].
Potentially, methanogenesis can also proceed at low pH levels. Peat environments are
host to Methanomicrobiales and Methanobacteriaceae Archaea able to produce methane at a
pH of around 4.5 [122]. Acidophilic methane-producing Archaea may also be present in
anaerobic sludge in closed digesters. After a long, gradual adaptation process, methane
can be produced at 4.5 or 6.5 pH [123]. However, it should be noted that acidophilic
methanogenesis has only been observed in the laboratory, with no reports to date on its
effective incorporation into full-scale installations [124].
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The pH determines synthesis and toxicity of intermediate and final metabolites during
AD (VFAs, ammonia, hydrogen sulfide). It has been noted that ionized volatile fatty acids
are toxic to methanogens at concentrations above 2000 mg CH3 COOH/dm3 , whereas
the non-ionized forms are harmful at levels as low as 10 mg CH3 COOH/dm3 , or even
lower [125]. For an optimal acid/base balance, it is generally accepted that total VFAs
should not exceed 500 mg CH3 COOH/dm3 , and the VFA-to-alkalinity ratio (expressed in
mg CaCO3 /dm3 ) should be around 0.3 [126]. The pH of the solution can also affect the
toxicity of ammoniacal nitrogen. The dissolved free ammonia and ammonium ions are at an
equilibrium point. Any change in pH results in an equilibrium shift with the non-ionized
ammonia being toxic to microbes at concentrations as little as 50–150 mg NH3 – N/dm3 .
Conversely, dissociated ammonia is far less toxic (1500–3000 mg NH4 + /dm3 ). Methanogens
can acquire an adaptive tolerance to ammoniacal nitrogen at levels as high as 5000 mg
NH4 + /dm3 . This also holds true for sulfur, as its non-dissociated form (H2 S) is approximately twice as toxic as the dissociated S2− [127].
The effect of pH on the dissociation of certain metabolites and their toxicity is also
determined by temperature. The dissociation degree will fall at higher temperatures,
meaning the metabolites can become more toxic. However, this is only the case for stepwise
changes. If the temperature is raised gradually, the microbes will progressively adapt to the
new equilibrium [128]. The effect of pH on anaerobic wastewater treatment performance is
presented in Table 5.
Table 5. Digestion performance vs. wastewater pH.
Reactor Type
Wastewater
pH Value
Initial COD
(mgO2 /dm3 )
COD Removal (%)
CH4 Yield
(m3 CH4 /kg
COD)
Reference
Whey
5.7–7.5
5–20
72–90.2
0.089–0.28
[129]
Whey
7.0
33
90
NA
[101]
Synthetic whey
6.9
1–10.2
85–93.8
0.158–0.35
[102]
Biofilm-supported
CSTR
Synthetic dairy
6.5–6.8
NA
46–59
NA
[103]
Dairy
6.8–7.4
3.2
85–98
0.37
[130]
Fluidized bed reactor
Ice-cream
wastewater
6.8–7.2
5.2–11.7
94.4
NA
[131]
Hybrid UASB
Dairy
5.9–7.6
5
65–93
0.25–0.31
[132]
Downflow–upflow
hybrid reactor
Whey
7.5
55–70
90–98.4
NA
[133]
Milk permeate
7.0
50–75
57.9–77
0.341
[108]
Whey
7.0–7.8
6–17
70–97
0.33
[109]
Whey
6.8–6.9
1–6
76–95
NA
[110]
Anaerobic filter
Anaerobic moving
biofilm reactor
Anaerobic sequencing
biofilm batch
reactor (ASBBR)
NA: not available.
3.3. Stirring
The anaerobic digestion of waste requires effective reactor stirring or flow control
to eliminate dead zones across the digester’s active volume [134]. Stirring increases the
contact surface between the wastewater and the bacterial biomass, while also agitating the
substrate and biochemical products [135]. It also serves to reduce temperature gradients
across the reactor [136], and prevents fouling, encrustation, and dead-zone formation [137].
The method and intensity of mixing anaerobically treated wastewater must also be chosen
with the specific characteristics of anaerobic sludge in mind. Anaerobic bacteria are small
and do not form dense conglomerates that can be separated from treated wastewater by
simple sedimentation [138]. Issues with anaerobic sludge also extend to thickening and
flocculation [139]. Design and process solutions aim to reduce the removal of biomass from
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the reactors (which can translate directly to reduced performance) [140]. This is achieved
by extending solid retention time (SRT) (extending sludge age) and reducing hydraulic
retention time (HRT). This is particularly important for methanogens due to their slow
generation rate [141].
Depending on the reactor design, stirring may be provided by agitators (reactors with
full stirring, e.g., CSTRs) or by the hydraulic pumping of wastewater (e.g., UASB reactors),
or wastewater with biogas (e.g., fluidized bed reactors). The stirring can be continuous or
intermittent [142].
3.4. Organic Load Rate (OLR)
One important aspect of anaerobic wastewater treatment is maintaining the delicate
balance between the general stages of the process: hydrolysis, acidogenesis, and methanogenesis [143]. Changes in flow rates and wastewater composition have been shown to be
critical in affecting anaerobic wastewater treatment performance [144]. This has a direct
influence on the organic load rate (OLR) for the digester and the sludge, the hydraulic
retention time (HRT), and the biomass retention time (SRT) in the reactor. Research is still
in progress on how these parameters relate to and impact the maintenance of wastewater
AD efficiency; it is still not fully known how anaerobic reactors perform under variable
conditions [145].
It has been noted across multiple studies that the accumulation of volatile fatty acids
(VFAs) is a common outcome of overloading and of sudden, uncontrolled changes in
hydraulic and organic loads [146]. Also considered important are variations in hydrogen partial pressure, which play an important role in maintaining the target proportions
between different anaerobic reaction products [147]. The stress of fluctuating OLR and
HRT may cause a shift in metabolism to a less favorable profile, upsetting the population balance between VFA producers (acids and acetogens) and consumers (methanogens,
sulfate- and nitrogen-reducing bacteria). This can lead to excessive carbon dioxide and
hydrogen generation. High hydrogen partial pressures (above 104 atm) can halt methanogenesis and permanently alter metabolic pathways [148]. Once this happens, sensitive and
slow-growing methanogens are unable to quickly and fully eliminate H2 , generated by
acidogenic bacteria [149]. This has been shown to inhibit the degradation of propionate,
malonate, and lactate [150].
Another typical outcome of stress brought on by excessive/unstable HRT and OLR is
a drastic increase in VFAs, which directly affects the biogas composition and production
rate [151]. Researchers agree that excessive OLR tends to cause VFA buildup in the medium
and has an inhibitory or even toxic effect on methanogenic bacteria [138,152,153]. However,
some authors downplay the role of high VFA levels, and instead point to skewed acid/base
balance as the major cause of reactor destabilization [154,155]. This is supported by studies
that did not observe any inhibition and toxicity of VFA after OLR increases and VFA
buildup under careful pH control [156,157]. Under these conditions, it is common to
observe significant accumulation of propionate and extensive hydrogen saturation in the
medium, which facilitates the alternative electron removal pathway [158].
One interesting effect of shortened HRT and elevated OLR, as reported in the literature,
is the sharp decrease in the count and length of filamentous microbes [159,160]. This is
attributed to flushing or disaggregation of these bacteria during shock loads [161]. There
have been investigations into the morphology of anaerobic biofilm as a function of organic
load, VFA levels, and biogas production in fluidized bed reactors. Filamentous bacteria
have been shown to be more populous at lower OLR, whereas high OLR and VFA led to
elevated counts of coliforms and cocci [162,163]. The effect of OLR and HRT on anaerobic
wastewater treatment performance is presented in Table 6.
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Table 6. Digestion performance vs. wastewater HRT and OLR.
Reactor Type
Wastewater
HRT
(d)
OLR
(kgCOD/m3 d)
Initial COD
(mgO2 /dm3 )
COD Removal
(%)
CH4 Yield
(m3 CH4 /kg COD)
Reference
Whey
1–5
1–4
5–20
72–90.2
0.089–0.28
[129]
Whey
10–15
2.2–3.3
33
90
NA
[101]
Synthetic whey
1
1–10.2
1–10.2
85–93.8
0.158–0.35
[102]
Biofilm-supported
CSTR
Synthetic dairy
10
NA
NA
46–59
NA
[103]
Dairy
7.5–11.3
10
3.2
85–98
0.37
[130]
Fluidized bed reactor
Simulated milk
0.75–5
2.22–31
10–77.5
78
0.27
[104]
Ice-cream
wastewater
1
1–12
5
70–90
0.157–0.193
[105]
Dairy
0.25
8–20
5
65–93
0.25–0.31
[132]
Dairy
1.9
0.97–2.82
1.9–5.34
91–97
0.27–0.359
[106]
Hybrid reactor
Dairy
0.5–1
1–2
1–2
64–76
0.03–0.24
[107]
Anaerobic moving
biofilm reactor
Milk permeate
9.7–116.5
0.5–6.5
50–75
57.9–77
0.341
[108]
Whey
0.6–17
1–21
6–17
70–97
0.33
[109]
Whey
0.33
2–12
1–6
76–95
NA
[110]
Anaerobic filter
Hybrid UASB
Anaerobic sequencing
biofilm batch
reactor (ASBBR)
NA: not available.
3.5. Microbial Community
When the microbial community in the anaerobic sludge is highly diverse taxonomically,
multiple species with the same or similar metabolic profile can co-exist [82]. This leads to
a more stable and productive operation of the digester [164]. If an uncontrolled change
in the medium conditions leads to the loss of some species, others will be able to sustain
and continue the given reaction in the methane production cascade [165]. The dominance
of a small number of species indicates that anaerobic bacteria in the reactor, especially
the selective and sensitive methane-generating Archaea, have been exposed to strong
environmental selection pressures (e.g., high temperature or high organic loadings) [166].
This results in the intensive growth of a few select, most adapted species [167].
The higher tolerance of anaerobic biomass to worsening environmental conditions is attributable to differences in the population structure [168]. Boonapatcharoen et al. (2007) [169]
used inoculum sludge sourced from a low-load anaerobic reactor, which was mainly colonized by Methanosaeta-like cells. The authors did not find this surprising, as Methanosaeta spp.
thrive in low-acetate conditions, unlike other acetoclastic methanogens, e.g., Methanosarcina
spp. Stabilized mesophilic reactors of wastewater treatment plants have low levels of
acetate, creating prime conditions for Methanosaeta microbes to grow in the inoculum.
Slow-growing Methanosaeta species have a higher affinity for acetate and lower uptake
threshold (5–15 µM); thus, high levels of acetate can inhibit their growth. Methanosarcina,
which are faster to grow, have a minimum acetate uptake threshold of 1–2 mM. This is
corroborated by Calli et al. (2010) [170], who operated mesophilic UASB reactors inoculated with Methanosaeta-heavy sludge. The Methanosaeta were replaced by Methanosarcina
in series with higher pollutant loadings. The resistance of Methanosarcina species to environmental changes may stem from their chondroitin structures (pseudo-chondroitin,
methano-chondroitin). In very specific growth conditions, chondroitin forms a thick and
rigid outer layer [171]. It can, therefore, be surmised that the constant exposure of anaerobic
biomass to a microwave electromagnetic field creates optimal conditions for the growth of
Methanosarcina strains with chondroitin walls.
3.6. Energetic and Economic Efficiency
The literature data indicates that the demand for energy in anaerobic wastewater
treatment systems is within a very broad range, from 0.20 kWh/m3 to 5.7 kWh/m3 of
wastewater [172]. Such a significant variability of energy consumption is due to the
construction and technological diversity of anaerobic reactors; climatic conditions; the
temperature of the methane fermentation process; and the characteristics, quantity, and
Energies 2023, 16, 83
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properties of the wastewater [173]. The analyses conducted so far have also demonstrated
significant differences in the unit energy demand for the removal of 1.0 kg of COD from
wastewater, with values ranging from 0.27 kWh/kg COD to 10 kWh/kg COD [21]. Apart
from the factors indicated above, these differences are affected by the susceptibility of
wastewater to biodegradation under anaerobic conditions and by the technological parameters of the treatment process, including mainly HRT and OLR. Ample studies have
proven a correlation between the initial concentration of organic compounds in wastewater,
its susceptibility to anaerobic degradation, as well as the applied organic load rate and
retention time in the digester [174,175]. In the case of anaerobic wastewater treatment, it is
possible to partially or fully cover the energy demand by recovering it from the methane
produced. The literature data indicate that the potential energy contained in biogas may
range from 0.3 kWh/m3 to 40 kWh/m3 of treated wastewater [176], whereas the energy
contained in methane reaches 9.17 kWh/m3 , and the maximum theoretical unit amount of
this biogas component is 0.35 m3 /kg of the COD removed. Thus, removing 1.0 kg of COD
can yield approximately 3.2 kWh of potential energy [177]. As in the case of energy demand,
the efficiency of methane production is affected by a huge number of variables. Therefore,
there is a need to optimize the anaerobic process of contaminant biodegradation [178–180].
It has been proven that in the case of highly concentrated biodegradable wastewater, the
production of methane not only covers the demand for energy, but also induces a positive
balance of heat and electricity. In terms of potential energy, it can yield from 5 to 20 kWh/m3
of treated wastewater for export [181]. There are also known examples of energetically
passive or positive anaerobic technological systems [182]. It should be emphasized that the
energy savings achieved during anaerobic wastewater treatment result mainly from the
lack of a need to aerate the wastewater and, in certain cases, from the lack of a need to mix
the digester’s content [183]. Profits are primarily related to the production of methane and
its conversion into heat and electricity [184]. Undoubtedly, a thorough economic balance
should also take into account the costs associated with the construction and design of
digesters and the fact that they are smaller in volume and require less space in terms of
the volume of sewage and the load of organic compounds [185]. It is also essential to take
account of the environmental costs related to the reduction of greenhouse gas emissions
and the production of renewable energy and biofertilizers. This has been confirmed by
LCA and LCC analyses [186,187].
4. Advantages and Disadvantages of Anaerobic Wastewater Treatment Systems
The anaerobic process has found wide use in treating highly concentrated wastewater,
especially that from the food industry [188]. It has been noted that anaerobic methods are
well suited for wastewater with COD levels (which primarily measure readily available
organics) of more than 4000 mgO2 /dm3 . In practice, the anaerobic technology is preferred
mainly for its cost efficiency, as it is considerably cheaper as a means of anaerobic wastewater treatment than traditional aerobic processes. The reduced cost of anaerobic treatment
stems mainly from the obvious lack of the need for wastewater aeration. It is estimated
that this can produce energy savings of up to 75% [183]. The sole energy input required for
anaerobic technologies is to power wastewater stirring/pumping mechanisms and temperature control systems [189]. According to the operators of anaerobic wastewater treatment
systems, heating accounts for approximately two-thirds of the energy consumption of an
anaerobic plant [190]. This portion of energy intake can be reduced by using reactors that
support the capture and combustion of biogas [191]. The methane produced after the full
conversion cycle can be used for fuel, e.g., for heat generation or for co-generation of heat
with electricity. In fact, 1 m3 of methane has a calorific value of 9.17 kWh, which means
that biogas containing 60–70% of methane (the average range) would have a calorific value
of 6–7 kWh [192]. It is estimated that 30% of the heat produced by a co-generation system
will be used to heat the reactor, whereas approximately 10–15% of the production will
be allocated for on-site power generation [193]. Biogas production from the AD of food
industry wastewater usually falls between 0.2 and 0.5 m3 /kg COD removed. The biogas
Energies 2023, 16, 83
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yield depends on a number of factors, the most important of which include the composition
and quality of the wastewater, as well as the type and design of the reactor [194].
The major advantage of anaerobic over aerobic wastewater treatment is the lower
generation of surplus sludge. Anaerobic treatment generates 0.02 to 0.15 g/g biomass per
1 g of COD removed, whereas aerobic systems can produce approx. 0.5–0.7 g/g biomass
for the same pollutant load. Seghezzo et al. (1998) [195] determined that anaerobic systems
produce three to twenty times less surplus sludge than aerobic systems.
Anaerobic wastewater treatment reactors also run at much higher biomass and pollutant loads [196]. This allows them to remove the same organic loads as aerobic ones at a
much smaller reactor size. The performance of anaerobic wastewater treatment is a function
of several factors, including reactor design, process temperature, and wastewater type. For
example, the COD removal from textile industry wastewater can range from 9% to 51%
in a UASB reactor [197], whereas the range for starch-containing wastewater is 77–93%
COD removed [198]. A well-designed, well-built, and well-operated anaerobic wastewater
treatment reactor does not disperse aerosols into the air, nor does it produce odors. A vital
advantage, especially for treating wastewater from seasonal/intermittent production, is
that the reactor can be quickly started, even after extended downtime. Anaerobic sludge
can be stored for months without nutrient media and still largely retains its biochemical
properties [199].
Of course, the anaerobic processes of wastewater treatment have their limitations and
shortcomings [200]. Therefore, they need to be continuously improved and supplemented
with novel technologies for better process stability and performance with regard to methane
yields and final effluent quality [201]. Some of the limitations of anaerobic reactors are
inherent to biological methods. AD entails a cascade of various biochemical conversions
to remove pollutants and synthesize biogas. The process is based on syntrophy (crossfeeding), where the metabolites from one part of the digestion process provide essential
nutrients for the organisms responsible for the subsequent step [202]. This is because the
microorganisms responsible for the successive digestion steps are highly specialized: from
hydrolysis, which can be carried out by various bacteria and Eucaryota, to methanogenesis,
which can only be conducted by specialized Archaea under very specific conditions [203].
Such close interdependence of the assorted digestion steps often leads to underperformance when products necessary to smoothly conduct the successive stages are produced in
too large or small quantities; for example, the overaccumulation of acidogenesis products
reduces pH and inhibits the activity of methanogenic bacteria [204]. The efficiency of the
entire process is ultimately determined by the most sensitive link in the conversion chain,
which is why methanogenic activity is used as the indicator for reactor efficiency [205].
One of the frequently indicated weaknesses of anaerobic technologies is the low
N and P removal rate [206]. In anaerobic processes, nutrients are only removed when
incorporated into the microbial biomass [207]. This issue limits the versatility of the
technology, which is why further advancement of anaerobic methods focuses on eliminating
this hurdle [208]. For this reason, anaerobic technologies are very often treated as the first
step in a larger processing chain [209]. In order to achieve final effluent quality sufficient
to discharge to a receiving water body, digesters are usually integrated and coupled with
other facilities and equipment [210]. Elaborate processing systems must control many
antagonistic operation parameters, such as oxygen concentration, VFA levels, loadings, and
concentrations of digestion microbes [211]. A typical treatment system alternates between
anaerobic and aerobic conditions to provide efficient organics removal, orthophosphate
fixation, ammonification, nitrification, and denitrification. In many cases, the process
needs to be modified or supplemented with other components to improve treatment
performance [212]. Therefore, there is real need to seek versatile methods that could serve as
a competitive alternative to current solutions, both in terms of technological and investment
attractiveness [213]. Various innovative solutions and methods can be used to overcome
this deficiency of anaerobic reactors and improve their technological and commercial
performance. These include active filling [214], dissolution of metals [215], zeolites [216],
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adsorption [217], absorption [218], inorganic coagulants [219], alkaline substances [220],
integration with microalgae cultivation systems [221,222], and many other physical and
chemical treatments [223]. A breakdown of the strengths and weaknesses most commonly
cited in the literature is given in Table 7.
Table 7. Breakdown of strengths and weaknesses of prevailing anaerobic reactors (as indicated in the
literature).
Reactor type
Advantages
Disadvantages
Reference
AD
Cost effective;
Fewer requirements to N and P;
Fewer space requirements;
High removal efficiency;
Lack of pathogenic organisms;
Less sludge generated;
Low energy requirements;
Production of methane, which can be
utilized as a heat or power source
Greater sensitivity to variables
loads and organic shocks;
High capital cost;
High energy requirements;
Long startup periods;
The strict control of
operating conditions;
Toxic compounds
[224–226]
Anaerobic contact process
Ethane as a useful end product;
High removal efficiency;
No oxygen requirements;
Various temperature ranges
Long retention time;
Poor settling properties
[14,227,228]
CSTR
Continuous operation;
Ease of operation;
Ease of cleaning;
High removal efficiency;
Minor operating cost;
No biomass retention;
Reasonable control;
Simple adaptions to two-phase runs
Bypassing and channeling,
probably with weak
agitation performance;
Low conversion per unit volume
[229–231]
Expanded-bed and/or
fluidized-bed
Can control and optimize the biological
film thickness;
Capital cost is lower;
Elimination of bed clogging;
Greater surface area;
High removal efficiency;
Low hydraulic head
Gas hold-up;
Plugging;
Problems of channeling
[232–234]
Fixed-bed digester
High removal efficiency
Difficult to design accurately;
Saturated region
[235–237]
Membrane anaerobic reactor
system (MARS)
Enhances biomass retention;
High removal efficiency
Long retention time
[238–240]
UASB
Biogas can be applied for energy
(generally needs scrubbing first);
Can tolerate high OLRs and hydraulic
loading rates;
Cost effective;
Great decrement in organics;
High removal efficiency;
Minor sludge generation;
No support material required
Long start-up period;
Reactor needs a skilled operation;
Sufficient amount of granular
seed sludge
[241–243]
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Table 7. Cont.
Reactor type
Upflow anaerobic filter
Advantages
Disadvantages
Reference
High OLRs;
High removal efficiency;
Minor sludge production (the sludge is
stabilized);
No electrical energy needed;
Short HRT;
Stable against organic and hydraulic
shock loading
Cleaning and removing the
clogged filter media are difficult;
Effluent and sludge require
further treatment and
proper discharge;
Low reduction of nutrients
and pathogens;
Requires expert design
and construction;
Risk of clogging, depending on
pre- and primary treatment
[244–246]
5. Reactors for Anaerobic Wastewater Treatment
Anaerobic wastewater treatment reactors are designed to minimize the inefficiencies
resulting from slow-growing biomass, which is a problem intrinsic to anaerobic digestion [247]. The main challenge is keeping the SRTs high and the HRTs low [248]. There
are many different designs of anaerobic wastewater treatment reactors currently in use.
Anaerobic-only designs include UASB (upflow anaerobic sludge blanket) and EGSB (expanded granular sludge bed) with high-density granulated anaerobic biomass [249]. In
reactors with full stirring (CSTR, continuous-flow stirred tank reactor, contactors), the
biomass takes the form of suspended sludge and is recirculated into the system by auxiliary
installations (settlement tanks, separators) [250]. In bed reactors, the anaerobic microbial
community forms a biofilm that adheres to the bed. However, in the case of anaerobic
beds, a large portion of the biomass may be suspended above the bed and not firmly attached [251]. The listed reactor types do not represent the totality of the available anaerobic
wastewater treatment solutions. New designs and improvements to existing ones continue
to be introduced. For example, a traditional CSTR can be expanded with a membrane
module [252], which dramatically changes its capabilities and applications. Wastewater
treatment parameters across different types of digestion bioreactors are given in Table 8.
Table 8. Wastewater treatment parameters across different types of digestion bioreactors.
Bioreactor
Normal Load
(kg COD/m3 /d)
Hydraulic Retention Time
(day)
Solid Retention Time
(day)
Reference
CSTR
0.25–4.0
10–60
10–60
[229–231]
Anaerobic filter
1–40
0.5–12
20
[232,253,254]
Fluidized bed reactor
1–100
0.2–5
30
[232–234]
UASB
10–30
0.5–7
20
[241–243]
Hybrid reactor
1–100
0.2–5
20
[255–257]
AnMBRs
0.5–12.5
3–30
20–40
[258–260]
Cubature constructions used in anaerobic wastewater treatment systems must be
resistant to aggressive environments; their wall surfaces must not corrode in contact with
wastewater fed to the installation [261]. An important feature of anaerobic digesters is their
sufficient mechanical resistance. Their construction should ensure the safety of their operation in terms of resistance to pressures related to the retention and transport of wastewater,
as well as resistance to loads related to the assembly of the necessary equipment and accompanying elements [262]. Gas-tightness is a prerequisite for anaerobic installations where the
biogas produced is stored, transported, and ultimately used for energetic purposes [263].
Leaks reduce the energy and economic efficiency of the technological system, pose a threat
to the occupational health and safety of personnel, and exert an adverse effect on the
Energies 2023, 16, 83
16 of 39
environment through the emission of CH4 , CO2 , and H2 S into the atmosphere [264]. Heat
losses should be minimized as well, which is particularly important in facilities operated in
cold climate zones [265]. The methane fermentation process is usually carried out under
mesophilic (30–40 ◦ C) or thermophilic (50–60 ◦ C) conditions. Therefore, it is necessary to
ensure the effective thermal insulation of the digester to reduce convective heat energy
losses [266].
Digesters and biogas tanks should be designed and constructed in a way that protects
against fire or explosion, freezing of gas supply and discharge pipes, gas condensation, and
corrosion caused by substances contained in the gas, including in particular ammonia and
hydrogen sulfide [267]. Shell tanks made of flexible materials should be secured with a
fence at least 1.8 m high. Metal and reinforced concrete biogas tanks may not be enclosed.
Underground biogas tanks can only be loaded with the soil above them [268]. Digesters
should be made of fire-proof materials [269]. The thermal insulation of digesters and biogas
lines should be made in a way that prevents fire from spreading. In addition, digesters and
biogas tanks should be equipped with lightning protection systems and protected against
static electricity [270]. Electrical installations should be carried out in pipes with a degree of
protection not lower than IP-54 or IP-68 if diving mixers are used. Flexible plastics intended
for the construction of biogas tanks should have a tearing strength of at least 15 N/cm, a
methane permeability not higher than 10 cm3 /m2 h bar, resistance to temperatures ranging
from −30 ◦ C to +50 ◦ C, as well as the ability to discharge electrostatic charges.
The basic input data for the design of anaerobic bioreactors include the computational
flow of wastewater and the physical and chemical characteristics of raw wastewater, mainly
in terms of the content of organic compounds (COD, BOD5 , TOC), the content and forms
of nutrients (P, N), and the presence of suspended lipid and protein fractions, sparingly
degradable substances, or substances toxic to anaerobic bacteria [271].
The preliminary calculations and dimensioning of reactors are made based on the
computations of:
Inflowing load of contaminants (COD) Equation (1):
L = Q·C [kg/d]
(1)
where:
L—load of organic contaminants expressed as the chemical oxygen demand (COD)
fed to the reactor within 24 h, kg/d,
Q—daily wastewater flow, m3 /d,
C—COD concentration in raw wastewater, kg O2 /m3 .
Removed load of contaminants (COD) Equation (2):
Lrem = Q·C ·η [kg/d]
(2)
where:
Lrem — load of organic contaminants expressed as the chemical oxygen demand (COD)
removed within 24 h, kg/d,
Q—daily wastewater flow, m3 /d,
C—COD concentration in raw wastewater, kg O2 /m3 ,
η—estimated wastewater treatment performance, %.
Biogas production Equation (3):
h
i
Qb = Lrem · y m3 /d
(3)
where:
Qb —biogas yield, m3 /d,
Lrem — load of organic contaminants expressed as the chemical oxygen demand (COD)
removed within 24 h, kg/d,
y—biogas yield coefficient (estimated empirically), m3 /kgCODrem .
Energies 2023, 16, 83
17 of 39
Energy value of biogas Equation (4):
E = Vb ·cCH4 ·e[kWh/d]
(4)
where:
E—energy value of biogas, kWh/d,
Vb —biogas yield, m3 /d,
CCH4 —methane concentration in biogas, %,
e—methane calorific value 9.17 [kWh/m3 ].
Daily energy load required to heat wastewater Equation (5):
Qc = m·c·∆T [kWh/d]
(5)
where:
Qc— daily energy load required to heat wastewater, kWh/d,
m—daily substrate load, Mg/d,
c—specific heat of sewage sludge, kWh/Mg·K,
∆T—temperature difference between sludge in the digester and substrate, K.
OLR—Organic Load Rate Equation (6):
OLR =
i
Ł h
kg/m3 ·d
VCz
(6)
where:
OLR—digester loading with organic contaminants expressed as COD, kg/m3 ·d,
VAct —active volume of internal tank, m3 ,
Ł—load of organic compounds expressed as COD, kg/d.
Hydraulic retention time (HRT) of wastewater in the digester Equation (7):
HRT =
Vh
[h]
Q
(7)
where:
HRT—hydraulic retention time, h,
Vh —active volume of the digester, m3 ,
Qh —wastewater flow per hour, m3 /h.
5.1. Continuous-Flow Stirred Tank Reactor (CSTR)
In contactors, the flocculated anaerobic sludge is suspended in the wastewater. At
their most basic, anaerobic contactors consist of a reaction vessel (digester) colonized by
anaerobic microflora, together with a stirring system and a settling tank [272]. The sludge
is recirculated from the settlement tank to the reaction chamber in order to age it and
maintain the target concentration (Figure 4). The advantage of this design is that it can
treat wastewater with high levels of suspensions and solids, e.g., proteins and lipids [273].
Therefore, contactors are cited as a potential option for treating meat-industry wastewater,
which other designs struggle with [24]. Compared to other reactor types, contactors are
cheap to build and easy to operate [274]. Their main drawback lies in the processing
limitations, particularly those relating to floating sludge in the settling tank, which can
be washed out in the treated effluent [229]. Due to the low concentrations of sludge in
the reaction vessel, these reactors can only handle smaller organic loads of 0.25–4.00 kg
COD/m3 /d [230]. CSTRs can be considered the progenitor of contactors. In this type of
reactor, the biomass retention time is the same as the hydraulic retention time. They are
rarely used for wastewater treatment, but are a common feature of agricultural biogas
plants [231].
Energies 2023, 16, 83
ergies 2023, 16, 83
the reaction vessel, these reactors can only handle smaller organic loads of 0.25–4.00 kg
COD/m3/d [230]. CSTRs can be considered the progenitor of contactors. In this type of
reactor, the biomass retention time is the same as the hydraulic retention time. They are
rarely used for wastewater treatment, but are a common feature of agricultural 18
biogas
of 39
plants [231].
Figure 4. Diagram of a contactor: 1—reactor, 2—settlement tank, 3—raw wastewater, 4—wastewater
Figure
4. Diagram
a contactor:
1—reactor,
2—settlement
and
sludge,
5—biogas,of6—treated
effluent,
7—recirculated
sludge. tank, 3—raw wastewater, 4—
wastewater and sludge, 5—biogas, 6—treated effluent, 7—recirculated sludge.
5.2. Anaerobic Filters
5.2. Anaerobic
Filters
In anaerobic
filters, the problem of biomass washout is addressed by capturing the
biomass
on top of afilters,
bed, forming
a biofilm
(Figurewashout
5). Filtration
media usually
consistthe
of
In anaerobic
the problem
of biomass
is addressed
by capturing
ceramic
plastic
[275].a Packed-bed
anaerobic
reactors
have usually
a very long
startbiomassand
on top
of astructures
bed, forming
biofilm (Figure
5). Filtration
media
consist
of
up
time, as
theplastic
biofilm
forms very
slowly
under anaerobic
conditions
[276].aWastewater
can
ceramic
and
structures
[275].
Packed‐bed
anaerobic
reactors have
very long start‐
be
through
anaerobic
filters
an upflow
oranaerobic
downflowconditions
configuration.
to
forms
veryinslowly
under
[276].According
Wastewater
uppassed
time, as
the biofilm
Rajeshwari
et al.
(2000)anaerobic
[232], the filters
organic
rate inorthe
reactor can
be as high as
40 kg
can be passed
through
in load
an upflow
downflow
configuration.
Accord‐
3 /d. Filtration reactors have multiple advantages: they are simple in design, easy
COD/m
ing to Rajeshwari
et al. (2000) [232], the organic load rate in the reactor can be as high as
to
do 3not
require stirring
saving
energy),they
run are
stably
even
high
40operate,
kg COD/m
/d. Filtration
reactors(thus
haveenable
multiple
advantages:
simple
inat
design,
OLRs,
have
a
high
ability
to
withstand
toxic
loadings,
and
can
accommodate
fluctuations
easy to operate, do not require stirring (thus enable saving energy), run stably even at
in
theOLRs,
pollutant
However,
oneloadings,
problematic
of such designs
is
high
haveconcentration
a high ability[277].
to withstand
toxic
and aspect
can accommodate
fluctu‐
the
relatively
size of
the digesters,
sinceHowever,
the packing
up a huge
portion
of their
ations
in the large
pollutant
concentration
[277].
onetakes
problematic
aspect
of such
de‐
volume.
Filtration-pore
blockage
also be ansince
issue,
especially
at high
levels
lipids
19
signs is the
relatively large
size of can
the digesters,
the
packing takes
upof
a 43
hugeofportion
and/or
suspended
solids
[278].
of their volume. Filtration‐pore blockage can also be an issue, especially at high levels of
lipids and/or suspended solids [278].
Figure 5. Diagram
anaerobic
biofilter:
1—biofilm
2—raw bed,
wastewater,
3—treated effluent,
Figure of
5. an
Diagram
of an
anaerobic
biofilter:bed,
1—biofilm
2—raw wastewater,
3—treated effluent,
4—biogas. 4—biogas.
The performance of full‐scale anaerobic beds in treating dairy wastewater was tested
by Omil et al. (2003) [253]. After almost two years of measurements, the authors found a
reduction of about 90% in organic matter (COD) at loads of 5–6 kg COD/m3/d and at an
industrial scale. The HRT was changed throughout the experiment, from 20 d at the onset
to 3.33 d at the end, without any significant changes in removal performance, indicating
Energies 2023, 16, 83
rgies 2023, 16, 83
19 of 39
The performance of full-scale anaerobic beds in treating dairy wastewater was tested
by Omil et al. (2003) [253]. After almost two years of measurements, the authors found
a reduction of about 90% in organic matter (COD) at loads of 5–6 kg COD/m3 /d and
at an industrial scale. The HRT was changed throughout the experiment, from 20 d at
the onset to 3.33 d at the end, without any significant changes in removal performance,
indicating that the reactor achieved a steady state [253]. An extremely high COD removal
efficiency (98%) was achieved by Mendez et al. (1989) [254] when using an anaerobic filter
for neutralizing whey, though at the expense of very long HRT (142 d). This very high
removal efficiency was achieved under mesophilic conditions, despite the relatively high
OLR of 9.8 kg COD/m3 /d [254].
5.3. UASB Reactors
A characteristic feature of UASB reactors, much like the structurally similar EGSB
reactors, is that they use anaerobic biomass in the form of granular sludge [279]. The
granules range from 0.14 to 5.00 mm in size [242]. The granulation of sludge makes it easier
to increase its levels in the digester, allowing installations to be operated at OLRs as high
as 10.0–40.0 kg COD/m3 /d [241]. The granulated microbial biomass in a UASB is kept
suspended by the upward-flowing wastewater and biogas [247]. The top of the device is
fitted with a three-phase separator, which separates the treated effluent from the sludge and
biogas bubbles [280]. UASB reactors are highly versatile, making them a popular choice
for treating a variety of industrial effluents [281]. On the other hand, they are beset by the
relatively long start-up times—3 to 5 months—needed to granulate the anaerobic sludge
and pre-treat the effluent if it contains large amounts of lipids and suspended solids [282]. In
practice, this means that UASB usually have to be accompanied by a flotation tank, further
complicating their exploitation. A diagram of a UASB reactor is presented in Figure 6.
UASB reactors have been successfully used to treat dairy effluent for over 50 years [283].
OLRs and HRTs vary greatly across the literature. For example, Najafpour et al. (2008) [243]
20 of 43compounds
tested OLRs ranging from 7.9 to 45.42 kg COD/m3 /d, achieving 98% organic
◦
removal from whey at 35 C.
Figure
6. UASB
reactor
diagram: 1—granules,
2—raw wastewater,
3—treated
effluent, 4—biogas,
Figure 6. UASB
reactor
diagram:
1—granules,
2—raw wastewater,
3—treated effluent,
4—biogas,
5—three‐phase
separator. separator.
5—three-phase
5.4. Fluidized-Bed
5.4. Fluidized‐Bed
Reactors Reactors
In fluidized‐bed
anaerobic fluidized-bed
reactors
7), thebiomass
microbial
biomass
grows on the
In anaerobic
reactors (Figure
7), (Figure
the microbial
grows
on the
surface
of
fine
media
particles
[284].
The
rapid
upward
flow
generated
by
the extensive
surface of fine media particles [284]. The rapid upward flow generated by the extensive
recirculation
keeps
the
biomass
suspended.
The
particles
that
make
up
the
fluidized
bed
recirculation keeps the biomass suspended. The particles that make up the fluidized bed
can
be
made
of
quartz,
pumice
granules,
or
activated
carbon
(the
particles
are
usually
0.2
to
can be made of quartz, pumice granules, or activated carbon (the particles are usually 0.2
3.0 mm in diameter) [233]. This structure allows fluidized-bed reactors to maintain a very
to 3.0 mm in diameter) [233]. This structure allows fluidized‐bed
reactors to maintain a
high concentration of biomass of over 40 kgDM /m3 (even 1003 kgDM /m3 in some cases),
very high concentration of biomass of over 40 kgDM/m3 (even
100 kgDM/m in some cases),
enabling very high OLRs. The values reported in the literature range from 1 to 100 kg
COD/m3/d, at HRTs of 0.5 to 7 d [232]. Unlike typical biofilters, fluidized‐bed reactors
usually do not suffer from blockage. However, they do require relatively high energy in‐
put for the extensive recirculation of effluent. This, in addition to the operational difficulty
5.4. Fluidized‐Bed Reactors
Energies 2023, 16, 83
In anaerobic fluidized‐bed reactors (Figure 7), the microbial biomass grows on the
surface of fine media particles [284]. The rapid upward flow generated by the extensive
recirculation keeps the biomass suspended. The particles that make up the fluidized bed
20 of 39
can be made of quartz, pumice granules, or activated carbon (the particles are usually 0.2
to 3.0 mm in diameter) [233]. This structure allows fluidized‐bed reactors to maintain a
very high concentration of biomass of over 40 kgDM/m3 (even 100 kgDM/m3 in some cases),
enabling
high
valuesinreported
in the range
literature
1 to 100 kg
enabling very
high very
OLRs.
TheOLRs.
valuesThe
reported
the literature
fromrange
1 to from
100 kg
3 /d, at HRTs of 0.5 to 7 d [232]. Unlike typical biofilters, fluidized-bed reactors
COD/m
COD/m3/d,
at HRTs
of 0.5 to 7 d [232]. Unlike typical biofilters, fluidized‐bed reactors
do not
suffer
from blockage.
they dorelatively
require relatively
high in‐
energy input
usually dousually
not suffer
from
blockage.
However,However,
they do require
high energy
the extensive
recirculation
of effluent.
in addition
to the operational
difficulty of
put for thefor
extensive
recirculation
of effluent.
This, inThis,
addition
to the operational
difficulty
fluidization
(keeping
the
fluid
suspended),
is
the
primary
flaw
of
this
design
[234].
of fluidization (keeping the fluid suspended), is the primary flaw of this design [234].
Figure 7. Fluidized-bed
reactor
diagram: 1—fluidized
2—raw wastewater,
3—treated effluent,
Figure 7. Fluidized‐bed
reactor diagram:
1—fluidized
bed, 2—rawbed,
wastewater,
3—treated effluent,
4—biogas, 5—recirculation
of treated effluent.
4—biogas, 5—recirculation
of treated effluent.
Hybrid Reactors
Anaerobic Reactors
5.5. Hybrid5.5.
Anaerobic
Hybridare
reactors
are
designs
merge
of other, ordinarily
discrete
Hybrid reactors
designs
that
mergethat
aspects
of aspects
other, ordinarily
discrete types
of types of
anaerobic
reactors
(Figure
8).
The
designers
of
such
solutions
typically
set
out
to
anaerobic reactors (Figure 8). The designers of such solutions typically set out to harnessharness the
strengths of the given reactor type while minimizing or eliminating its limitations [255]. The
most common hybrid process combines anaerobic filters with UASB reactors. This enables
large organic loadings while increasing biomass retention in the system. Such hybrid
reactors can also be quickly restarted after downtime [256]. Ramasamy et al. (2004) [257]
used this design to degrade synthetic effluent with a COD of 10 g/dm3 . The organic load
rate ranged from 0.82 to 6.11 kg COD/m3 /d and the hydraulic retention time from 4.1
to 1.7 d. Under these process parameters and mesophilic conditions, the COD removal
efficiencies varied between 90% and 97%. Another type of hybrid design calls for coupling
a CSTR with a membrane module for efficient anaerobic sludge separation [285], which
dramatically changes its capabilities and applications.
5.6. Anaerobic Membrane Bioreactors (AnMBRs)
Anaerobic membrane bioreactors (AnMBRs) have been used as an alternative to
traditional anaerobic digestion (Figure 9) [286]. Research and operational data show
that AnMBRs offer reduced sludge production and better biogas yields. It is believed that
incorporating membranes into an anaerobic bioreactor design reduces the total energy input
and facilitates microbial community retention for high-biomass, high-OLR processing [287].
The problem with this reactor type is membrane fouling, caused by foreign materials in
wastewater. Major membrane fouling factors include high concentrations of suspended
solids, protein, lipids, oils, and grease. Another limitation of AnMBRs is the high level
of extracellular polymeric substances (EPS) generated by anaerobic sludge (especially
granular sludge) microbes [288]. Depending on the target parameters of the treated effluent,
potential treatments include microfiltration, ultrafiltration, nanofiltration, and reverse
osmosis [289]. Buntner et al. (2013) [258] tested a combination of a UASB plus ultrafiltration
membrane reactor on dairy wastewater. The biogas production was 150 dm3 /kg COD with
Energies 2023, 16, 83
Energies 2023, 16, 83
the strengths of the given reactor type while minimizing or eliminating its limitations
[255]. The most common hybrid process combines anaerobic filters with UASB reactors.
This enables large organic loadings while increasing biomass retention in the system. Such
21 of 39
hybrid reactors can also be quickly restarted after downtime [256]. Ramasamy et al. (2004)
[257] used this design to degrade synthetic effluent with a COD of 10 g/dm3. The organic
load rate ranged from 0.82 to 6.11 kg COD/m3/d and the hydraulic retention time from 4.1
to 1.7 d. Under
these process
conditions,
the whereas
COD removal
a methane
fractionparameters
of 73%, theand
OLRmesophilic
was 4.85 kg
COD/m3 /d,
the COD removal
efficiencies varied
between
90%
and 97%.
Another
type ofwas
hybrid
designbycalls
for coupling
reached
95–99%.
Slightly
worse
performance
reported
Deowan
et al. (2013) [290] at
organic removal.
were pilot-scale
but the[285],
former
implementation
a CSTR with90%
a membrane
module Both
for efficient
anaerobicinstallations,
sludge separation
which
an external
membrane,
whereas the latter was immersed in the bioreactor.
dramaticallyused
changes
its capabilities
and applications.
22 of 43
Figure 8. Diagram
of 8.
a hybrid
reactor:
1—suspended
zone, 2—bed,
3—raw
4— wastewater,
Figure
Diagram
of a hybrid
reactor:sludge
1—suspended
sludge
zone,wastewater,
2—bed, 3—raw
treated effluent.
4—treated effluent.
5.6. Anaerobic Membrane Bioreactors (AnMBRs)
Anaerobic membrane bioreactors (AnMBRs) have been used as an alternative to tra‐
ditional anaerobic digestion (Figure 9) [286]. Research and operational data show that
AnMBRs offer reduced sludge production and better biogas yields. It is believed that in‐
corporating membranes into an anaerobic bioreactor design reduces the total energy input
and facilitates microbial community retention for high‐biomass, high‐OLR processing
[287]. The problem with this reactor type is membrane fouling, caused by foreign materi‐
als in wastewater. Major membrane fouling factors include high concentrations of sus‐
pended solids, protein, lipids, oils, and grease. Another limitation of AnMBRs is the high
level of extracellular polymeric substances (EPS) generated by anaerobic sludge (espe‐
cially granular sludge) microbes [288]. Depending on the target parameters of the treated
effluent, potential treatments include microfiltration, ultrafiltration, nanofiltration, and
reverse osmosis [289]. Buntner et al. (2013) [258] tested a combination of a UASB plus
ultrafiltration membrane reactor on dairy wastewater. The biogas production was 150
dm3/kg COD with a methane fraction of 73%, the OLR was 4.85 kg COD/m3/d, whereas
the COD removal reached 95–99%. Slightly worse performance was reported by Deowan
et al. (2013) [290] at 90% organic removal. Both were pilot‐scale installations, but the for‐
mer implementation used an external membrane, whereas the latter was immersed in the
bioreactor.
Figure
Figure9.9.Diagram
DiagramofofananAnMBR:
AnMBR:1—membrane
1—membranemodule,
module,2—biogas
2—biogasrecycle
recyclefor
formembrane
membranesourcing,
sourcing,
3—raw
3—rawwastewater
wastewaterinflow,
inflow,4—permeate,
4—permeate,5—biogas
5—biogasoutflow.
outflow.
Unconventionaland
andInnovative
InnovativeAnaerobic
AnaerobicReactors
Reactors
6.6.Unconventional
6.1. Multi-Section Hybrid Anaerobic Reactor (M-SHAR)
6.1. Multi‐Section Hybrid Anaerobic Reactor (M‐SHAR)
The structure of the M-SHAR consists of two separate sections, each with their own
The structure of the M‐SHAR consists of two separate sections, each with their own
function. The lower section, filled with granular sludge, is made up of two centrally
function. The lower section, filled with granular sludge, is made up of two centrally ar‐
arranged chambers (inner and middle) [32]. The design provides for separate hydrolranged chambers (inner and middle) [32]. The design provides for separate hydrolysis/ac‐
ysis/acidogenesis/methanogenesis zones and a meandering (labyrinthine) flow of the
idogenesis/methanogenesis zones and a meandering (labyrinthine) flow of the
wastewater being treated. The top part of the M‐SHAR houses a microwave‐heated bio‐
logical filter. Raw sewage is transported to the lower section, filled with granular sludge,
and then separated from it in the course of treatment. Afterwards, the liquid is pumped
onto the microwave‐heated anaerobic filter, where the dissolved organic matter is further
biodegraded. Studies have shown that microwave heating improves wastewater treat‐
Energies 2023, 16, 83
Energies 2023, 16, 83
22 of 39
wastewater being treated. The top part of the M-SHAR houses a microwave-heated biological filter. Raw sewage is transported to the lower section, filled with granular sludge,
and then separated from it in the course of treatment. Afterwards, the liquid is pumped
onto the microwave-heated anaerobic filter, where the dissolved organic matter is further
biodegraded. Studies have shown that microwave heating improves wastewater treatment
performance. At OLRs of 15 to 25 kg COD/m3 /d, the microwave-heated bioreactor boasted
of 43
5% higher COD removal and 12–20% higher biogas production [32]. The M-SHAR 23
reactor
is illustrated in Figure 10.
Figure 10.
(a)(a)
and
photograph
(b)(b)
of the
M-SHAR:
1—microwave
generator,
2—microwave
10. Diagram
Diagram
and
photograph
of the
M‐SHAR:
1—microwave
generator,
2—micro‐
wave generator
seal, 3—LDW
supply,
4—circulation
bed, 6—outlet,
generator
seal, 3—LDW
supply,
4—circulation
pump,pump,
5—US5—US
of bed,of6—outlet,
7—LS;7—LS;
sludgesludge
sepaseparation
area,
8—sludge
collector,
9—LS;
granular
sludge
area,
10—central
hose,
11—LDW
me‐
ration area, 8—sludge collector, 9—LS; granular sludge area, 10—central hose, 11—LDW metering
tering
pump,
12—water
jacket.
pump, 12—water jacket.
6.2. Anaerobic Fluidized Active Filling Reactor (FAF-R)
(FAF‐R)
The general FAF-R
FAF‐R process design consists of a retention (hydrolysis) tank equipped
with
(FAF)
is introduced
into
thethe
exact
digestion
with aa vertical
verticalagitator.
agitator.AAfluidized
fluidizedactive
activefilling
filling
(FAF)
is introduced
into
exact
diges‐
reactor
at a ratio
of 1/5
of the
digester’s
active
volume
tion reactor
at a ratio
of 1/5
of the
digester’s
active
volumeofofthe
thechamber
chamber[214].
[214]. Much
Much like
the
the hydrolysis
hydrolysis tank,
tank, the
the reactor
reactor is
is fitted
fitted with
with aa vertical
vertical agitator
agitator rotating
rotating at
at 60
60 rpm
rpm and
and
working
on
a
30
min
on/30
min
off
operating
scheme.
Due
to
the
density
of
the
FAF, the
working on a 30 min on/30 min off operating scheme. Due to the density of the FAF,
the
tank
tank acts
acts as
as aa contactor
contactor with
with fluidized
fluidized filling
filling during
during the
the mixing-on
mixing‐on periods.
periods. During
During the
the
mixing-off
the FAF
mixing‐off period,
period, the
FAF floats
floats to
to the
the top,
top, where
where aa filtration
filtration layer
layer forms
forms [214].
[214]. COD
COD
removal
have
been
reported
at over
74%.74%.
The The
biogas
yieldsyields
ranged
from
removal rates
ratesininthe
thereactor
reactor
have
been
reported
at over
biogas
ranged
3 /kg COD removed and the biogas contained approximately 70%
356
±
25
to
427
±
14
dm
3
from 356 ± 25 to 427 ± 14 dm /kg COD removed and the biogas contained approximately
3
methane.
Increasing
the organic
loadload
to 8.0
kg kg
COD/m
3/dled
70% methane.
Increasing
the organic
to 8.0
COD/m/d
ledto
toaasignificant
significant decrease
decrease
in
AD
and
effluent
treatment
efficiency,
which
correlated
with
decreased
pH
and
a rise
in AD and effluent treatment efficiency, which correlated with decreased pH
and
a rise
in
in
FOS/TAC
to
0.44
±
0.2.
Phosphorus
removal
rates
with
active
filling
ranged
FOS/TAC to 0.44 ± 0.2. Phosphorus removal rates with active filling ranged from 64.4from
± 2.4
64.4
± 2.4
to 81.2
± 8.2%, depending
onThe
the concentration
stages. The concentration
of totalsolids
suspended
to 81.2
± 8.2%,
depending
on the stages.
of total suspended
in the
solids
in
the
treated
effluent
was
low
[214].
A
diagram
of
the
M-SHAR
reactor
is
provided
treated effluent was low [214]. A diagram of the M‐SHAR reactor is provided in Figure
in Figure 11.
11.
Energies 2023, 16, 83
of 43
39
2423of
Figure 11. Diagram
Diagram of
of the experimental
experimental process system: 1—hydrolysis
1—hydrolysis tank,
tank, 2—fermentation
2—fermentation tank,
tank,
3—agitators,
3—agitators, 4—rotary
4—rotary lobe
lobe pump,
pump, 5—peristaltic
5—peristaltic pump,
pump, 6—magneto‐active
6—magneto-active filling,
filling, 7—limiting
7—limiting grid,
grid,
8—biogas
9—outflow of
of treated
treated effluent.
effluent.
8—biogas outflow
outflow with
with aa meter,
meter, 9—outflow
6.3. Multi‐Section
Multi-Section Horizontal Flow Anaerobic Reactor (HFAR)
This digester
into
four
sections,
each
for for
oneone
of the
digester is
ispartitioned
partitionedwith
withperforated
perforatedplates
plates
into
four
sections,
each
of
processing
steps
(hydrolysis,
acidogenesis,
acetogenesis,
andand
methanogenesis)
(Figure
12).
the
processing
steps
(hydrolysis,
acidogenesis,
acetogenesis,
methanogenesis)
(Figure
Stirring
in the
chamber
is provided
byby
a shaft
rotating
12).
Stirring
in the
chamber
is provided
a shaft
rotatingatat11rpm
rpmand
andfitted
fitted with
with stirring
Thereactor
reactorisisheated
heatedusing
usingmicrowave
microwavegenerators
generatorswith
withmagnetrons.
magnetrons.
Each
section
Each
section
is
rods. The
is equipped
with
a of
setfour
of four
20 kHz,
W ultrasonic
transducers
In thepart
upper
equipped
with
a set
20 kHz,
95 W95
ultrasonic
transducers
[21]. In[21].
the upper
of
partHFAR,
of the HFAR,
each partition
is equipped
with
for and
sludge
andcollection,
biogas collection,
the
each partition
is equipped
with ports
forports
sludge
biogas
as well
as
well
as
monitoring
temperature
and
pH.
The
COD
and
TOC
removal
in the
as monitoring temperature and pH. The COD and TOC removal in the HFAR
hasHFAR
been
3
has been measured
at approximately
at gOLR
of 2.03/d.
g The
COD/dm
measured
at approximately
85% at OLR85%
of 2.0
COD/dm
biogas /d.
yieldThe
wasbiogas
0.33 ±
yielddm
was
± removed,
0.03 dm3 /g
COD
removed,content
with a of
methane
content
of 68.1
± 5.8%. Peak
3/g0.33
0.03
COD
with
a methane
68.1 ± 5.8%.
Peak
performance
was
performance
achieved
with ultrasonic
generators
in a
min on/28
off
achieved
withwas
ultrasonic
generators
working
in a 2 minworking
on/28 min
off2 scheme.
Themin
nutri‐
scheme.
The
nutrient
removal
was
low
across
all
series
of
the
experiment,
ranging
from
ent removal was low across all series of the experiment, ranging from 2.04 ± 0.38 to 4.59 ±
2.04 ±for
0.38
to 4.59 ± and
0.68%
for 9.67
phosphorus
and from
9.67for
± nitrogen
3.36 to 20.36
0.68%
phosphorus
from
± 3.36 to 20.36
± 0.32%
[21]. ± 0.32% for
nitrogen [21].
6.4. Labyrinth-Flow Vertical Anaerobic Reactor with Magneto-Active Filling (LFAR–MAF)
A labyrinth-flow vertical anaerobic reactor (LFAR) consists of three parts, each having a different function: hydrolysis–acidogenesis, methanogenesis, and sedimentation–
clarification (Figure 13) [291]. The reactor is equipped with a two-pump system: the first
pump delivers raw wastewater to the reactor; the second, recirculating pump provides
complete mixing and circulation of sludge and wastewater in the hydrolysis tank. The flow
is directed downward in the methanogenesis tank, and upward in the clarifier. A diagram
of the LFAR is presented in Figure 13. The hydrolysis tank is packed with magneto-active
filling (MAF) comprising PE-HD beads. The beads contain neodymium magnets and are
coated with Cu-Fe powder. The metals (90 Fe/10 Cu) account for 5.0% of the weight
of a single bead [291]. The LFAR-MAF has been operated at the following parameters:
temperature = 35 ◦ C; HRT = 2 d; and organic load (in COD) = 6.0 g COD/dm3 /d. The
best AD performance was noted for 60% and 80% MAF in the filling [291]. The MAF had a
significant impact on methane fractions when the filling contained 60% to 100% magnets,
with over 70% CH4 [291]. The experiments showed that the MAF had a positive effect on
COD removal when the beads contained 20%, 40%, and 60% FM. The organic compounds
removal in these series reached 75–76%, but subsequent increases in the number of SMF
beads did not improve COD removal any further [291]. MAF was also shown to ensure
low levels of H2 S in the biogas.
Energies 2023, 16, 83
Energies 2023, 16, 83
25 of 43
24 of 39
12. Horizontal
Horizontalflow
flow
anaerobic
reactor
(HFAR)
schematic
diagram,
photo.
1—influ‐
Figure 12.
anaerobic
reactor
(HFAR)
(a)(a)
schematic
diagram,
(b)(b)
photo.
1—influent,
ent,
2—effluent,
3—sludge
recirculation,
4—mixing
blade,
5—thermal
insulation,
6—membrane
2—effluent, 3—sludge recirculation, 4—mixing blade, 5—thermal insulation, 6—membrane modmodule,
7—gas
meter,
8—digestate,
9—gearmotor,
10—samplingport,
port,GM—microwave
GM—microwave generator,
ule,
7—gas
meter,
8—digestate,
9—gearmotor,
10—sampling
generator,
GU—ultrasonic generator.
GU—ultrasonic generator.
6.4. Innovative
Labyrinth‐Flow
Vertical
Anaerobic
Reactor
with Magneto‐Active
Filling (LFAR–MAF)
6.5.
Anaerobic
Bioreactor
with
Fixed-Structured
Bed (ABFSB)
A labyrinth‐flow
anaerobic
(LFAR)
of three parts,
each
hav‐
In
this design, thevertical
digestion
processreactor
is coupled
withconsists
phase separation
in an
upflow
anaerobic
bioreactor
withhydrolysis–acidogenesis,
fixed-structured bed (ASTBR)
as the methanogenic
phase [236].
ing a different
function:
methanogenesis,
and sedimentation–
The
technology
was tested
on sugarcane
vinasse,
a type of
effluent
from ethanol
production.
clarification
(Figure
13) [291].
The reactor
is equipped
with
a two‐pump
system:
the first
3 /d [236]. The ASTBR was
The
organic
loadings
were
within
a
range
of
15–30
kg
COD/m
pump delivers raw wastewater to the reactor; the second, recirculating pump provides
successful
in removing
over 80% of
CODand
from
the vinasse,
demonstrated
stable
complete mixing
and circulation
of the
sludge
wastewater
in and
the hydrolysis
tank.
The
3 /d. A UASB
long-term
operation
(240 days),
for OLR values as
highand
as 30
kg COD/m
flow is directed
downward
in even
the methanogenesis
tank,
upward
in the
clarifier. A
performed
worseisunder
similar
conditions,
suffering
from
acid accumulation
diagram ofmuch
the LFAR
presented
in process
Figure 13.
The hydrolysis
tank
is packed
with mag‐
and
impaired
performance
[236].
neto‐active
filling
(MAF) comprising
PE‐HD beads. The beads contain neodymium mag‐
nets and are coated with Cu‐Fe powder. The metals (90 Fe/10 Cu) account for 5.0% of the
weight of a single bead [291]. The LFAR‐MAF has been operated at the following param‐
eters: temperature = 35 °C; HRT = 2 d; and organic load (in COD) = 6.0 g COD/dm3/d. The
best AD performance was noted for 60% and 80% MAF in the filling [291]. The MAF had
a significant impact on methane fractions when the filling contained 60% to 100% mag‐
nets, with over 70% CH4 [291]. The experiments showed that the MAF had a positive effect
on COD removal when the beads contained 20%, 40%, and 60% FM. The organic com‐
pounds removal in these series reached 75–76%, but subsequent increases in the number
Energies 2023, 16, 83
Energies 2023, 16, 83
26 of 43
25 of 39
of SMF beads did not improve COD removal any further [291]. MAF was also shown to
ensure low levels of H2S in the biogas.
Figure
(LFAR).
Figure 13.
13. Labyrinth‐flow
Labyrinth-flow vertical
vertical anaerobic
anaerobic reactor
reactor (LFAR).
6.5. Innovative
Anaerobic
Bioreactor
BedThe
(ABFSB)
The ABFSB
has been
tested with
on aFixed‐Structured
laboratory scale.
experimental reactor was
constructed
of PMMA
methacrylate))
a total
working
volume
4.77 L.
In this design,
the (poly(methyl
digestion process
is coupled to
with
phase
separation
in anofupflow
2 /g (98%
The
medium
had
a
total
weight
of
32
g,
which
translated
to
an
area
of
43.8
m
anaerobic bioreactor with fixed‐structured bed (ASTBR) as the methanogenic phase [236].
3 /min) so that the hydraulic
porosity).
The feed
pump on
output
was 0.4
dm3 /h
(6.6 cm
The
technology
was tested
sugarcane
vinasse,
a type
of effluent
from ethanol produc‐
retention
time would
be 12were
h. The
process
temperature
30 ± 1 3◦/d
C [236].
[292]. The
The ASTBR
reactor
tion.
The organic
loadings
within
a range
of 15–30 was
kg COD/m
was inoculated
with
granular
sludge
fromfrom
a UASB
treating
poultry
was
successful in
removing
over
80%sourced
of the COD
the reactor
vinasse,for
and
demonstrated
slaughterhouse
Thedays),
matrixeven
inoculation
out
by grinding
3/d. A
stable
long‐termwastewater.
operation (240
for OLRprocedure
values as was
highcarried
as 30 kg
COD/m
the
granules
and
immersing
the
medium
in
the
biomass
for
2
h
at
room
temperature.
UASB performed much worse under similar process conditions, suffering from acid accu‐
Synthetic and
sewage
was used
for the experiment
[292]. The ABFSB was operated at four
mulation
impaired
performance
[236].
2− ratios, namely, 6.1, 3.5, 1.7, and 0.72. The best COD and sulfate
different
COD/SO
The ABFSB has4 been tested on a laboratory scale. The experimental reactor 2was
con‐
− of 1.7.
removal of
performance
(82 and 89%,
respectively)towas
achieved
at avolume
COD/[SO
4 ]L.
structed
PMMA (poly(methyl
methacrylate))
a total
working
of 4.77
The
The
variants
also
boasted
a
higher
overall
kinetic
apparent
parameter
(0.96/h).
The
kinetic
2
medium had a total weight of 32 g, which translated to an area of 43.8 m /g (98% porosity).
parameters
indicate
thatwas
sulfate-reducing
bacteria
played
a major
part in removing
organic
The
feed pump
output
0.4 dm3/h (6.6
cm3/min)
so that
the hydraulic
retention
time
matter over a COD/[SO4 2− ] range of 6.1 to 1.7 [292].
would be 12 h. The process temperature was 30 ± 1 °C [292]. The reactor was inoculated
with
granular Multistage
sludge sourced
fromHythane
a UASB
reactor
for treating poultry slaughterhouse
6.6. Innovative
Anaerobic
Reactor
(MAHR)
wastewater. The matrix inoculation procedure was carried out by grinding the granules
Biohythane production from wastewater via AD is currently based on two-step, physand immersing the medium in the biomass for 2 h at room temperature. Synthetic sewage
ically separated processes that ultimately produce biohydrogen and biomethane [293].
was used for the experiment [292]. The ABFSB was operated at four different COD/SO42−
Such set-ups usually integrate two separate anaerobic reactors, which requires advanced
ratios, namely, 6.1, 3.5, 1.7, and 0.72. The best COD and sulfate removal performance (82
monitoring and costly, complex operation [293]. A multistage anaerobic hythane reactor
and 89%, respectively) was achieved at a COD/[SO42−] of 1.7. The variants also boasted a
(MAHR) is an innovative solution that integrates the two steps of the digestion in a single
higher overall kinetic apparent parameter (0.96/h). The kinetic parameters indicate that
digester [293]. An experimental MAHR has been constructed, consisting of an internal
sulfate‐reducing bacteria played a major part in removing organic matter over a
downflow reactor with an anaerobic filter and an external upflow sludge blanket to boost
COD/[SO42−] range of 6.1 to 1.7 [292].
microbial enrichment and thermodynamic efficiency of the associated bioreactions. The
MAHR was run for 160 d, with performance assessed on the basis of biogas production,
metabolic flux, and microbial population composition against a typical anaerobic high-rate
UASB. Biohythane production with an optimized hydrogen volume ratio (10–20%) and
high methane content (75–80%) was achieved in the hythane zone (MH) and methane
Energies 2023, 16, 83
26 of 39
zone (MM) of the MAHR, respectively. In addition, the MAHR showed better adaptability
at high organic loadings (120 g COD/dm3 /d) and provided better organic compounds
conversion, with a methane production rate 66% higher than the UASB. A thermodynamic
analysis indicated that the hydrogen extraction in MH significantly decreased the hydrogen
partial pressure (<0.1% vol.), which drove acetogenesis in MM. An analysis of the metabolic
flux and microbial function further demonstrated that the MAHR showed superior performance to the UASB, which was primarily attributed to improved acetogenesis and
acetoclastic methanogenesis [293].
The performance of unconventional and innovative anaerobic reactor designs is presented in Table 9.
Table 9. Performance of unconventional and innovative anaerobic reactor designs.
Reactor Type
Wastewater
HRT
(d)
OLR
(kg COD/m3 /d)
Initial COD (mgO2 /dm3 )
COD Removal
(%)
Biogas Yield
(dm3 /kg COD
rem.)
CH4 Fraction (%)
Reference
Multi-section
hybrid anaerobic
reactor
(M-SHAR)
Liquid dairy
waste (LDW)
largely
composed of
acid whey
2–10
5–25
50,000 ± 250
47–88
280–410
(dm3 /kg COD/d)
39–73
[32]
Anaerobic
fluidized active
filling reactor
(FAF-R)
Sugar-industry
effluent
20
4–8
3400 ± 201–6800 ± 270
>74
356 ± 25–427 ± 14
70
[214]
Multi-section
horizontal flow
anaerobic reactor
(HFAR)
Dairy
1
1–4
1029 ± 20.5–4046 ± 60.4
52.75 ± 0.42–
85.13 ± 1.05
30.2 ± 4.5–68.1 ± 5.8
[21]
Labyrinth-flow
vertical anaerobic
reactor with
magneto-active
filling
(LFAR–MAF)
Dairy
2
6
12,000 ± 130.6
75–76
317.1–319.0
66.1–75.8
[291]
Innovative
anaerobic
bioreactor with
fixed-structured
bed (ABFSB)
Sugarcane
vinasse
18–37
h
15–30
28.3 ± 4.6 (g/dm3 )
80
249 ± 93–301 ± 12
NA
[236]
Innovative
multistage
anaerobic
hythane reactor
(MAHR)
Synthetic
3
2–10 kg
COD/m3
120 g COD/dm3 /d
NA
NA
75–80
[293]
0.15 ± 0.01–0.33 ± 0.03
(dm3 /g CODrem.)
NA: not available.
7. Summary of Knowledge and Development Directions
Anaerobic wastewater treatment methods can efficiently biodegrade organic matter
and are still being actively developed. Such technologies are predominantly used in industries that generate high volumes of readily biodegradable waste. The popularity of
digesters stems from their multiple major advantages over aerobic-activated sludge or
biofilter processes. The most frequently cited benefits are the effective treatment of highOLR wastewater, the ability to accommodate high OLRs, relatively small size and space
requirements, markedly lower investment and operation costs compared with aerobic
technologies, and the ability to halt and resume operation (which is a priority for intermittent/seasonal producers). Anaerobic systems generate less surplus sludge, provide better
sludge stabilization, and lead to improved sanitary indicators, which directly contribute to
easier neutralization and management. Another crucial aspect is their ability to produce
methane-rich biogas that can be harnessed for energy purposes. The aforementioned benefits have been demonstrated not only in experimental research or pilot-scale units, but
predominantly in large-scale plants. There is currently a wide variety of solutions available
on the market, which greatly enhances the versatility of anaerobic wastewater treatment—
an important consideration for deployment. The high technical readiness level (TRL) and
thorough understanding of the technology makes it easier to select the appropriate process
for the type and volume of wastewater, pollutant load, biodegradability parameters, or
digestion inhibitors in the system.
Energies 2023, 16, 83
27 of 39
The nature of AD reactors and their advantages over aerobic technologies make them
an excellent choice from the standpoint of sustainable development, as well as for programs
intended to improve waste management, protect the environment, reduce energy waste,
and promote renewable energy. Since AD produces biogas and reduces carbon dioxide
emissions, its deployment is also in line with the principles of a circular economy, energy
and material recycling, bio-economy, and the European Union climate and energy package.
One of the limitations of anaerobic wastewater treatment is that the processing is
biologically mediated, making it susceptible to external physical and chemical factors.
Optimal and stable functioning of digestion reactors is a balancing act between environmental conditions and the rate/products of biochemical reactions. Achieving this balance
requires precise adherence to key process parameters, including temperature, organic
load rate, hydraulic retention time, anaerobic sludge levels in reactors, method and frequency of wastewater feeding, homogenization of waste in retention/equalization tanks,
stirring method, and wastewater amendment with microelements and essential nutrients
as necessary.
The low nitrogen and phosphorus removal rates of anaerobic wastewater treatment
are considered its biggest drawback. These biogenic substances are mainly responsible for
accelerating the eutrophication and degradation of natural water bodies. The fixation of
nitrogen and phosphorus in digesters is driven solely by the growth of bacterial biomass,
of which these substances are an important component. The N and P removal percentage
is usually within single digits. The resultant anaerobically treated effluent cannot be
discharged directly to a receiving water body, as it exceeds permissible nitrogen and
phosphorus concentrations.
Anaerobic digestion has been thoroughly explored in terms of its biochemical pathways, taxonomy of fermentation microorganisms, optimal medium conditions, inhibitors of
pollutant degradation, and final performance. The current understanding of the technology
and its shortcomings enables the identification of areas of growth and improvement. Granular sludge reactors are currently a popular choice, as they largely prevent sludge washout
and reduce the risk of effluent recontamination. There have also been bioreactors built
and tested with structurally and functionally separated zones for specific steps of the AD
cascade. Various packings have been explored to increase the active surface for anaerobic
microbe growth, boost nutrient removal, and stimulate biochemical conversion. Finally,
physical factors have been utilized as well, including microwave radiation, static magnetic
field, electromagnetic field, and ultrasound. Such treatments ensure optimal medium conditions and promote biochemical reactions, as exemplified by the demonstrated non-thermal
effects of electromagnetic microwave radiation.
Integrating AD with microalgal biomass production systems is an important next step
in the development of anaerobic wastewater treatment. This is borne from the need to
remove N and P, as well as to reduce the costs of microalgal culture via novel, cost-effective
technologies. Numerous studies have shown that nitrogen and phosphorus in pre-treated
wastewater can serve as valuable sources of nutrients. After all, pre-treated sewage has
low concentrations of organic matter and contains readily available mineralized nitrogen
and phosphorus. Incorporating such methods in microalgae cultivation systems becomes
even more justified when considering the fact that anaerobically treated effluent cannot
be discharged directly into the environment, and requires complex, costly post-processing
(including nitrification, denitrification, and biological/chemical phosphorus removal).
8. Conclusions
Anaerobic wastewater treatment is the main avenue of wastewater treatment technology development, especially with regard to industrial effluents. The currently increasing
number of large-scale units speaks to the considerable benefits and rising popularity of
anaerobic digestion. However, despite the clear advantages, there is still a need to seek
new technologies and to upgrade/improve the existing ones, as evidenced by the num-
Energies 2023, 16, 83
28 of 39
ber of experimental and operational studies conducted by scientific and technological
institutions worldwide.
Author Contributions: Conceptualization, M.Z. and M.D.; methodology, M.Z. and M.D.; validation,
M.Z.; formal analysis, M.Z. and J.K.; investigation, M.Z., J.K., and M.D.; resources, M.Z., J.K., and
M.D.; data curation, M.Z. and J.K.; writing—original draft preparation, J.K. and M.D.; writing—
review and editing, M.Z., J.K., and M.D.; visualization, M.D and J.K.; supervision, M.Z.; project
administration, M.Z.; funding acquisition, M.D. All authors have read and agreed to the published
version of the manuscript.
Funding: The manuscript was financially supported by the Minister of Education and Science in the
range of the program entitled “Regional Initiative of Excellence” for the years 2019–2023, project no.
010/RID/2018/19, amount of funding: PLN 12,000,000, and the work WZ/WB-IIŚ/3/2022, funded
by the Minister of Education and Science.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Viet, N.D.; Jang, D.; Yoon, Y.; Jang, A. Enhancement of Membrane System Performance Using Artificial Intelligence Technologies
for Sustainable Water and Wastewater Treatment: A Critical Review. Crit. Rev. Environ. Sci. Technol. 2021, 52, 3689–3719.
[CrossRef]
Kosek, K.; Luczkiewicz, A.; Fudala-Ksia˛żek, S.; Jankowska, K.; Szopińska, M.; Svahn, O.; Tränckner, J.; Kaiser, A.; Langas, V.;
Björklund, E. Implementation of Advanced Micropollutants Removal Technologies in Wastewater Treatment Plants (WWTPs)—
Examples and Challenges Based on Selected EU Countries. Environ. Sci. Policy 2020, 112, 213–226. [CrossRef]
Michailos, S.; Walker, M.; Moody, A.; Poggio, D.; Pourkashanian, M. Biomethane Production Using an Integrated Anaerobic
Digestion, Gasification and CO2 Biomethanation Process in a Real Waste Water Treatment Plant: A Techno-Economic Assessment.
Energy Convers. Manag. 2020, 209, 112663. [CrossRef]
Morello, R.; Di Capua, F.; Esposito, G.; Pirozzi, F.; Fratino, U.; Spasiano, D. Sludge Minimization in Mainstream Wastewater
Treatment: Mechanisms, Strategies, Technologies, and Current Development. J. Environ. Manage. 2022, 319, 115756. [CrossRef]
Pillai, B.B.K.; Meghvansi, M.K.; Sudha, M.C.; Sreenivasulu, M. Microbial Community Dynamics in Anaerobic Digester Treating
Human Waste: A Review. In Anaerobic Biodigesters for Human Waste Treatment; Springer: Singapore, 2022; pp. 95–111. [CrossRef]
Di Berardino, S.E.; Di Berardino, M.A. Application of Anaerobic Hybrid Filters for Sewage Treatment. In The Handbook of
Environmental Chemistry; Springer: Berlin/Heidelberg, Germany, 2022; pp. 1–42. [CrossRef]
Aghel, B.; Behaein, S.; Wongwises, S.; Shadloo, M.S. A Review of Recent Progress in Biogas Upgrading: With Emphasis on Carbon
Capture. Biomass Bioenergy 2022, 160, 106422. [CrossRef]
Nguyen, L.N.; Kumar, J.; Vu, M.T.; Mohammed, J.A.H.; Pathak, N.; Commault, A.S.; Sutherland, D.; Zdarta, J.; Tyagi, V.K.;
Nghiem, L.D. Biomethane Production from Anaerobic Co-Digestion at Wastewater Treatment Plants: A Critical Review on
Development and Innovations in Biogas Upgrading Techniques. Sci. Total Environ. 2021, 765, 142753. [CrossRef]
Tekerlekopoulou, A.G.; Economou, C.N.; Tatoulis, T.I.; Akratos, C.S.; Vayenas, D.V. Wastewater Treatment and Water Reuse in the
Food Industry. In The Interaction of Food Industry and Environment; Academic Press: Cambridge, MA, USA, 2020; pp. 245–280.
[CrossRef]
Ng, M.; Dalhatou, S.; Wilson, J.; Kamdem, B.P.; Temitope, M.B.; Paumo, H.K.; Djelal, H.; Assadi, A.A.; Nguyen-Tri, P.; Kane, A.
Characterization of Slaughterhouse Wastewater and Development of Treatment Techniques: A Review. Processes 2022, 10, 1300.
[CrossRef]
Capodaglio, A.G.; Olsson, G. Energy Issues in Sustainable Urban Wastewater Management: Use, Demand Reduction and
Recovery in the Urban Water Cycle. Sustainaibilty 2019, 12, 266. [CrossRef]
Karamichailidou, D.; Alexandridis, A.; Anagnostopoulos, G.; Syriopoulos, G.; Sekkas, O. Modeling Biogas Production from
Anaerobic Wastewater Treatment Plants Using Radial Basis Function Networks and Differential Evolution. Comput. Chem. Eng.
2022, 157, 107629. [CrossRef]
Anijiofor, S.C.; Azreen, N.; Jamil, M.; Jabbar, S.; Sakyat, S.; Gomes, C. Aerobic and Anaerobic Sewage Biodegradable Processes:
The Gap Analysis. Int. J. Res. Environ. Sci. 2017, 3, 2454–9444. [CrossRef]
Goli, A.; Shamiri, A.; Khosroyar, S.; Talaiekhozani, A.; Sanaye, R.; Azizi, K. A Review on Different Aerobic and Anaerobic
Treatment Methods in Dairy Industry Wastewater. J. Environ. Treat. Tech. 2019, 7, 113–141.
Liu, W.; Song, X.; Huda, N.; Xie, M.; Li, G.; Luo, W. Comparison between Aerobic and Anaerobic Membrane Bioreactors for Trace
Organic Contaminant Removal in Wastewater Treatment. Environ. Technol. Innov. 2020, 17, 100564. [CrossRef]
Energies 2023, 16, 83
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
29 of 39
Cashman, S.; Ma, X.; Mosley, J.; Garland, J.; Crone, B.; Xue, X. Energy and Greenhouse Gas Life Cycle Assessment and Cost
Analysis of Aerobic and Anaerobic Membrane Bioreactor Systems: Influence of Scale, Population Density, Climate, and Methane
Recovery. Bioresour. Technol. 2018, 254, 56–66. [CrossRef] [PubMed]
Martin, I.; Pidou, M.; Soares, A.; Judd, S.; Jefferson, B. Modelling the Energy Demands of Aerobic and Anaerobic Membrane
Bioreactors for Wastewater Treatment. Environ. Technol. 2011, 32, 921–932. [CrossRef]
Show, K.Y.; Lee, D.J. Anaerobic Treatment Versus Aerobic Treatment. In Current Developments in Biotechnology and Bioengineering;
Elsevier: Amsterdam, The Netherlands, 2017; pp. 205–230. [CrossRef]
Dereli, R.K. Modeling Long-Term Performance of Full-Scale Anaerobic Expanded Granular Sludge Bed Reactor Treating Confectionery Industry Wastewater. Environ. Sci. Pollut. Res. 2019, 26, 25037–25045. [CrossRef]
Kazimierowicz, J.; Zieliński, M.; D˛ebowski, M. Influence of the Heating Method on the Efficiency of Biomethane Production from
Expired Food Products. Fermentation 2021, 7, 12. [CrossRef]
D˛ebowski, M.; Zieliński, M.; Kisielewska, M.; Kazimierowicz, J. Evaluation of Anaerobic Digestion of Dairy Wastewater in an
Innovative Multi-Section Horizontal Flow Reactor. Energies 2020, 13, 2392. [CrossRef]
Sivaprakasam, S.; Balaji, K. A Review of Upflow Anaerobic Sludge Fixed Film (UASFF) Reactor for Treatment of Dairy Wastewater.
Mater. Today Proc. 2021, 43, 1879–1883. [CrossRef]
Shi, C.; Wang, K.; Zheng, M.; Liu, Y.; Ma, J.; Li, K. The Efficiencies and Capacities of Carbon Conversion in Fruit and Vegetable
Waste Two-Phase Anaerobic Digestion: Ethanol-Path vs. Butyrate-Path. Waste Manag. 2021, 126, 737–746. [CrossRef]
Harris, P.W.; McCabe, B.K. Process Optimisation of Anaerobic Digestion Treating High-Strength Wastewater in the Australian
Red Meat Processing Industry. Appl. Sci. 2020, 10, 7947. [CrossRef]
Sangeetha, T.; Rajneesh, C.P.; Yan, W.M. Integration of Microbial Electrolysis Cells with Anaerobic Digestion to Treat Beer Industry
Wastewater. In Integrated Microbial Fuel Cells for Wastewater Treatment; Butterworth-Heinemann: Oxford, UK, 2020; pp. 313–346.
[CrossRef]
Jiang, Q.; Xin, Y.; Jiang, Y.; Huang, L.; Shen, P. Improving the Efficiency of Anaerobic Digestion of Molasses Alcohol Wastewater
Using Cassava Alcohol Wastewater as a Mixed Feedstock. Bioresour. Technol. 2022, 344, 126179. [CrossRef]
Bakraoui, M.; Karouach, F.; Ouhammou, B.; Lahboubi, N.; El Gnaoui, Y.; Kerrou, O.; Aggour, M.; El Bari, H. Kinetics Study of
Methane Production from Anaerobic Digestion of Sludge and Wastewater Recycled Pulp and Paper. IOP Conf. Ser. Mater. Sci.
Eng. 2020, 946, 012009. [CrossRef]
Vítězová, M.; Kohoutová, A.; Vítěz, T.; Hanišáková, N.; Kushkevych, I. Methanogenic Microorganisms in Industrial Wastewater
Anaerobic Treatment. Processes 2020, 8, 1546. [CrossRef]
Song, Q.; Chen, X.; Zhou, W.; Xie, X. Application of a Spiral Symmetric Stream Anaerobic Bioreactor for Treating Saline Heparin
Sodium Pharmaceutical Wastewater: Reactor Operating Characteristics, Organics Degradation Pathway and Salt Tolerance
Mechanism. Water Res. 2021, 205, 117671. [CrossRef] [PubMed]
Khoshnevisan, B.; Duan, N.; Tsapekos, P.; Awasthi, M.K.; Liu, Z.; Mohammadi, A.; Angelidaki, I.; Tsang, D.C.W.; Zhang, Z.; Pan,
J.; et al. A Critical Review on Livestock Manure Biorefinery Technologies: Sustainability, Challenges, and Future Perspectives.
Renew. Sustain. Energy Rev. 2021, 135, 110033. [CrossRef]
Yuan, H.; Guan, R.; Li, X.; Zhu, C.; Wachemo, A.C.; Zou, D. Investigation of Anaerobic Digestion Performance and System
Stability of CaO-Ultrasonic Pretreated Dewatered Activated Sludge. Waste Biomass Valorization 2019, 10, 2445–2453. [CrossRef]
Zieliński, M.; D˛ebowski, M.; Kazimierowicz, J. Microwave Radiation Influence on Dairy Waste Anaerobic Digestion in a
Multi-Section Hybrid Anaerobic Reactor (M-SHAR). Processes 2021, 9, 1772. [CrossRef]
Yue, L.; Cheng, J.; Tang, S.; An, X.; Hua, J.; Dong, H.; Zhou, J. Ultrasound and Microwave Pretreatments Promote Methane
Production Potential and Energy Conversion during Anaerobic Digestion of Lipid and Food Wastes. Energy 2021, 228, 120525.
[CrossRef]
Kisielewska, M.; Rusanowska, P.; Dudek, M.; Nowicka, A.; Krzywik, A.; D˛ebowski, M.; Joanna, K.; Zieliński, M. Evaluation of
Ultrasound Pretreatment for Enhanced Anaerobic Digestion of Sida Hermaphrodita. Bioenergy Res. 2020, 13, 824–832. [CrossRef]
Kuşçu, Ö.S.; Çömlekçi, S.; Çört, N. Disintegration of Sewage Sludge Using Pulsed Electrical Field Technique: PEF Optimization,
Simulation, and Anaerobic Digestion. Environ. Technol. 2021, 43, 2809–2824. [CrossRef] [PubMed]
Wang, Z.; Sun, Y.; Qu, J.; Yang, F.; Li, J.; Yan, B. Effects on Mesophilic Anaerobic Digestion Performance of Corn Stalk with the
Addition/ Pretreatment of Depolymerization Wastewater. Fuel 2022, 322, 124234. [CrossRef]
Wang, S.; Yu, S.; Lu, Q.; Liao, Y.; Li, H.; Sun, L.; Wang, H.; Zhang, Y. Development of an Alkaline/Acid Pre-Treatment and
Anaerobic Digestion (APAD) Process for Methane Generation from Waste Activated Sludge. Sci. Total Environ. 2020, 708, 134564.
[CrossRef] [PubMed]
Zou, X.; Yang, R.; Zhou, X.; Cao, G.; Zhu, R.; Ouyang, F. Effects of Mixed Alkali-Thermal Pretreatment on Anaerobic Digestion
Performance of Waste Activated Sludge. J. Clean. Prod. 2020, 259, 120940. [CrossRef]
Brough, D.; Jouhara, H. The Aluminium Industry: A Review on State-of-the-Art Technologies, Environmental Impacts and
Possibilities for Waste Heat Recovery. Int. J. 2020, 1–2, 100007. [CrossRef]
Ren, Y.; Wang, C.; He, Z.; Qin, Y.; Li, Y.Y. Enhanced Biomethanation of Lipids by High-Solid Co-Digestion with Food Waste: Biogas
Production and Lipids Degradation Demonstrated by Long-Term Continuous Operation. Bioresour. Technol. 2022, 348, 126750.
[CrossRef]
Energies 2023, 16, 83
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
58.
59.
60.
61.
62.
63.
64.
65.
66.
67.
30 of 39
Loganath, R.; Senophiyah-Mary, J. Critical Review on the Necessity of Bioelectricity Generation from Slaughterhouse Industry
Waste and Wastewater Using Different Anaerobic Digestion Reactors. Renew. Sustain. Energy Rev. 2020, 134, 110360. [CrossRef]
Wiśniewska, M.; Kulig, A.; Lelicińska-Serafin, K. Odour Nuisance at Municipal Waste Biogas Plants and the Effect of Feedstock
Modification on the Circular Economy—A Review. Energies 2021, 14, 6470. [CrossRef]
Obaideen, K.; Abdelkareem, M.A.; Wilberforce, T.; Elsaid, K.; Sayed, E.T.; Maghrabie, H.M.; Olabi, A.G. Biogas Role in
Achievement of the Sustainable Development Goals: Evaluation, Challenges, and Guidelines. J. Taiwan Inst. Chem. Eng. 2022,
131, 104207. [CrossRef]
Stolecka, K.; Rusin, A. Potential Hazards Posed by Biogas Plants. Renew. Sustain. Energy Rev. 2021, 135, 110225. [CrossRef]
Natividad Pérez-Camacho, M.; Curry, R.; Cromie, T. Life Cycle Environmental Impacts of Biogas Production and Utilisation
Substituting for Grid Electricity, Natural Gas Grid and Transport Fuels. Waste Manag. 2019, 95, 90–101. [CrossRef]
Sevillano, C.A.; Pesantes, A.A.; Peña Carpio, E.; Martínez, E.J.; Gómez, X. Anaerobic Digestion for Producing Renewable
Energy—The Evolution of This Technology in a New Uncertain Scenario. Entropy 2021, 23, 145. [CrossRef] [PubMed]
Harb, M.; Lou, E.; Smith, A.L.; Stadler, L.B. Perspectives on the Fate of Micropollutants in Mainstream Anaerobic Wastewater
Treatment. Curr. Opin. Biotechnol. 2019, 57, 94–100. [CrossRef] [PubMed]
Hallaji, S.M.; Kuroshkarim, M.; Moussavi, S.P. Enhancing Methane Production Using Anaerobic Co-Digestion of Waste Activated
Sludge with Combined Fruit Waste and Cheese Whey. BMC Biotechnol. 2019, 19, 1–10. [CrossRef] [PubMed]
Olugbemide, A.D.; Likozar, B. Assessment of Liquid and Solid Digestates from Anaerobic Digestion of Rice Husk as Potential
Biofertilizer and Nutrient Source for Microalgae Cultivation. Processes 2022, 10, 1007. [CrossRef]
Gülşen, H.; Yapıcıoğlu, P.; Gülşen, H.; Yapıcıoğlu, P. Greenhouse Gas Emission Estimation for a UASB Reactor in a Dairy
Wastewater Treatment Plant. Int. J. Glob. Warm. 2019, 17, 373–388. [CrossRef]
Gray, N.; O’Shea, R.; Smyth, B.; Lens, P.N.L.; Murphy, J.D. What Is the Energy Balance of Electrofuels Produced through
Power-to-Fuel Integration with Biogas Facilities? Renew. Sustain. Energy Rev. 2022, 155, 111886. [CrossRef]
Pierie, F.; van Someren, C.E.J.; Kruse, S.N.M.; Laugs, G.A.H.; Benders, R.M.J.; Moll, H.C. Local Balancing of the Electricity
Grid in a Renewable Municipality; Analyzing the Effectiveness and Cost of Decentralized Load Balancing Looking at Multiple
Combinations of Technologies. Energies 2021, 14, 4926. [CrossRef]
Chew, K.W.; Chia, S.R.; Yen, H.W.; Nomanbhay, S.; Ho, Y.C.; Show, P.L. Transformation of Biomass Waste into Sustainable Organic
Fertilizers. Sustainability 2019, 11, 2266. [CrossRef]
Hussain, Z.; Mishra, J.; Vanacore, E. Waste to Energy and Circular Economy: The Case of Anaerobic Digestion. J. Enterp. Inf.
Manag. 2020, 33, 817–838. [CrossRef]
Liu, X.; Wang, D.; Chen, Z.; Wei, W.; Mannina, G.; Ni, B.-J. Advances in Pretreatment Strategies to Enhance the Biodegradability
of Waste Activated Sludge for the Conversion of Refractory Substances. Bioresour. Technol. 2022, 362, 127804. [CrossRef]
Joshi, P.; Visvanathan, C. Sustainable Management Practices of Food Waste in Asia: Technological and Policy Drivers. J. Environ.
Manage. 2019, 247, 538–550. [CrossRef] [PubMed]
Kumar, V.; Nabaterega, R.; Khoei, S.; Eskicioglu, C. Insight into Interactions between Syntrophic Bacteria and Archaea in
Anaerobic Digestion Amended with Conductive Materials. Renew. Sustain. Energy Rev. 2021, 144, 110965. [CrossRef]
Xiao, K.; Abbt-Braun, G.; Horn, H. Changes in the Characteristics of Dissolved Organic Matter during Sludge Treatment: A
Critical Review. Water Res. 2020, 187, 116441. [CrossRef] [PubMed]
Pasalari, H.; Gholami, M.; Rezaee, A.; Esrafili, A.; Farzadkia, M. Perspectives on Microbial Community in Anaerobic Digestion
with Emphasis on Environmental Parameters: A Systematic Review. Chemosphere 2021, 270, 128618. [CrossRef] [PubMed]
Yadav, M.; Joshi, C.; Paritosh, K.; Thakur, J.; Pareek, N.; Masakapalli, S.K.; Vivekanand, V. Reprint of Organic Waste Conversion
through Anaerobic Digestion: A Critical Insight into the Metabolic Pathways and Microbial Interactions. Metab. Eng. 2022, 71,
62–76. [CrossRef]
Kazimierowicz, J.; D˛ebowski, M. Aerobic Granular Sludge as a Substrate in Anaerobic Digestion—Current Status and Perspectives.
Sustainability 2022, 14, 10904. [CrossRef]
Paul Choudhury, S.; Panda, S.; Haq, I.; Kalamdhad, A.S. Enhanced Methane Production and Hydrocarbon Removal from
Petroleum Refinery Sludge after Pseudomonas Putida Pretreatment and Process Scale-Up. Bioresour. Technol. 2022, 343, 126127.
[CrossRef]
Khanh Nguyen, V.; Kumar Chaudhary, D.; Hari Dahal, R.; Hoang Trinh, N.; Kim, J.; Chang, S.W.; Hong, Y.; Duc La, D.; Nguyen,
X.C.; Hao Ngo, H.; et al. Review on Pretreatment Techniques to Improve Anaerobic Digestion of Sewage Sludge. Fuel 2021,
285, 119105. [CrossRef]
Ma, Y.; Gu, J.; Liu, Y. Evaluation of Anaerobic Digestion of Food Waste and Waste Activated Sludge: Soluble COD versus Its
Chemical Composition. Sci. Total Environ. 2018, 643, 21–27. [CrossRef]
Światek,
˛
M.; Lewicki, A.; Szymanowska, D.; Kubiak, P. The Effect of Introduction of Chicken Manure on the Biodiversity and
Performance of an Anaerobic Digester. Electron. J. Biotechnol. 2019, 37, 25–33. [CrossRef]
De Prá, M.C.; Anschau, A.; Busso, C.; Gabiatti, N.; Bortoli, M. Effect of Short-Chain Fatty Acid Production on Biogas Generation.
Adv. Biol. Chem. 2019, 9, 199–216. [CrossRef]
Granatto, C.F.; Grosseli, G.M.; Sakamoto, I.K.; Fadini, P.S.; Varesche, M.B.A. Influence of Metabolic Cosubstrates on Methanogenic
Potential and Degradation of Triclosan and Propranolol in Sanitary Sewage. Environ. Res. 2021, 199, 111220. [CrossRef] [PubMed]
Energies 2023, 16, 83
68.
69.
70.
71.
72.
73.
74.
75.
76.
77.
78.
79.
80.
81.
82.
83.
84.
85.
86.
87.
88.
89.
90.
91.
92.
93.
94.
95.
96.
31 of 39
García-Depraect, O.; Diaz-Cruces, V.F.; León-Becerril, E. Upgrading of Anaerobic Digestion of Tequila Vinasse by Using an
Innovative Two-Stage System with Dominant Lactate-Type Fermentation in Acidogenesis. Fuel 2020, 280, 118606. [CrossRef]
Gao, T.; Zhang, H.; Xu, X.; Teng, J. Integrating Microbial Electrolysis Cell Based on Electrochemical Carbon Dioxide Reduction
into Anaerobic Osmosis Membrane Reactor for Biogas Upgrading. Water Res. 2021, 190, 116679. [CrossRef] [PubMed]
Anukam, A.; Mohammadi, A.; Naqvi, M.; Granström, K. A Review of the Chemistry of Anaerobic Digestion: Methods of
Accelerating and Optimizing Process Efficiency. Processes 2019, 7, 504. [CrossRef]
Pan, X.; Zhao, L.; Li, C.; Angelidaki, I.; Lv, N.; Ning, J.; Cai, G.; Zhu, G. Deep Insights into the Network of Acetate Metabolism in
Anaerobic Digestion: Focusing on Syntrophic Acetate Oxidation and Homoacetogenesis. Water Res. 2021, 190, 116774. [CrossRef]
Zhu, X.; Campanaro, S.; Treu, L.; Seshadri, R.; Ivanova, N.; Kougias, P.G.; Kyrpides, N.; Angelidaki, I. Metabolic Dependencies
Govern Microbial Syntrophies during Methanogenesis in an Anaerobic Digestion Ecosystem. Microbiome 2020, 8, 1–14. [CrossRef]
Pramanik, S.K.; Suja, F.B.; Zain, S.M.; Pramanik, B.K. The Anaerobic Digestion Process of Biogas Production from Food Waste:
Prospects and Constraints. Bioresour. Technol. Rep. 2019, 8, 100310. [CrossRef]
Wang, S.; An, Z.; Wang, Z.W. Bioconversion of Methane to Chemicals and Fuels by Methane-Oxidizing Bacteria. Adv. Bioenergy
2020, 5, 169–247. [CrossRef]
Ray, S.; Kuppam, C.; Pandit, S.; Kumar, P. Biogas Upgrading by Hydrogenotrophic Methanogens: An Overview. Waste Biomass
Valorization 2022, 1, 1–16. [CrossRef]
Sogodogo, E.; Drancourt, M.; Grine, G. Methanogens as Emerging Pathogens in Anaerobic Abscesses. Eur. J. Clin. Microbiol. Infect.
Dis. 2019, 38, 811–818. [CrossRef] [PubMed]
Wintsche, B.; Jehmlich, N.; Popp, D.; Harms, H.; Kleinsteuber, S. Metabolic Adaptation of Methanogens in Anaerobic Digesters
upon Trace Element Limitation. Front. Microbiol. 2018, 9, 405. [CrossRef] [PubMed]
Kurth, J.M.; Op den Camp, H.J.M.; Welte, C.U. Several Ways One Goal—Methanogenesis from Unconventional Substrates.
Appl. Microbiol. Biotechnol. 2020, 104, 6839–6854. [CrossRef] [PubMed]
Chen, S.; He, J.; Wang, H.; Dong, B.; Li, N.; Dai, X. Microbial Responses and Metabolic Pathways Reveal the Recovery Mechanism
of an Anaerobic Digestion System Subjected to Progressive Inhibition by Ammonia. Chem. Eng. J. 2018, 350, 312–323. [CrossRef]
Kazimierowicz, J.; Dzienis, L.; D˛ebowski, M.; Zieliński, M. Optimisation of Methane Fermentation as a Valorisation Method for
Food Waste Products. Biomass Bioenergy 2021, 144, 105913. [CrossRef]
Manchala, K.R.; Sun, Y.; Zhang, D.; Wang, Z.W. Anaerobic Digestion Modelling. Adv. Bioenergy 2017, 2, 69–141. [CrossRef]
Rossi, A.; Morlino, M.S.; Gaspari, M.; Basile, A.; Kougias, P.; Treu, L.; Campanaro, S. Analysis of the Anaerobic Digestion
Metagenome under Environmental Stresses Stimulating Prophage Induction. Microbiome 2022, 10, 1–21. [CrossRef]
Hardoim, C.C.P.; Ramaglia, A.C.M.; Lôbo-Hajdu, G.; Custódio, M.R. Community Composition and Functional Prediction of
Prokaryotes Associated with Sympatric Sponge Species of Southwestern Atlantic Coast. Sci. Rep. 2021, 11, 1–16. [CrossRef]
Daniels, L.; Hanson, R.S.; Phillips, J.A. Chemical Analysis. In Methods for General and Molecular Microbiology; John Wiley & Sons:
Hoboken, NJ, USA, 2014; pp. 462–503. [CrossRef]
Tang, Y.Q.; Shigematsu, T.; Morimura, S.; Kida, K. Dynamics of the Microbial Community during Continuous Methane Fermentation in Continuously Stirred Tank Reactors. J. Biosci. Bioeng. 2015, 119, 375–383. [CrossRef]
Wade, M.J. Not Just Numbers: Mathematical Modelling and Its Contribution to Anaerobic Digestion Processes. Processes 2020,
8, 888. [CrossRef]
Arcus, V.L.; van der Kamp, M.W.; Pudney, C.R.; Mulholland, A.J. Enzyme Evolution and the Temperature Dependence of Enzyme
Catalysis. Curr. Opin. Struct. Biol. 2020, 65, 96–101. [CrossRef] [PubMed]
Dhaked, R.K.; Singh, P.; Singh, L. Biomethanation under Psychrophilic Conditions. Waste Manag. 2010, 30, 2490–2496. [CrossRef]
[PubMed]
Aguilar-Muñoz, P.; Lavergne, C.; Chamy, R.; Cabrol, L. The Biotechnological Potential of Microbial Communities from Antarctic
Soils and Sediments: Application to Low Temperature Biogenic Methane Production. J. Biotechnol. 2022, 351, 38–49. [CrossRef]
[PubMed]
Rathour, R.; Gupta, J.; Mishra, A.; Rajeev, A.C.; Dupont, C.L.; Thakur, I.S. A Comparative Metagenomic Study Reveals Microbial
Diversity and Their Role in the Biogeochemical Cycling of Pangong Lake. Sci. Total Environ. 2020, 731, 139074. [CrossRef]
Sukma Safitri, A.; Michelle Kaster, K.; Kommedal, R. Effect of Low Temperature and Municipal Wastewater Organic Loading on
Anaerobic Granule Reactor Performance. Bioresour. Technol. 2022, 360, 127616. [CrossRef]
Yusof, N.A.; Hashim, N.H.F.; Bharudin, I. Cold Adaptation Strategies and the Potential of Psychrophilic Enzymes from the
Antarctic Yeast, Glaciozyma Antarctica PI12. J. Fungi 2021, 7, 528. [CrossRef]
Nie, E.; He, P.; Zhang, H.; Hao, L.; Shao, L.; Lü, F. How Does Temperature Regulate Anaerobic Digestion? Renew. Sustain. Energy
Rev. 2021, 150, 111453. [CrossRef]
Dalby, F.R.; Hafner, S.D.; Petersen, S.O.; VanderZaag, A.C.; Habtewold, J.; Dunfield, K.; Chantigny, M.H.; Sommer, S.G.
Understanding Methane Emission from Stored Animal Manure: A Review to Guide Model Development. J. Environ. Qual. 2021,
50, 817–835. [CrossRef]
Basinas, P.; Rusín, J.; Chamrádová, K. Assessment of High-Solid Mesophilic and Thermophilic Anaerobic Digestion of
Mechanically-Separated Municipal Solid Waste. Environ. Res. 2021, 192, 110202. [CrossRef]
Zhang, Q.; Zeng, L.; Fu, X.; Pan, F.; Shi, X.; Wang, T. Comparison of Anaerobic Co-Digestion of Pig Manure and Sludge at
Different Mixing Ratios at Thermophilic and Mesophilic Temperatures. Bioresour. Technol. 2021, 337, 125425. [CrossRef]
Energies 2023, 16, 83
97.
98.
99.
100.
101.
102.
103.
104.
105.
106.
107.
108.
109.
110.
111.
112.
113.
114.
115.
116.
117.
118.
119.
120.
121.
32 of 39
Ruffino, B.; Cerutti, A.; Campo, G.; Scibilia, G.; Lorenzi, E.; Zanetti, M. Thermophilic vs. Mesophilic Anaerobic Digestion of Waste
Activated Sludge: Modelling and Energy Balance for Its Applicability at a Full Scale WWTP. Renew. Energy 2020, 156, 235–248.
[CrossRef]
Rocamora, I.; Wagland, S.T.; Villa, R.; Simpson, E.W.; Fernández, O.; Bajón-Fernández, Y. Dry Anaerobic Digestion of Organic
Waste: A Review of Operational Parameters and Their Impact on Process Performance. Bioresour. Technol. 2020, 299, 122681.
[CrossRef] [PubMed]
Wang, L.; Hao, J.; Wang, C.; Li, Y.; Yang, Q. Carbohydrate-to-Protein Ratio Regulates Hydrolysis and Acidogenesis Processes
during Volatile Fatty Acids Production. Bioresour. Technol. 2022, 355, 127266. [CrossRef] [PubMed]
Alves, M.; Pereira, A.; Mota, M.; Novais, J.M.; Colleran, E. Staged and Non-staged Anaerobic Filters: Microbial Activity
Segregation, Hydrodynamic Behaviour and Performance. J. Chem. Technol. Biotechnol. Int. Res. Process. Environ. Clean Technol.
1998, 73, 99–108. [CrossRef]
Zayed, G.; Winter, J. Inhibition of Methane Production from Whey by Heavy Metals—Protective Effect of Sulfide. Appl. Microbiol.
Biotechnol. 2000, 53, 726–731. [CrossRef]
Córdoba, P.R.; Sanchez Riera, F.; Siñeriz, F. Treatment of Dairy Industry Wastewater with an Anaerobic Filter. Biotechnol. Lett.
1984, 6, 753–758. [CrossRef]
Ramasamy, E.V.; Abbasi, S.A. Energy Recovery from Dairy Waste-Waters: Impacts of Biofilm Support Systems on Anaerobic CST
Reactors. Appl. Energy 2000, 65, 91–98. [CrossRef]
Kundu, K.; Bergmann, I.; Hahnke, S.; Klocke, M.; Sharma, S.; Sreekrishnan, T.R. Carbon Source—A Strong Determinant of
Microbial Community Structure and Performance of an Anaerobic Reactor. J. Biotechnol. 2013, 168, 616–624. [CrossRef]
Da Motta Marques, D.M.; Cayless, S.M.; Lester, J.N. Process Aiders for Start-up of Anaerobic Fluidised Bed Systems. Environ.
Technol. 1990, 11, 1093–1106. [CrossRef]
Strydom, J.P.; Britz, T.J.; Mostert, J.F. Two-Phase Anaerobic Digestion of Three Different Dairy Effluents Using a Hybrid Bioreactor.
Water S. A. 1997, 23, 151–156.
Zielińska, M.; Cydzik-Kwiatkowska, A.; Zieliński, M.; D˛ebowski, M. Impact of Temperature, Microwave Radiation and Organic
Loading Rate on Methanogenic Community and Biogas Production during Fermentation of Dairy Wastewater. Bioresour. Technol.
2013, 129, 308–314. [CrossRef] [PubMed]
Wang, S.; Rao, N.C.; Qiu, R.; Moletta, R. Treatability and Kinetic Analysis of Anaerobic Moving Bed Biofilm Reactor Treating
High Strength Milk Permeate. Desalin. Water Treat. 2012, 4, 191–197. [CrossRef]
Rodgers, M.; Zhan, X.M.; Dolan, B. Mixing Characteristics and Whey Wastewater Treatment of a Novel Moving Anaerobic Biofilm
Reactor. J. Environ. Sci. Health Part A 2007, 39, 2183–2193. [CrossRef] [PubMed]
Damasceno, L.H.S.; Rodrigues, J.A.D.; Ratusznei, S.M.; Zaiat, M.; Foresti, E. Effects of Feeding Time and Organic Loading in an
Anaerobic Sequencing Batch Biofilm Reactor (ASBBR) Treating Diluted Whey. J. Environ. Manag. 2007, 85, 927–935. [CrossRef]
[PubMed]
Zamri, M.F.M.A.; Hasmady, S.; Akhiar, A.; Ideris, F.; Shamsuddin, A.H.; Mofijur, M.; Fattah, I.M.R.; Mahlia, T.M.I. A Comprehensive Review on Anaerobic Digestion of Organic Fraction of Municipal Solid Waste. Renew. Sustain. Energy Rev. 2021, 137, 110637.
[CrossRef]
Wainaina, S.; Lukitawesa; Kumar Awasthi, M.; Taherzadeh, M.J. Bioengineering of Anaerobic Digestion for Volatile Fatty Acids,
Hydrogen or Methane Production: A Critical Review. Bioengineered 2019, 10, 437–458. [CrossRef]
Arras, W.; Hussain, A.; Hausler, R.; Guiot, S.R. Mesophilic, Thermophilic and Hyperthermophilic Acidogenic Fermentation of
Food Waste in Batch: Effect of Inoculum Source. Waste Manag. 2019, 87, 279–287. [CrossRef]
He, K.; Li, W.; Tang, L.; Li, W.; Lv, S.; Xing, D. Suppressing Methane Production to Boost High-Purity Hydrogen Production in
Microbial Electrolysis Cells. Environ. Sci. Technol. 2022, 56, 11931–11951. [CrossRef]
Jürgensen, L.; Ehimen, E.A.; Born, J.; Holm-Nielsen, J.B. A Combination Anaerobic Digestion Scheme for Biogas Production from
Dairy Effluent—CSTR and ABR, and Biogas Upgrading. Biomass Bioenergy 2018, 111, 241–247. [CrossRef]
Lu, Y.; Zhang, Q.; Wang, X.; Zhou, X.; Zhu, J. Effect of PH on Volatile Fatty Acid Production from Anaerobic Digestion of Potato
Peel Waste. Bioresour. Technol. 2020, 316, 123851. [CrossRef]
Greses, S.; Tomás-Pejó, E.; González-Fernández, C. Short-Chain Fatty Acids and Hydrogen Production in One Single Anaerobic
Fermentation Stage Using Carbohydrate-Rich Food Waste. J. Clean. Prod. 2021, 284, 124727. [CrossRef]
Fuess, L.T.; Zaiat, M.; do Nascimento, C.A.O. Thermophilic Biodigestion of Fermented Sugarcane Molasses in High-Rate
Structured-Bed Reactors: Alkalinization Strategies Define the Operating Limits. Energy Convers. Manag. 2021, 239, 114203.
[CrossRef]
Holl, E.; Steinbrenner, J.; Merkle, W.; Krümpel, J.; Lansing, S.; Baier, U.; Oechsner, H.; Lemmer, A. Two-Stage Anaerobic Digestion:
State of Technology and Perspective Roles in Future Energy Systems. Bioresour. Technol. 2022, 360, 127633. [CrossRef] [PubMed]
Zhang, L.; Guo, K.; Wang, L.; Xu, R.; Lu, D.; Zhou, Y. Effect of Sludge Retention Time on Microbial Succession and Assembly
in Thermal Hydrolysis Pretreated Sludge Digesters: Deterministic versus Stochastic Processes. Water Res. 2022, 209, 117900.
[CrossRef] [PubMed]
Tabatabaei, M.; Aghbashlo, M.; Valijanian, E.; Kazemi Shariat Panahi, H.; Nizami, A.S.; Ghanavati, H.; Sulaiman, A.; Mirmohamadsadeghi, S.; Karimi, K. A Comprehensive Review on Recent Biological Innovations to Improve Biogas Production, Part 1:
Upstream Strategies. Renew. Energy 2020, 146, 1204–1220. [CrossRef]
Energies 2023, 16, 83
33 of 39
122. Neogi, S.; Chatterjee, P.K. Acidophilic Methanogenesis for Treatment of Wastewater in Food Processing Industries. Handb. Adv.
Approaches Towar. Pollut. Prev. Control 2021, 2, 249–273. [CrossRef]
123. Han, W.; He, P.; Lin, Y.; Shao, L.; Lü, F. A Methanogenic Consortium Was Active and Exhibited Long-Term Survival in an
Extremely Acidified Thermophilic Bioreactor. Front. Microbiol. 2019, 10, 2757. [CrossRef]
124. Salama, E.S.; Saha, S.; Kurade, M.B.; Dev, S.; Chang, S.W.; Jeon, B.H. Recent Trends in Anaerobic Co-Digestion: Fat, Oil, and
Grease (FOG) for Enhanced Biomethanation. Prog. Energy Combust. Sci. 2019, 70, 22–42. [CrossRef]
125. Kuglarz, M.; Grübel, K.; Bohdziewicz, J. Co-Digestion of Sewage Sludge and Glycerol Fraction Mixture Facilitated by Microwave
Pretreatment. Ecol. Chem. Eng. A 2017, 24, 87–100. [CrossRef]
126. Kanchanapiya, P.; Jantharadej, K.; Chavanparit, O. Performance of Precast Anaerobic Digestion Tank Combined with a Food
Waste Disposer System. A Case Study of Household Food Waste in Thailand. Environ. Prot. Eng. 2017, 43, 93–108. [CrossRef]
127. Fazzino, F. A Novel Integrated Treatment for Mature Landfill Leachate via Active Filtration and Anaerobic Digestion. Ph.D.
Thesis, University of Messina, Messina, Italy, 2021.
128. Madigou, C.; Lê Cao, K.A.; Bureau, C.; Mazéas, L.; Déjean, S.; Chapleur, O. Ecological Consequences of Abrupt Temperature
Changes in Anaerobic Digesters. Chem. Eng. J. 2019, 361, 266–277. [CrossRef]
129. Gannoun, H.; Khelifi, E.; Bouallagui, H.; Touhami, Y.; Hamdi, M. Ecological Clarification of Cheese Whey Prior to Anaerobic
Digestion in Upflow Anaerobic Filter. Bioresour. Technol. 2008, 99, 6105–6111. [CrossRef] [PubMed]
130. Haridas, A.; Suresh, S.; Chitra, K.R.; Manilal, V.B. The Buoyant Filter Bioreactor: A High-Rate Anaerobic Reactor for Complex
Wastewater—Process Dynamics with Dairy Effluent. Water Res. 2005, 39, 993–1004. [CrossRef] [PubMed]
131. Borja, R.; Banks, C.J. Response of an Anaerobic Fluidized Bed Reactor Treating Ice-Cream Wastewater to Organic, Hydraulic,
Temperature and PH Shocks. J. Biotechnol. 1995, 39, 251–259. [CrossRef]
132. Rajesh Banu, J.; Anandan, S.; Kaliappan, S.; Yeom, I.T. Treatment of Dairy Wastewater Using Anaerobic and Solar Photocatalytic
Methods. Sol. Energy 2008, 82, 812–819. [CrossRef]
133. Malaspina, F.; Cellamare, C.M.; Stante, L.; Tilche, A. Anaerobic Treatment of Cheese Whey with a Downflow-Upflow Hybrid
Reactor. Bioresour. Technol. 1996, 55, 131–139. [CrossRef]
134. Singh, B.; Kovács, K.L.; Bagi, Z.; Nyári, J.; Szepesi, G.L.; Petrik, M.; Siménfalvi, Z.; Szamosi, Z. Enhancing Efficiency of Anaerobic
Digestion by Optimization of Mixing Regimes Using Helical Ribbon Impeller. Ferment. 2021, 7, 251. [CrossRef]
135. Stronach, S.M.; Rudd, T.; Lester, J.N. Anaerobic Digestion Processes in Industrial Wastewater Treatment—Google Ksia˛żki; Springer
Science & Business Media: Berlin/Heidelberg, Germany, 2012; Volume 2.
136. Wang, H.; Larson, R.A.; Borchardt, M.; Spencer, S. Effect of Mixing Duration on Biogas Production and Methanogen Distribution
in an Anaerobic Digester. Environ. Technol. 2019, 42, 93–99. [CrossRef]
137. Singh, R.; Pareek, N.; Kumar, R.; Vivekanand, V. Anaerobic Biodigesters for the Treatment of High-Strength Wastewater. In
Anaerobic Biodigesters for Human Waste Treatment; Springer: Singapore, 2022; pp. 75–94. [CrossRef]
138. Mahat, S.B.; Omar, R.; Che Man, H.; Mohamad Idris, A.I.; Mustapa Kamal, S.M.; Idris, A.; Shreeshivadasan, C.; Jamali, N.S.;
Abdullah, L.C. Performance of Dynamic Anaerobic Membrane Bioreactor (DAnMBR) with Phase Separation in Treating High
Strength Food Processing Wastewater. J. Environ. Chem. Eng. 2021, 9, 105245. [CrossRef]
139. Shahid, M.A.; Maqbool, N.; Khan, S.J. An Integrated Investigation on Anaerobic Membrane-Based Thickening of Fecal Sludge and
the Role of Extracellular Polymeric Substances (EPS) in Solid-Liquid Separation. J. Environ. Manag. 2022, 305, 114350. [CrossRef]
140. Stazi, V.; Tomei, M.C. Enhancing Anaerobic Treatment of Domestic Wastewater: State of the Art, Innovative Technologies and
Future Perspectives. Sci. Total Environ. 2018, 635, 78–91. [CrossRef]
141. Zhang, W.; Lang, Q.; Pan, Z.; Jiang, Y.; Liebetrau, J.; Nelles, M.; Dong, H.; Dong, R. Performance Evaluation of a Novel Anaerobic
Digestion Operation Process for Treating High-Solids Content Chicken Manure: Effect of Reduction of the Hydraulic Retention
Time at a Constant Organic Loading Rate. Waste Manag. 2017, 64, 340–347. [CrossRef] [PubMed]
142. Kariyama, I.D.; Zhai, X.; Wu, B. Influence of Mixing on Anaerobic Digestion Efficiency in Stirred Tank Digesters: A Review. Water
Res. 2018, 143, 503–517. [CrossRef] [PubMed]
143. Induchoodan, T.G.; Haq, I.; Kalamdhad, A.S. Factors Affecting Anaerobic Digestion for Biogas Production: A Review. In Advanced
Organic Waste Management Sustainable Practices and Approaches; Elsevier: Amsterdam, The Netherlands, 2022; pp. 223–233.
[CrossRef]
144. Mahmood, Z.; Cheng, H.; Tian, M. A Critical Review on Advanced Anaerobic Membrane Bioreactors (AnMBRs) for Wastewater
Treatment: Advanced Membrane Materials and Energy Demand. Environ. Sci. Water Res. Technol. 2022, 8, 2126–2144. [CrossRef]
145. Uri-Carreño, N.; Nielsen, P.H.; Gernaey, K.V.; Flores-Alsina, X. Long-Term Operation Assessment of a Full-Scale MembraneAerated Biofilm Reactor under Nordic Conditions. Sci. Total Environ. 2021, 779, 146366. [CrossRef]
146. Jo, Y.; Rhee, C.; Choi, H.; Shin, J.; Shin, S.G.; Lee, C. Long-Term Effectiveness of Bioaugmentation with Rumen Culture in
Continuous Anaerobic Digestion of Food and Vegetable Wastes under Feed Composition Fluctuations. Bioresour. Technol. 2021,
338, 125500. [CrossRef]
147. Zhu, X.; Yellezuome, D.; Liu, R.; Wang, Z.; Liu, X. Effects of Co-Digestion of Food Waste, Corn Straw and Chicken Manure in
Two-Stage Anaerobic Digestion on Trace Element Bioavailability and Microbial Community Composition. Bioresour. Technol.
2022, 346, 126625. [CrossRef]
148. Braga Nan, L.; Trably, E.; Santa-Catalina, G.; Bernet, N.; Delgenès, J.P.; Escudié, R. Biomethanation Processes: New Insights on the
Effect of a High H2partial Pressure on Microbial Communities. Biotechnol. Biofuels 2020, 13, 1–17. [CrossRef]
Energies 2023, 16, 83
34 of 39
149. Rafrafi, Y.; Laguillaumie, L.; Dumas, C. Biological Methanation of H2 and CO2 with Mixed Cultures: Current Advances, Hurdles
and Challenges. Waste Biomass Valorization 2021, 12, 5259–5282. [CrossRef]
150. Amin, F.R.; Khalid, H.; El-Mashad, H.M.; Chen, C.; Liu, G.; Zhang, R. Functions of Bacteria and Archaea Participating in the
Bioconversion of Organic Waste for Methane Production. Sci. Total Environ. 2021, 763, 143007. [CrossRef]
151. Maurus, K.; Kremmeter, N.; Ahmed, S.; Kazda, M. High-Resolution Monitoring of VFA Dynamics Reveals Process Failure and
Exponential Decrease of Biogas Production. Biomass Convers. Biorefinery 2021, 1, 1–11. [CrossRef]
152. Arif, S.; Liaquat, R.; Adil, M. Applications of Materials as Additives in Anaerobic Digestion Technology. Renew. Sustain. Energy
Rev. 2018, 97, 354–366. [CrossRef]
153. Ghanavati, H. Biogas Production Systems: Operation, Process Control, and Troubleshooting. Biogas. Biofuel Biorefinery Technol.
2018, 6, 199–219. [CrossRef]
154. Aboudi, K.; Álvarez-Gallego, C.J.; Romero-García, L.I. Evaluation of Methane Generation and Process Stability from Anaerobic
Co-Digestion of Sugar Beet by-Product and Cow Manure. J. Biosci. Bioeng. 2016, 121, 566–572. [CrossRef]
155. Oliveira, J.T.; de Sousa, M.C.; Martins, I.A.; de Sena, L.M.G.; Nogueira, T.R.; Vidal, C.B.; Neto, E.F.A.; Romero, F.B.; Campos, O.S.;
do Nascimento, R.F. Electrocoagulation/Oxidation/Flotation by Direct Pulsed Current Applied to the Removal of Antibiotics
from Brazilian WWTP Effluents. Electrochim. Acta 2021, 388, 138499. [CrossRef]
156. Angelidaki, I.; Chen, X.; Cui, J.; Kaparaju, P.; Ellegaard, L. Thermophilic Anaerobic Digestion of Source-Sorted Organic Fraction
of Household Municipal Solid Waste: Start-up Procedure for Continuously Stirred Tank Reactor. Water Res. 2006, 40, 2621–2628.
[CrossRef]
157. Filer, J.; Ding, H.H.; Chang, S. Biochemical Methane Potential (BMP) Assay Method for Anaerobic Digestion Research. Water
2019, 11, 921. [CrossRef]
158. Sasidhar, K.B.; Kumar, P.S.; Xiao, L. A Critical Review on the Two-Stage Biohythane Production and Its Viability as a Renewable
Fuel. Fuel 2022, 317, 123449. [CrossRef]
159. Brito, G.C.B.; Lange, L.C.; Santos, V.L.; Amaral, M.C.S.; Moravia, W.G. Long-Term Evaluation of Membrane Bioreactor Inoculated
with Commercial Baker’s Yeast Treating Landfill Leachate: Pollutant Removal, Microorganism Dynamic and Membrane Fouling.
Water Sci. Technol. 2019, 79, 398–410. [CrossRef]
160. Chung, C.M.; Yamamoto, K.; Cho, K. A Submerged Membrane Bioreactor under Unprecedentedly Short Hydraulic Retention
Time Enabled by Non-Woven Fabric Pre-Filtration and Electrochemical Membrane Cleaning. J. Memb. Sci. 2019, 592, 117355.
[CrossRef]
161. Blansaer, N.; Alloul, A.; Verstraete, W.; Vlaeminck, S.E.; Smets, B.F. Aggregation of Purple Bacteria in an Upflow Photobioreactor
to Facilitate Solid/Liquid Separation: Impact of Organic Loading Rate, Hydraulic Retention Time and Water Composition.
Bioresour. Technol. 2022, 348, 126806. [CrossRef] [PubMed]
162. Garber, A.; Hastie, P.; Murray, J.A. Factors Influencing Equine Gut Microbiota: Current Knowledge. J. Equine Vet. Sci. 2020,
88, 102943. [CrossRef] [PubMed]
163. Lei, Y.; Sun, D.; Dang, Y.; Feng, X.; Huo, D.; Liu, C.; Zheng, K.; Holmes, D.E. Metagenomic Analysis Reveals That Activated
Carbon Aids Anaerobic Digestion of Raw Incineration Leachate by Promoting Direct Interspecies Electron Transfer. Water Res.
2019, 161, 570–580. [CrossRef]
164. Zhang, L.; Gong, X.; Xu, R.; Guo, K.; Wang, L.; Zhou, Y. Responses of Mesophilic Anaerobic Sludge Microbiota to Thermophilic
Conditions: Implications for Start-up and Operation of Thermophilic THP-AD Systems. Water Res. 2022, 216, 118332. [CrossRef]
[PubMed]
165. Behera, B.; Venkata Supraja, K.; Paramasivan, B. Integrated Microalgal Biorefinery for the Production and Application of
Biostimulants in Circular Bioeconomy. Bioresour. Technol. 2021, 339, 125588. [CrossRef] [PubMed]
166. Rashama, C.; Ijoma, G.N.; Matambo, T.S. The Effects of Phytochemicals on Methanogenesis: Insights from Ruminant Digestion
and Implications for Industrial Biogas Digesters Management. Phytochem. Rev. 2021, 20, 1245–1271. [CrossRef]
167. Raut, M.P.; Pandhal, J.; Wright, P.C. Effective Pretreatment of Lignocellulosic Co-Substrates Using Barley Straw-Adapted Microbial
Consortia to Enhanced Biomethanation by Anaerobic Digestion. Bioresour. Technol. 2021, 321, 124437. [CrossRef] [PubMed]
168. Alalawy, A.I.; Guo, Z.; Almutairi, F.M.; El Rabey, H.A.; Al-Duais, M.A.; Mohammed, G.M.; Almasoudi, F.M.; Alotaibi, M.A.;
Salama, E.S.; Abomohra, A.E.F.; et al. Explication of Structural Variations in the Bacterial and Archaeal Community of Anaerobic
Digestion Sludges: An Insight through Metagenomics. J. Environ. Chem. Eng. 2021, 9, 105910. [CrossRef]
169. Boonapatcharoen, N.; Meepian, K.; Chaiprasert, P.; Techkarnjanaruk, S. Molecular Monitoring of Microbial Population Dynamics
during Operational Periods of Anaerobic Hybrid Reactor Treating Cassava Starch Wastewater. Microb. Ecol. 2007, 54, 21–30.
[CrossRef]
170. Calli, B.; Mertoglu, B.; Inanc, B.; Yenigun, O. Community Changes During Start-up in Methanogenic Bioreactors Exposed to
Increasing Levels of Ammonia. Environ. Technol. 2010, 26, 85–91. [CrossRef]
171. Lange, M.; Ahring, B.K. A Comprehensive Study into the Molecular Methodology and Molecular Biology of Methanogenic
Archaea. FEMS Microbiol. Rev. 2001, 25, 553–571. [CrossRef] [PubMed]
172. Crone, B.C. Reductions of Mass Transfer Resistance in Membrane Systems Used for Dissolved Methane Recovery during
Anaerobic Treatment of Domestic Wastewater. Ph.D. Thesis, University of Cincinnati, Cincinnati, OH, USA, 2020.
173. Jegede, A.O.; Zeeman, G.; Bruning, H. A Review of Mixing, Design and Loading Conditions in Household Anaerobic Digesters.
Crit. Rev. Environ. Sci. Technol. 2019, 49, 2117–2153. [CrossRef]
Energies 2023, 16, 83
35 of 39
174. Hameed, S.A.; Riffat, R.; Li, B.; Naz, I.; Badshah, M.; Ahmed, S.; Ali, N. Microbial Population Dynamics in Temperature-Phased
Anaerobic Digestion of Municipal Wastewater Sludge. J. Chem. Technol. Biotechnol. 2019, 94, 1816–1831. [CrossRef]
175. Sun, M.; Xiao, K.; Zhu, Y.; Ou, B.; Yu, W.; Liang, S.; Hou, H.; Yuan, S.; Gan, F.; Mi, R.; et al. Deciphering the Role of Microplastic
Size on Anaerobic Sludge Digestion: Changes of Dissolved Organic Matter, Leaching Compounds and Microbial Community.
Environ. Res. 2022, 214, 114032. [CrossRef]
176. Lopes, T.A.S.; Queiroz, L.M.; Torres, E.A.; Kiperstok, A. Low Complexity Wastewater Treatment Process in Developing Countries:
A LCA Approach to Evaluate Environmental Gains. Sci. Total Environ. 2020, 720, 137593. [CrossRef]
177. Wojcieszak, D.; Przybył, J.; Mazurkiewicz, J.; Janczak, D.; Zaborowicz, M. Section Modern Energy and Power Sources increasing
the energy value of corn stover used in biogas plant without pre-processing. Int. Multidiscip. Sci. GeoConference SGEM 2018, 18,
495–501. [CrossRef]
178. Chollom, M.N.; Rathilal, S.; Swalaha, F.M.; Bakare, B.F.; Tetteh, E.K. Comparison of Response Surface Methods for the Optimization of an Upflow Anaerobic Sludge Blanket for the Treatment of Slaughterhouse Wastewater. Environ. Eng. Res. 2020, 25, 114–122.
[CrossRef]
179. Khan, M.A.; Ngo, H.H.; Guo, W.; Liu, Y.; Nghiem, L.D.; Chang, S.W.; Nguyen, D.D.; Zhang, S.; Luo, G.; Jia, H. Optimization of
Hydraulic Retention Time and Organic Loading Rate for Volatile Fatty Acid Production from Low Strength Wastewater in an
Anaerobic Membrane Bioreactor. Bioresour. Technol. 2019, 271, 100–108. [CrossRef]
180. Zhou, P.; Meshref, M.N.A.; Dhar, B.R. Optimization of Thermal Hydrolysis Process for Enhancing Anaerobic Digestion in a
Wastewater Treatment Plant with Existing Primary Sludge Fermentation. Bioresour. Technol. 2021, 321, 124498. [CrossRef]
181. Yan, P.; Shi, H.X.; Chen, Y.P.; Gao, X.; Fang, F.; Guo, J.S. Optimization of Recovery and Utilization Pathway of Chemical Energy
from Wastewater Pollutants by a Net-Zero Energy Wastewater Treatment Model. Renew. Sustain. Energy Rev. 2020, 133, 110160.
[CrossRef]
182. Masłon, A.; Czarnota, J.; Szaja, A.; Szulzyk-Cieplak, J.; Łagód, G. The Enhancement of Energy Efficiency in a Wastewater
Treatment Plant through Sustainable Biogas Use: Case Study from Poland. Energies 2020, 13, 6056. [CrossRef]
183. Thongsai, A.; Phuttaro, C.; Saritpongteeraka, K.; Charnnok, B.; Bae, J.; Noophan, P.L.; Chaiprapat, S. Efficacy of Anaerobic
Membrane Bioreactor under Intermittent Liquid Circulation and Its Potential Energy Saving against a Conventional Activated
Sludge for Industrial Wastewater Treatment. Energy 2022, 244, 122556. [CrossRef]
184. Sodri, A.; Septriana, F.E. Biogas Power Generation from Palm Oil Mill Effluent (POME): Techno-Economic and Environmental
Impact Evaluation. Energies 2022, 15, 7265. [CrossRef]
185. Calicioglu, O.; Femeena, P.V.; Mutel, C.L.; Sills, D.L.; Richard, T.L.; Brennan, R.A. Techno-Economic Analysis and Life Cycle
Assessment of an Integrated Wastewater-Derived Duckweed Biorefinery. ACS Sustain. Chem. Eng. 2021, 9, 9395–9408. [CrossRef]
186. Cartes, J.; Neumann, P.; Hospido, A.; Vidal, G. Life Cycle Assessment of Management Alternatives for Sludge from Sewage
Treatment Plants in Chile: Does Advanced Anaerobic Digestion Improve Environmental Performance Compared to Current
Practices? J. Mater. Cycles Waste Manag. 2018, 20, 1530–1540. [CrossRef]
187. Tabesh, M.; Feizee Masooleh, M.; Roghani, B.; Motevallian, S.S. Life-Cycle Assessment (LCA) of Wastewater Treatment Plants: A
Case Study of Tehran, Iran. Int. J. Civ. Eng. 2019, 17, 1155–1169. [CrossRef]
188. Chuka-ogwude, D.; Ogbonna, J.; Moheimani, N.R. A Review on Microalgal Culture to Treat Anaerobic Digestate Food Waste
Effluent. Algal Res. 2020, 47, 101841. [CrossRef]
189. Rajesh Banu, J.; Godvin Sharmila, V.; Kavitha, S.; Rajajothi, R.; Gunasekaran, M.; Angappane, S.; Kumar, G. TiO2—Chitosan Thin
Film Induced Solar Photocatalytic Deflocculation of Sludge for Profitable Bacterial Pretreatment and Biofuel Production. Fuel
2020, 273, 117741. [CrossRef]
190. Mistretta, M.; Gulotta, T.M.; Caputo, P.; Cellura, M. Bioenergy from Anaerobic Digestion Plants: Energy and Environmental
Assessment of a Wide Sample of Italian Plants. Sci. Total Environ. 2022, 843, 157012. [CrossRef]
191. Lisbona, P.; Pascual, S.; Pérez, V. Evaluation of Synergies of a Biomass Power Plant and a Biogas Station with a Carbon Capture
System. Energies 2021, 14, 908. [CrossRef]
192. Tauber, J.; Ramsbacher, A.; Svardal, K.; Krampe, J.; Steinmetz, H.; Schafer, M.; Wiese, J. Energetic Potential for Biological
Methanation in Anaerobic Sewage Sludge Digesters in Austria. Energies 2021, 14, 6618. [CrossRef]
193. Iglesias, R.; Muñoz, R.; Polanco, M.; Díaz, I.; Susmozas, A.; Moreno, A.D.; Guirado, M.; Carreras, N.; Ballesteros, M. Biogas from
Anaerobic Digestion as an Energy Vector: Current Upgrading Development. Energies 2021, 14, 2742. [CrossRef]
194. Gunes, B.; Stokes, J.; Davis, P.; Connolly, C.; Lawler, J. Pre-Treatments to Enhance Biogas Yield and Quality from Anaerobic
Digestion of Whiskey Distillery and Brewery Wastes: A Review. Renew. Sustain. Energy Rev. 2019, 113, 109281. [CrossRef]
195. Seghezzo, L.; Zeeman, G.; Van Lier, J.B.; Hamelers, H.V.M.; Lettinga, G. A Review: The Anaerobic Treatment of Sewage in UASB
and EGSB Reactors. Bioresour. Technol. 1998, 65, 175–190. [CrossRef]
196. Chan, Y.J.; Chong, M.F.; Law, C.L.; Hassell, D.G. A Review on Anaerobic–Aerobic Treatment of Industrial and Municipal
Wastewater. Chem. Eng. J. 2009, 155, 1–18. [CrossRef]
197. Işik, M.; Sponza, D.T. Anaerobic/Aerobic Sequential Treatment of a Cotton Textile Mill Wastewater. J. Chem. Technol. Biotechnol.
2004, 79, 1268–1274. [CrossRef]
198. Sklyar, V.; Epov, A.; Gladchenko, M.; Danilovich, D.; Kalyuzhnyi, S. Combined Biologic (Anaerobic-Aerobic) and Chemical
Treatment of Starch Industry Wastewater. Appl. Biochem. Biotechnol. 2003, 109, 253–262. [CrossRef]
Energies 2023, 16, 83
36 of 39
199. Wu, J.; Kong, Z.; Luo, Z.; Qin, Y.; Rong, C.; Wang, T.; Hanaoka, T.; Sakemi, S.; Ito, M.; Kobayashi, S.; et al. A Successful
Start-up of an Anaerobic Membrane Bioreactor (AnMBR) Coupled Mainstream Partial Nitritation-Anammox (PN/A) System: A
Pilot-Scale Study on in-Situ NOB Elimination, AnAOB Growth Kinetics, and Mainstream Treatment Performance. Water Res.
2021, 207, 117783. [CrossRef]
200. Maaz, M.; Yasin, M.; Aslam, M.; Kumar, G.; Atabani, A.E.; Idrees, M.; Anjum, F.; Jamil, F.; Ahmad, R.; Khan, A.L.; et al. Anaerobic
Membrane Bioreactors for Wastewater Treatment: Novel Configurations, Fouling Control and Energy Considerations. Bioresour.
Technol. 2019, 283, 358–372. [CrossRef]
201. Chan, P.C.; Lu, Q.; de Toledo, R.A.; Gu, J.D.; Shim, H. Improved Anaerobic Co-Digestion of Food Waste and Domestic Wastewater
by Copper Supplementation—Microbial Community Change and Enhanced Effluent Quality. Sci. Total Environ. 2019, 670,
337–344. [CrossRef]
202. De Bernardini, N.; Basile, A.; Zampieri, G.; Kovalovszki, A.; De, B.; Diaz, D.; Offer, E.; Wongfaed, N.; Angelidaki, I.; Kougias,
P.G.; et al. Integrating Metagenomic Binning with Flux Balance Analysis to Unravel Syntrophies in Anaerobic CO2 Methanation.
Microbiome 2022, 10, 1–18. [CrossRef] [PubMed]
203. Borrel, G.; Parisot, N.; Harris, H.M.B.; Peyretaillade, E.; Gaci, N.; Tottey, W.; Bardot, O.; Raymann, K.; Gribaldo, S.; Peyret, P.; et al.
Comparative Genomics Highlights the Unique Biology of Methanomassiliicoccales, a Thermoplasmatales-Related Seventh Order
of Methanogenic Archaea That Encodes Pyrrolysine. BMC Genom. 2014, 15, 1–24. [CrossRef] [PubMed]
204. Meegoda, J.N.; Li, B.; Patel, K.; Wang, L.B. A Review of the Processes, Parameters, and Optimization of Anaerobic Digestion.
Int. J. Environ. Res. Public Heal. 2018, 15, 2224. [CrossRef] [PubMed]
205. Callejas, C.; Fernández, A.; Passeggi, M.; Wenzel, J.; Bovio, P.; Borzacconi, L.; Etchebehere, C. Microbiota Adaptation after an
Alkaline PH Perturbation in a Full-Scale UASB Anaerobic Reactor Treating Dairy Wastewater. Bioprocess. Biosyst. Eng. 2019, 42,
2035–2046. [CrossRef]
206. Abuabdou, S.M.A.; Ahmad, W.; Aun, N.C.; Bashir, M.J.K. A Review of Anaerobic Membrane Bioreactors (AnMBR) for the
Treatment of Highly Contaminated Landfill Leachate and Biogas Production: Effectiveness, Limitations and Future Perspectives.
J. Clean. Prod. 2020, 255, 120215. [CrossRef]
207. Ma, H.; Guo, Y.; Qin, Y.; Li, Y.Y. Nutrient Recovery Technologies Integrated with Energy Recovery by Waste Biomass Anaerobic
Digestion. Bioresour. Technol. 2018, 269, 520–531. [CrossRef]
208. Rout, P.R.; Shahid, M.K.; Dash, R.R.; Bhunia, P.; Liu, D.; Varjani, S.; Zhang, T.C.; Surampalli, R.Y. Nutrient Removal from Domestic
Wastewater: A Comprehensive Review on Conventional and Advanced Technologies. J. Environ. Manage. 2021, 296, 113246.
[CrossRef]
209. Aziz, A.; Basheer, F.; Sengar, A.; Irfanullah; Khan, S.U.; Farooqi, I.H. Biological Wastewater Treatment (Anaerobic-Aerobic)
Technologies for Safe Discharge of Treated Slaughterhouse and Meat Processing Wastewater. Sci. Total Environ. 2019, 686, 681–708.
[CrossRef]
210. Ellacuriaga, M.; García-Cascallana, J.; Gómez, X. Biogas Production from Organic Wastes: Integrating Concepts of Circular
Economy. Fuels 2021, 2, 9. [CrossRef]
211. Selvaraj, P.S.; Periasamy, K.; Suganya, K.; Ramadass, K.; Muthusamy, S.; Ramesh, P.; Bush, R.; Vincent, S.G.T.; Palanisami, T.
Novel Resources Recovery from Anaerobic Digestates: Current Trends and Future Perspectives. Crit. Rev. Environ. Sci. Technol.
2021, 52, 1915–1999. [CrossRef]
212. Hasan, M.N.; Altaf, M.M.; Khan, N.A.; Khan, A.H.; Khan, A.A.; Ahmed, S.; Kumar, P.S.; Naushad, M.; Rajapaksha, A.U.; Iqbal,
J.; et al. Recent Technologies for Nutrient Removal and Recovery from Wastewaters: A Review. Chemosphere 2021, 277, 130328.
[CrossRef]
213. Kehrein, P.; Van Loosdrecht, M.; Osseweijer, P.; Garfí, M.; Dewulf, J.; Posada, J. A Critical Review of Resource Recovery from
Municipal Wastewater Treatment Plants—Market Supply Potentials, Technologies and Bottlenecks. Environ. Sci. Water Res.
Technol. 2020, 6, 877–910. [CrossRef]
214. D˛ebowski, M.; Zieliński, M. Technological Effectiveness of Sugar-Industry Effluent Methane Fermentation in a Fluidized Active
Filling Reactor (FAF-R). Energies 2020, 13, 6626. [CrossRef]
215. Kumar, M.; Gogoi, A.; Mukherjee, S. Metal Removal, Partitioning and Phase Distributions in the Wastewater and Sludge:
Performance Evaluation of Conventional, Upflow Anaerobic Sludge Blanket and Downflow Hanging Sponge Treatment Systems.
J. Clean. Prod. 2020, 249, 119426. [CrossRef]
216. Montalvo, S.; Huiliñir, C.; Borja, R.; Sánchez, E.; Herrmann, C. Application of Zeolites for Biological Treatment Processes of Solid
Wastes and Wastewaters—Review. Bioresour. Technol. 2020, 301, 122808. [CrossRef] [PubMed]
217. Cheng, D.; Ngo, H.H.; Guo, W.; Chang, S.W.; Nguyen, D.D.; Liu, Y.; Shan, X.; Nghiem, L.D.; Nguyen, L.N. Removal Process of
Antibiotics during Anaerobic Treatment of Swine Wastewater. Bioresour. Technol. 2020, 300, 122707. [CrossRef] [PubMed]
218. Shi, X.; Zuo, J.; Zhang, M.; Wang, Y.; Yu, H.; Li, B. Enhanced Biogas Production and in Situ Ammonia Recovery from Food Waste
Using a Gas-Membrane Absorption Anaerobic Reactor. Bioresour. Technol. 2019, 292, 121864. [CrossRef]
219. Liu, X.; Wu, Y.; Xu, Q.; Du, M.; Wang, D.; Yang, Q.; Yang, G.; Chen, H.; Zeng, T.; Liu, Y.; et al. Mechanistic Insights into the Effect
of Poly Ferric Sulfate on Anaerobic Digestion of Waste Activated Sludge. Water Res. 2021, 189, 116645. [CrossRef]
220. He, D.; Xiao, J.; Wang, D.; Liu, X.; Fu, Q.; Li, Y.; Du, M.; Yang, Q.; Liu, Y.; Wang, Q.; et al. Digestion Liquid Based Alkaline
Pretreatment of Waste Activated Sludge Promotes Methane Production from Anaerobic Digestion. Water Res. 2021, 199, 117198.
[CrossRef]
Energies 2023, 16, 83
37 of 39
221. Barreiro-Vescovo, S.; Barbera, E.; Bertucco, A.; Sforza, E. Integration of Microalgae Cultivation in a Biogas Production Process
from Organic Municipal Solid Waste: From Laboratory to Pilot Scale. ChemEngineering 2020, 4, 25. [CrossRef]
222. Ferreira, L.O.; Astals, S.; Passos, F. Anaerobic Co-Digestion of Food Waste and Microalgae in an Integrated Treatment Plant.
J. Chem. Technol. Biotechnol. 2022, 97, 1545–1554. [CrossRef]
223. Agrawal, K.; Bhatt, A.; Bhardwaj, N.; Kumar, B.; Verma, P. Integrated Approach for the Treatment of Industrial Effluent by
Physico-Chemical and Microbiological Process for Sustainable Environment. In Combined Application of Physico-Chemical &
Microbiological Processes for Industrial Effluent Treatment Plant; Springer: Singapore, 2020; pp. 119–143. [CrossRef]
224. Duan, N.; Zhang, D.; Lin, C.; Zhang, Y.; Zhao, L.; Liu, H.; Liu, Z. Effect of Organic Loading Rate on Anaerobic Digestion of Pig
Manure: Methane Production, Mass Flow, Reactor Scale and Heating Scenarios. J. Environ. Manag. 2019, 231, 646–652. [CrossRef]
[PubMed]
225. Fu, S.; Angelidaki, I.; Zhang, Y. In Situ Biogas Upgrading by CO2-to-CH4 Bioconversion. Trends Biotechnol. 2021, 39, 336–347.
[CrossRef] [PubMed]
226. Ma, C.; Liu, J.; Ye, M.; Zou, L.; Qian, G.; Li, Y.Y. Towards Utmost Bioenergy Conversion Efficiency of Food Waste: Pretreatment,
Co-Digestion, and Reactor Type. Renew. Sustain. Energy Rev. 2018, 90, 700–709. [CrossRef]
227. Mao, C.; Feng, Y.; Wang, X.; Ren, G. Review on Research Achievements of Biogas from Anaerobic Digestion. Renew. Sustain.
Energy Rev. 2015, 45, 540–555. [CrossRef]
228. Van, D.P.; Fujiwara, T.; Tho, B.L.; Toan, P.P.S.; Minh, G.H. A Review of Anaerobic Digestion Systems for Biodegradable Waste:
Configurations, Operating Parameters, and Current Trends. Environ. Eng. Res. 2020, 25, 1–17. [CrossRef]
229. Holohan, B.C.; Duarte, M.S.; Szabo-Corbacho, M.A.; Cavaleiro, A.J.; Salvador, A.F.; Pereira, M.A.; Ziels, R.M.; Frijters, C.T.M.J.;
Pacheco-Ruiz, S.; Carballa, M.; et al. Principles, Advances, and Perspectives of Anaerobic Digestion of Lipids. Environ. Sci.
Technol. 2022, 56, 4749–4775. [CrossRef]
230. Bianco, F.; Race, M.; Forino, V.; Pacheco-Ruiz, S.; Rene, E.R. Bioreactors for Wastewater to Energy Conversion: From Pilot to Full
Scale Experiences. Waste Biorefinery Value Addit. Resour. Util. 2021, 16, 103–124. [CrossRef]
231. Wu, D.; Peng, X.; Li, L.; Yang, P.; Peng, Y.; Liu, H.; Wang, X. Commercial Biogas Plants: Review on Operational Parameters and
Guide for Performance Optimization. Fuel 2021, 303, 121282. [CrossRef]
232. Rajeshwari, K.V.; Balakrishnan, M.; Kansal, A.; Lata, K.; Kishore, V.V.N. State-of-the-Art of Anaerobic Digestion Technology for
Industrial Wastewater Treatment. Renew. Sustain. Energy Rev. 2000, 4, 135–156. [CrossRef]
233. Cayetano, R.D.A.; Kim, G.B.; Park, J.; Yang, Y.H.; Jeon, B.H.; Jang, M.; Kim, S.H. Biofilm Formation as a Method of Improved
Treatment during Anaerobic Digestion of Organic Matter for Biogas Recovery. Bioresour. Technol. 2022, 344, 126309. [CrossRef]
[PubMed]
234. Meena, M.; Sonigra, P.; Yadav, G.; Barupal, T. Wastewater Treatment Techniques: An Introduction. In Removal of emerging
Contaminants through Microbial Processes; Springer: Singapore, 2021; pp. 161–182. [CrossRef]
235. de Aquino, S.; Fuess, L.T.; Pires, E.C. Media Arrangement Impacts Cell Growth in Anaerobic Fixed-Bed Reactors Treating
Sugarcane Vinasse: Structured vs. Randomic Biomass Immobilization. Bioresour. Technol. 2017, 235, 219–228. [CrossRef] [PubMed]
236. Fuess, L.T.; Kiyuna, L.S.M.; Ferraz, A.D.N.; Persinoti, G.F.; Squina, F.M.; Garcia, M.L.; Zaiat, M. Thermophilic Two-Phase
Anaerobic Digestion Using an Innovative Fixed-Bed Reactor for Enhanced Organic Matter Removal and Bioenergy Recovery
from Sugarcane Vinasse. Appl. Energy 2017, 189, 480–491. [CrossRef]
237. Meng, X.; Yuan, X.; Ren, J.; Wang, X.; Zhu, W.; Cui, Z. Methane Production and Characteristics of the Microbial Community in a
Two-Stage Fixed-Bed Anaerobic Reactor Using Molasses. Bioresour. Technol. 2017, 241, 1050–1059. [CrossRef]
238. Dereli, R.K.; van der Zee, F.P.; Ozturk, I.; van Lier, J.B. Treatment of Cheese Whey by a Cross-Flow Anaerobic Membrane
Bioreactor: Biological and Filtration Performance. Environ. Res. 2019, 168, 109–117. [CrossRef]
239. Guo, W.; Ngo, H.H.; Chen, C.; Pandey, A.; Tung, K.L.; Lee, D.J. Anaerobic Membrane Bioreactors for Future Green Bioprocesses.
In Green Technologies for Sustainable Water Management; ASCE Press: Baltimore, MD, USA, 2016; pp. 867–902. [CrossRef]
240. Jain, M. Anaerobic Membrane Bioreactor as Highly Efficient and Reliable Technology for Wastewater Treatment—A Review.
Adv. Chem. Eng. Sci. 2018, 08, 82–100. [CrossRef]
241. Upadhyay, A.; Pareek, N.; Vivekanand, V. Characteristics, parameters, and process design of anaerobic digesters. In Biomethane;
Apple Academic Press: Jersey City, NJ, USA, 2022; pp. 13–42. [CrossRef]
242. Lin, L.; Luo, Z.; Ishida, K.; Urasaki, K.; Kubota, K.; Li, Y.Y. Fast Formation of Anammox Granules Using a NitrificationDenitrification Sludge and Transformation of Microbial Community. Water Res. 2022, 221, 118751. [CrossRef]
243. Najafpour, G.; Najafpour, G.D.; Hashemiyeh, B.A.; Asadi, M.; Ghasemi, M.B. Biological Treatment of Dairy Wastewater in an
Upflow Anaerobic Sludge-Fixed Film Bioreactor. J. Agric. Environ. Sci 2008, 4, 251–257.
244. Del Castillo, A.F.; Garibay, M.V.; Senés-Guerrero, C.; Yebra-Montes, C.; de Anda, J.; Gradilla-Hernández, M.S. Mathematical
Modeling of a Domestic Wastewater Treatment System Combining a Septic Tank, an Up Flow Anaerobic Filter, and a Constructed
Wetland. Water 2020, 12, 3019. [CrossRef]
245. Huang, J.; Wu, M.; Chen, J.; Liu, X.; Chen, T.; Wen, Y.; Tang, J.; Xie, Z. Enhanced Azo Dye Removal in a Continuously Operated
Up-Flow Anaerobic Filter Packed with Henna Plant Biomass. J. Hazard. Mater. 2015, 299, 158–164. [CrossRef]
246. Zhou, H.; Xu, G. Integrated Effects of Temperature and COD/N on an up-Flow Anaerobic Filter-Biological Aerated Filter:
Performance, Biofilm Characteristics and Microbial Community. Bioresour. Technol. 2019, 293, 122004. [CrossRef] [PubMed]
Energies 2023, 16, 83
38 of 39
247. Ngwenya, N.; Gaszynski, C.; Ikumi, D. A Review of Winery Wastewater Treatment: A Focus on UASB Biotechnology Optimisation
and Recovery Strategies. J. Environ. Chem. Eng. 2022, 10, 108172. [CrossRef]
248. Jiang, M.; Westerholm, M.; Qiao, W.; Wandera, S.M.; Dong, R. High Rate Anaerobic Digestion of Swine Wastewater in an
Anaerobic Membrane Bioreactor. Energy 2020, 193, 116783. [CrossRef]
249. Goddek, S.; Delaide, B.P.L.; Joyce, A.; Wuertz, S.; Jijakli, M.H.; Gross, A.; Eding, E.H.; Bläser, I.; Reuter, M.; Keizer, L.C.P.; et al.
Nutrient Mineralization and Organic Matter Reduction Performance of RAS-Based Sludge in Sequential UASB-EGSB Reactors.
Aquac. Eng. 2018, 83, 10–19. [CrossRef]
250. Arora, A.S.; Nawaz, A.; Qyyum, M.A.; Ismail, S.; Aslam, M.; Tawfik, A.; Yun, C.M.; Lee, M. Energy Saving Anammox TechnologyBased Nitrogen Removal and Bioenergy Recovery from Wastewater: Inhibition Mechanisms, State-of-the-Art Control Strategies,
and Prospects. Renew. Sustain. Energy Rev. 2021, 135, 110126. [CrossRef]
251. Lee, J.; Kwon, D.; Kim, J. Long-Term Performance Evaluation of Granular Activated Carbon Fluidization and Biogas Sparging in
Anaerobic Fluidized Bed Membrane Bioreactor: Membrane Fouling and Micropollutant Removal. Process Saf. Environ. Prot. 2021,
154, 425–432. [CrossRef]
252. Hu, Y.; Cai, X.; Du, R.; Yang, Y.; Rong, C.; Qin, Y.; Li, Y.Y. A Review on Anaerobic Membrane Bioreactors for Enhanced
Valorization of Urban Organic Wastes: Achievements, Limitations, Energy Balance and Future Perspectives. Sci. Total Environ.
2022, 820, 153284. [CrossRef] [PubMed]
253. Omil, F.; Garrido, J.M.; Arrojo, B.; Méndez, R. Anaerobic Filter Reactor Performance for the Treatment of Complex Dairy
Wastewater at Industrial Scale. Water Res. 2003, 37, 4099–4108. [CrossRef]
254. Méndez, R.; Blázquez, R.; Lorenzo, F.; Lema, J.M. Anaerobic Treatment of Cheese Whey: Start-up and Operation. Water Sci.
Technol. 1989, 21, 1857–1860. [CrossRef]
255. Shahzad, H.M.A.; Khan, S.J.; Zeshan; Jamal, Y.; Habib, Z. Evaluating the Performance of Anaerobic Moving Bed Bioreactor and
Upflow Anaerobic Hybrid Reactor for Treating Textile Desizing Wastewater. Biochem. Eng. J. 2021, 174, 108123. [CrossRef]
256. Fernández del Castillo, A.; Garibay, M.V.; Senés-Guerrero, C.; Orozco-Nunnelly, D.A.; de Anda, J.; Gradilla-Hernández, M.S.
A Review of the Sustainability of Anaerobic Reactors Combined with Constructed Wetlands for Decentralized Wastewater
Treatment. J. Clean. Prod. 2022, 371, 133428. [CrossRef]
257. Ramasamy, E.V.; Gajalakshmi, S.; Sanjeevi, R.; Jithesh, M.N.; Abbasi, S.A. Feasibility Studies on the Treatment of Dairy Wastewaters
with Upflow Anaerobic Sludge Blanket Reactors. Bioresour. Technol. 2004, 93, 209–212. [CrossRef] [PubMed]
258. Buntner, D.; Sánchez, A.; Garrido, J.M. Feasibility of Combined UASB and MBR System in Dairy Wastewater Treatment at
Ambient Temperatures. Chem. Eng. J. 2013, 230, 475–481. [CrossRef]
259. Szabo-Corbacho, M.A.; Pacheco-Ruiz, S.; Míguez, D.; Hooijmans, C.M.; García, H.A.; Brdjanovic, D.; van Lier, J.B. Impact of Solids
Retention Time on the Biological Performance of an AnMBR Treating Lipid-Rich Synthetic Dairy Wastewater. Environ. Technol.
2019, 42, 597–608. [CrossRef]
260. Wandera, S.M.; Qiao, W.; Jiang, M.; Gapani, D.E.; Bi, S.; Dong, R. AnMBR as Alternative to Conventional CSTR to Achieve
Efficient Methane Production from Thermal Hydrolyzed Sludge at Short HRTs. Energy 2018, 159, 588–598. [CrossRef]
261. Mucha, Z.; Wójcik, W.; Polus, M. Brief Review of Operation of Anaerobic Wastewater Treatment with Membrane Bioreactors.
E3S Web Conf. 2019, 86, 00020. [CrossRef]
262. Panigrahi, S.; Dubey, B.K. A Critical Review on Operating Parameters and Strategies to Improve the Biogas Yield from Anaerobic
Digestion of Organic Fraction of Municipal Solid Waste. Renew. Energy 2019, 143, 779–797. [CrossRef]
263. Semiyaga, S.; Nakagiri, A.; Niwagaba, C.B.; Manga, M. Application of Anaerobic Digestion in Decentralized Faecal Sludge
Treatment Plants. In Anaerobic Biodigesters for Human Waste Treatment; Springer: Singapore, 2022; pp. 263–281. [CrossRef]
264. Wang, S.; Sahoo, K.; Jena, U.; Dong, H.; Bergman, R.; Runge, T. Life-Cycle Assessment of Treating Slaughterhouse Waste Using
Anaerobic Digestion Systems. J. Clean. Prod. 2021, 292, 126038. [CrossRef]
265. Calise, F.; Cappiello, F.L.; Cimmino, L.; Dentice d’Accadia, M.; Vicidomini, M. Dynamic Analysis and Investigation of the Thermal
Transient Effects in a CSTR Reactor Producing Biogas. Energy 2023, 263, 126010. [CrossRef]
266. Liu, C.; Sun, Y.; Li, D.; Bian, J.; Wu, Y.; Li, P.; Sun, Y. Influence of Enclosure Filled with Phase Change Material on PhotoThermal Regulation of Direct Absorption Anaerobic Reactor: Numerical and Experimental Study. Appl. Energy 2022, 313, 118885.
[CrossRef]
267. Krentowski, J.R. Assessment of Destructive Impact of Different Factors on Concrete Structures Durability. Materials 2021, 15, 225.
[CrossRef] [PubMed]
268. Mahmudul, H.M.; Rasul, M.G.; Akbar, D.; Narayanan, R.; Mofijur, M. A Comprehensive Review of the Recent Development and
Challenges of a Solar-Assisted Biodigester System. Sci. Total Environ. 2021, 753, 141920. [CrossRef] [PubMed]
269. Cahyani, D.; Haryanto, A.; Putra, G.A.; Fil’Aini, R.; Marpaung, D.S. Life Cycle Assessment of Biogas Digester in Small Scale
Tapioca Industry. IOP Conf. Ser. Earth Environ. Sci. 2019, 258, 012017. [CrossRef]
270. Akbulut, A.; Arslan, O.; Arat, H.; Erbas, O. Important Aspects for the Planning of Biogas Energy Plants: Malatya Case Study.
Case Stud. Therm. Eng. 2021, 26, 101076. [CrossRef]
271. Zieliński, M.; D˛ebowski, M.; Kazimierowicz, J. Outflow from a Biogas Plant as a Medium for Microalgae Biomass Cultivation—
Pilot Scale Study and Technical Concept of a Large-Scale Installation. Energies 2022, 15, 2912. [CrossRef]
272. Vendruscolo, E.C.G.; Mesa, D.; Rissi, D.V.; Meyer, B.H.; de Oliveira Pedrosa, F.; de Souza, E.M.; Cruz, L.M. Microbial Communities
Network Analysis of Anaerobic Reactors Fed with Bovine and Swine Slurry. Sci. Total Environ. 2020, 742, 140314. [CrossRef]
Energies 2023, 16, 83
39 of 39
273. Paçal, M.; Semerci, N.; Çallı, B. Treatment of Synthetic Wastewater and Cheese Whey by the Anaerobic Dynamic Membrane
Bioreactor. Environ. Sci. Pollut. Res. 2019, 26, 32942–32956. [CrossRef]
274. Saikat, B.; Naveen, P.; Sivamani, S. Reactor Design for Biogas Production-A Short Review. J. Energy Power Technol. 2022, 4, 1–22.
[CrossRef]
275. Shankar, K.R.; Theresa, V.; Brindha, R.; Renganathan, S. Bioprocessing for Enhanced Biological Textile Wastewater Treatment. Bioprocess
Engineering a Green Environment; CRC Press: Boca Raton, FL, USA, 2018; pp. 99–130. [CrossRef]
276. Li, Y.Y.; Huang, X.W.; Li, X.Y. Use of a Packed-Bed Biofilm Reactor to Achieve Rapid Formation of Anammox Biofilms for
High-Rate Nitrogen Removal. J. Clean. Prod. 2021, 321, 128999. [CrossRef]
277. Cremonez, P.A.; Teleken, J.G.; Weiser Meier, T.R.; Alves, H.J. Two-Stage Anaerobic Digestion in Agroindustrial Waste Treatment:
A Review. J. Environ. Manag. 2021, 281, 111854. [CrossRef]
278. Küçükağa, Y.; Facchin, A.; Torri, C.; Kara, S. An Original Arduino-Controlled Anaerobic Bioreactor Packed with Biochar as a
Porous Filter Media. Methods X 2022, 9, 101615. [CrossRef] [PubMed]
279. Faria, C.V.; Souza, D.F.; Pontes, T.M.; Amaral, M.C.S.; Fonseca, F.V. Strategies of Anaerobic Sludge Granulation in an EGSB
Reactor. J. Environ. Manag. 2019, 244, 69–76. [CrossRef] [PubMed]
280. Hao, F.; Shen, M. Simulation Investigation of the Baffle Overlapping Rate on Three-Phase Separation Efficiency in a Typical UASB
Reactor. IOP Conf. Ser. Earth Environ. Sci. 2019, 227, 052032. [CrossRef]
281. Shah, A.; Shah, M. Characterisation and Bioremediation of Wastewater: A Review Exploring Bioremediation as a Sustainable
Technique for Pharmaceutical Wastewater. Groundw. Sustain. Dev. 2020, 11, 100383. [CrossRef]
282. Rattier, M.; Jimenez, J.A.; Miller, M.W.; Dhanasekar, A.; Willis, J.; Keller, J.; Batstone, D. Long-Term Comparison of Pilot UASB
and AnMBR Systems Treating Domestic Sewage at Ambient Temperatures. J. Environ. Chem. Eng. 2022, 10, 108489. [CrossRef]
283. Ahmad, T.; Aadil, R.M.; Ahmed, H.; Rahman, U.U.; Soares, B.C.V.; Souza, S.L.Q.; Pimentel, T.C.; Scudino, H.; Guimarães, J.T.;
Esmerino, E.A.; et al. Treatment and Utilization of Dairy Industrial Waste: A Review. Trends Food Sci. Technol. 2019, 88, 361–372.
[CrossRef]
284. Biologie, A.X.; Vorobyova, O. Anaerobic filters, anaerobic fluidized bed reactors, upflow anaerobic sludge blanket reactors—
anaerobic reactor for industrial wastewater treatment. In Proceedings of the III International Scientific and Practical Conference,
Zurich, Switzerland, 12 August 2022; pp. 84–88. [CrossRef]
285. Anjum, F.; Khan, I.M.; Kim, J.; Aslam, M.; Blandin, G.; Heran, M.; Lesage, G. Trends and Progress in AnMBR for Domestic
Wastewater Treatment and Their Impacts on Process Efficiency and Membrane Fouling. Environ. Technol. Innov. 2021, 21, 101204.
[CrossRef]
286. Musa, M.A.; Idrus, S.; Man, H.C.; Daud, N.N.N. Wastewater Treatment and Biogas Recovery Using Anaerobic Membrane
Bioreactors (AnMBRs): Strategies and Achievements. Energies 2018, 11, 1675. [CrossRef]
287. Pasaoglu, M.E.; Kaya, R.; Korkut, S.; Turken, T.; Zeytuncu, B.; Kose Mutlu, B.; Ozgun, H.; Ersahin, M.E.; Koyuncu, I. Novel
Anaerobic Biological Treatment Systems. In Current Developments in Biotechnology and Bioengineering; Elsevier: Amsterdam, The
Netherlands, 2022; pp. 529–560. [CrossRef]
288. Balcıoğlu, G.; Yilmaz, G.; Gönder, Z.B. Evaluation of Anaerobic Membrane Bioreactor (AnMBR) Treating Confectionery Wastewater at Long-Term Operation under Different Organic Loading Rates: Performance and Membrane Fouling. Chem. Eng. J. 2021,
404, 126261. [CrossRef]
289. Magalhães, N.C.; Silva, A.F.R.; Cunha, P.V.M.; Drewes, J.E.; Amaral, M.C.S. Role of Nanofiltration or Reverse Osmosis Integrated
to Ultrafiltration-Anaerobic Membrane Bioreactor Treating Vinasse for the Conservation of Water and Nutrients in the Ethanol
Industry. J. Water Process Eng. 2020, 36, 101338. [CrossRef]
290. Deowan, S.A.; Galiano, F.; Hoinkis, J.; Figoli, A.; Drioli, E. Submerged Membrane Bioreactor (SMBR) for Treatment of Textile Dye
Wastewatertowards Developing Novel MBR Process. APCBEE Procedia 2013, 5, 259–264. [CrossRef]
291. D˛ebowski, M.; Zieliński, M.; Krzemieniewski, M.; Brudniak, A. Effect of Magneto-Active Filling on the Effectiveness of Methane
Fermentation of Dairy Wastewaters. Int. J. Green Energy 2022, 19, 455–462. [CrossRef]
292. Camiloti, P.R.; Mockaitis, G.; Domingues Rodrigues, J.A.; Rissato Zamariolli Damianovic, M.H.; Foresti, E.; Zaiat, M. Innovative
Anaerobic Bioreactor with Fixed-Structured Bed (ABFSB) for Simultaneous Sulfate Reduction and Organic Matter Removal.
J. Chem. Technol. Biotechnol. 2014, 89, 1044–1050. [CrossRef]
293. Si, B.; Yang, H.; Huang, S.; Watson, J.; Zhang, Y.; Liu, Z. An Innovative Multistage Anaerobic Hythane Reactor (MAHR): Metabolic
Flux, Thermodynamics and Microbial Functions. Water Res. 2020, 169, 115216. [CrossRef]
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