Ecological Engineering 152 (2020) 105882
Contents lists available at ScienceDirect
Ecological Engineering
journal homepage: www.elsevier.com/locate/ecoleng
Review
Prospectives and challenges of wastewater treatment technologies to combat
contaminants of emerging concerns
T
⁎
Aamir Ishaq Shaha, Mehraj U. Din Dara, Rouf Ahmad Bhatb, , J.P. Singha, Kuldip Singhc,
Shakeel Ahmad Bhatd
a
Department of Soil and Water Engineering, Punjab Agricultural University, Ludhiana, Punjab 141004, India
Division of Environmental Science, Sher- e- Kashmir University of Agricultural Sciences and Technology of Kashmir, Shalimar, Srinagar 190025, India
c
Department of Soil Science, Punjab Agricultural University, Ludhiana, Punjab 141004, India
d
College of Agricultural Engineering and Technology, Sher- e- Kashmir University of Agricultural Sciences and Technology of Kashmir, Shalimar, Srinagar 190025, India
b
A R T I C LE I N FO
A B S T R A C T
Keywords:
Ultraviolet irradiation
Biofilters
Hydrogels
Algal technology
Membrane bioreactors
Nanofiltration
Various anthropogenic activities result in a continuous discharge of contaminants of emerging concerns (CECs)
into the natural environs. The remediation of these substances is an emerging concern to safeguard life on earth.
The main aim of this research article is to provide a deep imminent into the available conventional and advanced
wastewater treatment processes and to analyze their removal efficiencies, long term application prospects while
comparing them technically and economically. Several traditional approaches and advanced oxidation processes
(AOPs) experimented in the recent past. Ozonation and powdered activated charcoal remove iohexol, iomeprol,
and iopromide with an efficiency of > 97% and 90% for diatrizoate. Algal technologies have excellent removal
efficiency for heavy metals (37–100%) and remove noxious nutrients. AOPs significantly remove hazardous
contaminants from wastewaters. Interestingly, engineered biochar cleans heavy metals, toxic dyes, COD, pesticides, and harmful aromatic compounds effectively. Moreover, nanofiltration nowadays considered as the boon
for treating the wastewater with a dye removal efficiency achieved to be 98%. The removal efficiencies by
exploring AOPs vary 45–100% for specific contaminants but, increase the energy consumption cost by 60–150%.
Undoubtedly, AOPs overweighed to conventional remediation technologies for efficiency, but are specific to
remove a particular contaminant. Furthermore, based on past research, these techniques appreciably remove one
or more kinds of pollutants but are inadequate to remove most of the toxic substances efficiently from wastewater. Therefore, a comprehensive research is required to find an appropriate low cost, ecofriendly, and efficient
technology to remediate different kinds of CECs from wastewater.
1. Introduction
Various anthropogenic activities result in a continuous discharge of
contaminants of emerging concerns (CECs) into the natural environs.
Urban wastewater treatment plants (WWTP) result in land-based pollution of water bodies due to the release of organic contaminants into
the aquatic systems. A study based on a systematic comparison of raw
wastewaters in the Asian region and those in European and North
American for the existence of 60 most common emerging contaminants
was conducted. The study reported higher concentration of
contaminants in the Asian region as compared to the European and
North American countries (Qi et al., 2015). It is well known fact that
substantial variation in the removal efficiencies of the contaminants
exist due to differences in operating conditions of the treatment plants,
high structural diversity, and variability of chemical and physicochemical properties of the organic contaminants. As a result, it has been
observed that predicted-no-effect concentrations (PNECs) level of numerous emerging contaminants for aquatic organisms always exceeded
in treated wastewaters (Tran et al., 2018). Plasticizers, pharmaceutical
drugs, artificial sweeteners, personal care products (PCPs), insect
Abbreviations: CEC, Contaminants of emerging concern; DBP, Disinfection By Products; EBC, Endocrine Disrupting Chemicals; RO, Reverse osmosis; FO, Forward
osmosis; UF, Ultra filtration; MF, Microfiltration; PSB, Photosynthetic bacteria; WWTP, Wastewater treatment plant; CAPEX, Capital expenditure; OPEX, Operating
expenditures; ICPS, Inductively coupled plasma mass spectrometry; EBPR, Enhanced biological phosphorus removal; LAS, Linear alkylbenzeneB214 sulfonate; NF,
Nanofiltration; AOP, Advanced oxidation processes; ARB, Antibiotic Resistant Bacteria; TSS, Total suspended solids; PhAC, Pharmaceuticals; PCP, Personal Care
Products; PNEC, Perpredicted Noeffect Concentration
⁎
Corresponding author.
E-mail address: rufi.bhat@gmail.com (R.A. Bhat).
https://doi.org/10.1016/j.ecoleng.2020.105882
Received 15 December 2019; Received in revised form 23 April 2020; Accepted 30 April 2020
0925-8574/ © 2020 Elsevier B.V. All rights reserved.
Ecological Engineering 152 (2020) 105882
A.I. Shah, et al.
repellents, hormones, and flame retardants are categorized as emerging
contaminants found mainly in municipal sewage (Rodil et al., 2012;
Loos et al., 2013). Most of the available studies are focused on investigating a preselected set of target analytes which may disregard
many organic contaminants potentially harmful in nature. A more
comprehensive logical evaluation and assessment of emerging contaminants is therefore obvious and needed (Petrie et al., 2015). A more
holistic approach is non-target screening analyses of sources such as in
municipal wastewaters, which involves identifying a wide range of
structurally diverse emerging contaminants. In any case, very few such
studies based either on liquid chromatography/mass spectrometry (LC/
MS) or gas chromatography/ mass spectrometry (GC/MS) analyses
have been conducted. Gros et al. (2017) recently performed a comprehensive chemical screening of municipal wastewaters by applying
high-resolution MS-LC and identified a total of 79 emerging contaminants including pesticides, fluoroalkyl substances, pharmaceuticals
(PhACs), PCPs and flame retardants. Based on MS/GC technique, several studies involving non-target screening of municipal wastewaters
were conducted in the last decades (e.g. Paxeus and Schroder, 1996;
Eriksson et al., 2003; Wluka et al., 2017; Blum et al., 2017). The results
not only indicated the presence of industrial chemicals in the wastewaters but also high structural variety compounds such as PCPs and
PhACs used in the households were also found (Eriksson et al., 2003;
Wluka et al., 2017). A recent prioritization study of identified contaminants revealed the most relevant compounds being emitted by onsite WWTPs categorized as the UV absorber octocrylene, 7,9-tetramethyl-5-decyne-4,7-diol, synthetic fragrance galaxolide, linear alkylbenzenes from detergent residues and α-tocopheryl acetate (Blum et al.,
2017). A growing source of concern for the environment is the presence
of contaminants of emerging concern (CEC) and pathogens in the
treated wastewater effluents discharged from the WWTP's (Krzeminksi
et al., 2019; Onesios et al., 2009). A wide range of infections in humans
with diverse symptoms may be a result of the presence of pathogens
and enteric viruses in the treated water (Purnell et al., 2015). Endocrine
system of living organisms may be affectedby either inhibiting, disrupting or mimicking of hormone functions due to the presence of trace
concentrations of endocrine-disrupting chemicals (EDCs) (Bolong et al.,
2009). The presence of PCPs and PHACs in water resources may lead
these contaminants to end up in plants, terrestrial-aquatic organisms,
and other food sources, thus affecting the overall food chain (Bolong
et al., 2009; Krzeminksi et al., 2019). The magnitude of the adverse
effects of these contaminants has led to the emergence of new design
criteria and upgradation of conventional technologies in WWTPs
(Krzeminksi et al., 2019). Activated carbon due to its ability to adsorb
most CECs has shown more favorable results as compared to the traditional coagulation-flocculation methodologies which had limited
success (Luo et al., 2014a, 2014b; Meinel et al., 2016). Pore blocking
and competition for adsorption sites, however, undermine the long
term performance of activated carbon (Luo et al., 2014a, 2014b). The
effectiveness of membrane filtration techniques have also been investigated (Bolong et al., 2009; Luo et al., 2015; Tang et al., 2017). Due
to their average membrane pore size being greater than the molecular
mean diameter of the target CEC, both ultrafiltration (UF) and microfiltration (MF) have performed poorly (Luo et al., 2014a, 2014b). On
the other hand, RO and NF systems despite their higher removal efficiencies suffer increased membrane fouling and remain permeable to
small contaminants (Bolong et al., 2009; Luo et al., 2014a, 2014b).
Further treatment of the filtrate is required for activated carbon and
other methods of filtration. Both UV irradiation and advanced oxidation
although being expensive require an aggressive application (Bolong
et al., 2009; Luo et al., 2015). UV irradiation and advanced oxidation
both are susceptible of producing unwanted disinfection byproducts
(DBPs) that can pose human health and environmental risks (Bradley
et al., 2011; Luo et al., 2014a). Improved reduction of FRNA coliphage
concentrations with membrane bioreactor (MBR) or bio-sand systems
relative to traditional activated sludge have also been reported (Bradley
et al., 2011; Purnell et al., 2015). Traditional wastewater treatment
processes are ineffective in degrading micropollutants such as enteric
pathogens and CEC (Luo et al., 2014a, 2014b; Bradley et al., 2011).
Hence there is a need of development of new methods and technologies
that can effectively remove both micropollutants as well meet the
conventional treatment metrics at minimum capital and operating costs
(Krzeminksi et al., 2019; Bolong et al., 2009; Luo et al., 2014a, 2014b;
Onesios et al., 2009; Rizzo et al., 2019). Various physical, chemical, and
biological technologies have been proposed for the degradation of
contaminants as well for reduction of contaminant levels in the discharged waters to safe levels. Among them, the most favorable results
have been observed in advanced tertiary treatment technologies although the adoption of these techniques remains a cost and energyintensive task (Christofilopoulos, 2017). Data regarding possible
changes in their antibiotic resistance profile post-treatment and antibiotic-resistant bacteria (ARB) elimination are limited, although these
technologies tend to reduce the bacterial load (Rizzo et al., 2013).
Emerging environmental issues related to the quality environment
have been raised due to the interaction of harmful products generated
by the ways of enhanced anthropogenic activities. These products (toxic
substances) pose the foremost threat to quality environs. The CECs viz.,
pesticides, fertilizers, heavy metals, microplastics, pharmaceutically
active compounds (PhACs), cosmetic products, wastes from industries,
natural and synthetic hormones usually end up in the soil (Enick, 2006;
Barboza and Gimenez, 2015). The toxic contamination in the freshwater ecosystem indirectly affects human health and the surrounding
ecosystem (Daughton and Ternes, 1999a, 1999b; Malaj et al., 2014;
Rizzo et al., 2019). CECs are usually detected in low concentrations in
wastewaters and contaminated aquatic ecosystems. The CECs removal
with the application of conventional treatment methods (Li and Zhang,
2011) viz., activated sludge process (Rizzo et al., 2015), filtration
(Krzeminski et al., 2019), and disinfection (Rizzo et al., 2019) usually
remains ineffective in urban WWTPs. The effluent discharges from
WWTPs become a major source of CECs into the natural environs
(Petrie et al., 2014). The uptake of CECs by plants from contaminated
soils and it's culmination up in the food chain is another issue of concern to life-supporting ecosystems and particularly to the human health
(Paz et al., 2016). The acceptable concentration of CECs in wastewater
discharged from ineffective WWTPs or utilized for the agricultural
purpose have not been effectively monitored and regulated. Regulations
relating to wastewater reuse exist only in a few developed countries
e.g., Switzerland (Paranychianakis et al., 2015). A related policy to be
followed by the majority of the European countries is still under progress (Rizzo et al., 2019). The CECs concentration monitoring and
regulation are major debated subjects nowadays among the policymakers and stakeholders (Rizzo et al., 2018; Rizzo et al., 2019). CECs
can be effectively removed through RO (Petersen, 1993) membranes;
activated carbon adsorption (Lee et al., 2006) and ozonation process
(Bailey, 2012). The application of activated carbon and ozonation
methods for CEC removal from wastewater has been increasingly
growing in countries like Switzerland, Germany, etc. In Switzerland,
implementation of a new water protection act in 2016 has led to regulation of CECs removal from urban wastewater, while in Germany the
implementation of regulations on CECs is being done voluntarily
(Eggen et al., 2014; FOEN, 2015; Rizzo et al., 2019). However, activated carbon treatment, unlike ozonation, is ineffective for the disinfection of bacterial colonies (Moore et al., 2000). A post-treatment
polishing with a bioactive sand filter is recommended for the ozonation
process due to the formation of disinfection/ oxidation byproducts such
as Nitroso-dimethylamine (NDMA) (Hollender et al., 2009; Rizzo et al.,
2019). Hence additional disinfection stages are required due to the
imposition of stringent concentration limits on CECs in wastewater.
High energy demand has been reported for membrane filtration technology with dense membranes, such as RO or NF. The technique,
however, aids in delivering additional water quality benefits such as
salt removal from the wastewater. Consolidated treatment methods
2
Ecological Engineering 152 (2020) 105882
A.I. Shah, et al.
reported (Samaras et al., 2013; Yang et al., 2014; Cabeza et al., 2012).
Municipal wastewater has been identified as a principal source of the
release of ECs from the point and non-point sources such as industries
and stormwater, water treatment facilities and wastewater from
households into the environment (Ternes et al., 2004). The higher levels of ECs are also a growing concern for sludge management (Wu
et al., 2010). The current designs of WWTPs are unable to restrict or
eliminate the ECs and their metabolites, where they are released as
sewage effluents into streams or rivers having high biodiversity. With
regard to the performance evaluation of wastewater technologies for
nutrient removal, a substantial amount of work has been done so far.
(Molinos-Senante et al., 2012; Daughton and Ternes, 1999a, 1999b;
Heberer, 2002; Barcelo, 2003; Daughton, 2004; Petrovic et al., 2009),
however, there is an unavailability of data on the adverse ecotoxicological impacts of these contaminants on the aquatic ecosystems.
Efficiencies of WWTPs have a significant influx on the concentration
of CECs discharged into nearby public sewer systems. Prevalent substances and CECs with high concentrations in WWTP secondary effluents have also been investigated (Venkatesan and Halden, 2014). A
summary of concentrations of major CECs in WWTPs effluents from
Asia (China, Korea), the US and Europe was reported by Luo et al.,
2014a, 2014b. Different compounds removed along with the efficiencies of the methods used are listed in Table 3 and Table 4. PhACs
concentrations were found to be extended from0.001 to 10 μg/L; with
reported PhACs concentration as high as 1 μg/L (Luo et al. (2014a,
2014b)). Currently, there are no limits and regulations on concentrations of CECs in wastewater discharge (Barbosa et al., 2016). An international priority list based on potential risks of PhACs was developed
by the Global Water Research Coalition with 44 compounds classified
into three classes- Class I (10), Class II (18) and Class III (16) (GWRC,
2008). The classification of PhACs into different groups is based on
their relative occurrence, resistance to treatment, degradability, human
toxicity, and eco-toxicity. There is a need to control the CECs discharge
limits and persisting scientific research in the field of advanced wastewater treatment to arrive at new treatment techniques (Bui et al.,
2016; Ahmed et al., 2017; Rizzo et al., 2019).
Table 1
Categories of wastewater treatment methods.
Conventional methods of wastewater
treatment
Advanced methods of wastewater
treatment
Ozonation and powdered AC
Membrane filtration technology
Chitosen based hydrogels
Ozone treatment and electro
coagulation
Nanotechnology for wastewater
Nanofiltration process
High pressure membrane
Wastewater treatment by PSBs and
photobioreactors
Algal Technology
Microplastic detection technology
Engineered biochar
Artificial wetlands (Biofilters)
available including ozonation, membranes and activated charcoal (AC)
along with new methods of advanced oxidation processes (AOPs), have
been reviewed critically in this review article to examine (i) CECs removal efficiency (ii) barriers in homogenous AOP application (iii) advantages and limitations (iv) possible obstacles towards medium to
longtime adoption of heterogeneous processes, and (v) comparison
based on technical and economical perspectives. Various shortcomings
and research gaps are discussed at the end to evolve at the most appropriate technique (s) for the degradation of CECs from wastewaters.
The various techniques discussed in this review article has been divided
into conventional and advanced oxidation processes based on their
recovery efficiencies, as tabulated, Table 1 which has been discussed
under their respective categories throughout this article.
2. Urban wastewater and CECs
The presence of different types of new compounds of human origin
has been identified in recent decades, drawing attention towards their
possible environmental implications. The wastewater characteristics of
industrial effluents have been listed as in Table 2. The occurrence of
these new compounds also known as the “emerging pollutants” have
become a concern among the engineers, researchers, and the common
people. The pollutants have a harmful impact on human health and
affect both terrestrial and aquatic ecosystem even in trace quantities.
With the help of advanced analytical technologies, non-regulated trace
pollutants of organic nature called as emerging micro-pollutants have
also been detected (Richardson, 2007). Contaminants that have a new
origin and require new treatment and detection techniques are termed
as “emerging” and are classified based on the probable and apprehensible risk to the environment and human health (US EPA, 2012). They
can originate either from industries or may be sourced from agricultural, municipal (domestic), laboratory, or hospital wastewater. To a
greater extent, these contaminants may be derived from three broad
sources, viz. a) Personal Care Products (PCPs) b) Endocrine Disrupting
Compounds (EDCs) and c) Pharmaceuticals (PhACs). The emerging
contaminants, however, may not be confined to the above categories
only and may consist of metabolites of ECs, illegal drugs, engineered
genes, nanomaterials (NMs), etc. During the process of wastewater
treatment, the NMs decrease the biological activity of the bacterial
biomass which eventually leads to decreased EC removal efficiency
(Wang et al., 2012). The presence of ECs in the discharge from WWTPs
as well as in drinking water, groundwater and surface water has been
3. Conventional approaches for CECs removal
Several studies related to the fate of CECs have been carried out on
various scales (laboratory and pilot) during advanced and biological
wastewater treatment in aquatic ecosystems (Halling-Sørensen et al.,
1998). Among various available techniques, AC (activated charcoal)
treatment and ozonation have emerged to be the most economically
feasible and promising technologies for the efficient working of
WWTPs. Currently, Switzerland is the only country that regulates CEC
removal from urban wastewater with both AC treatment and ozonation
implemented at full scale (Eggen et al., 2014; FOEN, 2015; Rizzo et al.,
2019). The membrane techniques for wastewater treatment include RO
and NF, which are pressure-driven for the safe treatment of wastewaters. The RO and NF techniques are sensitive for the removal of
pathogenic microorganisms and are suitable for water reclamation
processes (Shon et al., 2013). The efficiency of post-treatment processes
based on germicidal light in the UV spectrum range as well as using
chemicals (e.g. chlorine, ozone) improves exponentially due to pretreatment using membrane filtration process (Rizzo et al., 2019). In
Australia, the USA, Israel, Singapore, and Netherlands specifically
adopt high-pressure membranes in a number of potable reuse schemes
(Tang et al., 2018; Rizzo et al., 2019). Advanced wastewater treatment
in WWTPs which include pressure-driven processes, ozonation, and AC
treatment are classified as consolidated processes in this review article.
Table 2
Wastewater characteristics of Industrial Effluents (Bilińska et al., 2019).
Indicator
RB5 aqueous solution
Industrial wastewater
pH
Conductivity (mS /cm)
NaCl (g/L)
COD (mgO2/L)
TOC (mgO2/L)
Dye (mg/L)
11.5–12.5
35–40
30–40
910–920
150–160
450–550
11–12
50–60
50–55
1310–1325
260–265
780–790
3.1. Ozonation and powdered activated carbon
Ozonation and Powdered Activated Carbon (PACs) are established
technologies in the full-scale treatment of CECs in wastewater
3
Ecological Engineering 152 (2020) 105882
A.I. Shah, et al.
Table 3
Removal efficiencies of PhACs, PCPs and EDCs in environment media by different biological treatment processes.
Treatment processes
Matrix
Compounds detected
Removal efficiency (%)
References
Activated sludge process
Membrane bioreactor (MBR)
Wastewater
Wastewater
Waste stabilization ponds
Wastewater
96, 38–99.8
99
90
80
99
99
92
97
96
Costanzo et al. (2005)
Urase et al. (2005)
Lesjean et al. (2005)
Carballa et al. (2007)
Hai et al. (2011)
Matamoros et al. (2016)
Constructed wetlands (CWs)
Wastewater
Anaerobic treatment
Activated sludge
Cephalein
Ketoprofen
Pharmaceuticals
Steriods
(Sulfamethoxazole Trimethoprim, 4-nonylphenol, Caffeine)
Caffeine,
Naproxen
Ibuprofen
Triclosan
Galaxolide Tonalide
Caffeine
Enrofloxacin
Tetracycline
Sulfomethoxazole
Estrone
Nonylphenol
Ibuprofen, Naproxen
99
94
98
99
96/68
50/100
> 80
Matamoros et al. (2007)
Matamoros et al. (2009)
Carvalho et al. (2013)
Carballa et al. (2006)
Paterakis et al. (2012)
Samaras et al. (2013)
treatment (Sun et al., 2017) and PACs (Margot et al., 2013) are highly
effective in the removal of steroid hormones (E2 and EE2). It is although impossible to conclude whether ozonation or AC treatment is
superior, as both the techniques have an edge over the other in several
respects (Knopp et al., 2016).
Iodinated contrast media (ICM), personal care and pharmaceutical
products which occur in concentrations from ng to μg L-1 in secondary
effluents from sewage treatment plants (STPs) are regarded as persistent compounds. Forrez et al. (2011), in order to remove micropollutants from STP-effluent, applied biogenic metals bio-palladium
(Bio-Pd) and manganese oxides (BioMnOx) onto lab-scale membrane
bioreactors (MBR) as reductive and oxidative technologies. A total of 14
substances out of the 29 substances detected in the STP-effluent were
removed in the BioMnOx MBR Fig. 1. The biological removal by Pseudomonas putida and associated bacteria in the enriched biofilm and
chemical oxidation by BioMnOx are the supposed removal mechanisms.
In order to be competitive with the ozonation technique for contaminant removal, the removal rates in this technique (highest value:
2.6 mg diclofenac L-1 d-1) needs improvement by a factor of 10. Using
Bio-Pd as a nanosized catalyst, ICM (Iodinated Contrast Media) was
successfully dehalogenated with a novel reductive technique. Iohexol,
Iomeprol, and Iopromide were removed with an efficiency of > 97%
and the more recalcitrant diatrizoate with anefficiencyof90% (Forrez
et al., 2011).
treatment and hence require a detailed comparison (Rizzo et al., 2018).
The treatment using AC offers an edge over AOPs with the former resulting in no by-product formation and low energy consumption (Knopp
et al., 2016; Mousel et al., 2017; Rizzo et al., 2019). However, the AC
production process is a highly energy-intensive process. Furthermore,
the CECs removed from wastewater and adsorbed on the surface are
usually considered hazardous to the environment, and hence require
adequate disposal strategies (Rajasulochana and Preethy, 2016; Rizzo
et al., 2019). Granular Activated Carbon (GAC) has a smaller carbon
dioxide footprint in comparison to the PACs due to their ability to be
reactivated and reused again. However, regeneration and reuse of GAC
require high energy to desorb the adsorbed compounds of higher molecular weight (Bui et al., 2016) which undermines the advantage of
exhausted GAC reuse potential (Rizzo et al., 2019). GAC regeneration
also requires the management of a hot stream containing desorbed
pollutants to be treated as hazardous waste and disposed off accordingly. Ozone treatment for CECs removal performs better for certain
compounds such as diclofenac, gabapentin, and sulfamethoxazole,
while as PACs were found to be effective for some CECs (benzotriazole,
fluconazole, valsartan) (Margot et al., 2013; Kovalova et al., 2013;
Jekel et al., 2015; Rizzo et al., 2019). Both the techniques were ineffective in the removal of negatively charged iodinated contrast media
with high efficiency; however, AC performed better for the removal of
neutral contrast media like iopromide (Knopp et al., 2016). CECs such
as perfluorooctanesulfonic acid (PFOS), Acesulfame and Perfluorooctanoic acid (PFOA) are not effectively removed by either of the
processes (Thompson et al., 2011; Margot et al., 2013; Altmann et al.,
2015; Mailler et al., 2015; Altmann et al., 2016). However, both ozone
3.2. Membrane filtration and its engineering aspects
The removal of microorganisms and total suspended solids (TSS)
Table 4
Removal efficiencies (%) of PhACs, PCPs and EDCs in environment media by different physico-chemical treatment processes.
Treatment processes
Matrix
Compounds detected
Removal efficiency
(%)
References
Activated carbon adsorption
Wastewater water
Wastewater water
Ozonation
River water
Drinking water
Secondary effluent of a sewage
treatment plant
Wastewater
50–100
80–100
50–100
> 90
> 90
> 90
98–99
96
45–65
20–45
~ 100
Nguyen et al. (2016)
Advanced oxidation process
“Bisphenol-A, Diclofenac”
“Carbamazepine”
“Sulfamethoxazole”
“Acetaminophen Diclofenac Sulfamethoxazole”
Ultraviolet irradiation
Nanofiltration
“Estrone, Estradiol, Estriol, 17α ethynyl Estradiol”
“Estrone, Estradiol, Estriol, 17α-ethynyl estradiol”
“Sulfonamides”
“Macrolides”
“Diclofenac, Ibuprofen, Metronidazole, Moxifloxacin,
Telmisartan, Tramadol”
“Roxithromycin, Azithromycin “
4
> 87
Rosario-Ortiz et al.
(2010)
Reungoat et al. (2011)
Westerhoff et al. (2005)
Broséus et al. (2009)
Kim et al. (2009)
Beier et al. (2010)
Liu et al. (2014)
Ecological Engineering 152 (2020) 105882
A.I. Shah, et al.
“Naproxen (>95%)
Diuron (>94%)”
“Ibuprofen (>95%)
Naproxen (>95%)
Iomeprol (63%)”
Biogenic Manganese
Oxide and Bio
Palladium Membrane
Bioreactors (Oxidative
and Reductive
Technologies)
Micro pollutants
Removed
“Clarithromycin, (75%)
Iohexol (72%)
Iopromide (68%)
Sulfamethoxazole
(52%)”
“Codeine
(>93%)
N-acetylsulfamethoxaz
ole (92%)”
“Chlorophene (>89%)
Diclofenac (86%)
Mecoprop (81%)
Triclosan (>78%)”
Fig. 1. Techniques for removal of negatively charged iodinated contrast media.
(Urase et al., 2005). The retention stream decreases in volume with the
movement of water across the membrane from feed to permeate and the
cross-flow velocity increases through the pressure vessel. Practically
50% of the flow feed remaining after filtration is fed and mixed in
another pressure vessel, which at the subsequent stage is fed into another pressure vessel to keep cross-flow velocity within required limits
(Lesjean et al., 2005). Water recoveries of 70%–85% can be achieved
employing a range of stage designs with two or three stages being used
predominantly (Xue et al., 2010). The membranes employed for RO or
NF commercial filtration designs are also referred to as thin-film composites. The membrane composite consists of a polyester layer for
giving structural strength, a polyethersulfone layer identical to the UF
membrane and a third top ultrathin layer of cross-linked polyamide.
The third top ultrathin layer has a size range between 10 and 100 nm
and forms a part of the membrane retaining TDS (Adams et al., 2002).
The resistance of this layer to chemical substances is comparatively low
when compared to the materials used in UF and MF membranes (Beier
et al., 2010). Therefore, this layer remains susceptible to the damage
due to the use of ozone or hypo-chlorous acid, both of which are considered to be strong oxidants. The vulnerability of membrane to chemicals thereby limits the use of in-situ bio-fouling control with germicidal chemical agents as well as other cleaning agents (Westerhoff et al.,
2005). Hydraulic backwashing is also not feasible for the top polyamide
layer due to its dense structure, which otherwise may shrug off from the
primary support layer (Gerrity et al., 2011). A general list of available
membrane separation processes along with associated driving forces
and suitability of each process to various size ranges is given in Table 5.
CECs molecular weight typically lies between 100 and 400 Da, with
some macrolide antibiotics being considerably larger in size (YangaliQuintanilla, 2010). The molecular radii of CECs corresponding to this
from the urban wastewater utilizing a physical barrier tend to be two
main objectives of the ultra-filtration (UF) and low-pressure membrane
or microfiltration technique (MF) (Falsanisi et al., 2010). The variation
in nominal pore sizes of the filtration membranes ranges from
0.01–0.04 μm in UF and 0.1–1 μm in MF (Crittenden et al., 2012). The
chemical resistance over a pH range, oxidation condition, and hydrophilic character tend to be common properties of the polymer chemistries employed (Kabir et al., 2018). These material characteristics such
as resistance to chemical treatments along with engineering approaches
make these membranes quite hard and robust. Chemical soaking, air
scouring, and the use of hydraulic backwashing control fouling reversibly, thereby maintain the functionality of the membrane. A highwater recovery ranging between 96 and 98% is observed in low-pressure membrane filtration processes in wastewater treatments (Wintgens
et al., 2003). Comparatively, RO and NF membranes require pre-filtered
water with less TSS concentration. The treatment objectives in water
treatment can range from removal of main inorganic solutes to reduction in hardness, trace metal contamination or electrical conductivity
along with the removal of organic contaminants. Pre-treatment using
membranes may, however, be effective for a reduction in total dissolved
solids (TDS) in the wastewater (Akhondi et al., 2017).
Cross-flow filtration is a predominant design for membrane filtration from an engineering point of view and is a dominant design from
industrial standards as well. A cross-flow design results from the geometry of spiral wound membrane modules, whereby leaves of the
membranes are packed (Rizzo et al., 2019). The spiral wound membrane modules are successively placed in pressure vessels. These modules are installed sequentially in pressure vessels. The flow requirements due to the resulting design being highly modular are
considerably addressed with pressure vessels placed parallel to flow
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rejection rate, while a high rejection rate through membrane have been
specifically observed for negatively charged organic contaminants
(Bellona et al., 2004).
Table 5
Classification of membrane separation processes (Bowen and Mukhtar, 1996).
Name of process
Driving force
Separation size range
Microfiltration
Ultra-filtration
Reverse Osmosis (Hyperfiltrator)
Electro-dialysis
Dialysis
“Pressure gradient”
“Pressure gradient”
“Pressure gradient”
10–0.1 μm
< 0.1 μm-5 nm
< 5 nm
“Electric field gradient”
“Concentration
gradient”
< 5 nm
< 5 nm
3.3. CECs remediation with photosynthetic bacteria
The wastewaters are laided with a high concentration of different
types of nutrients such as Carbon (C), Phosphorus (P), and Nitrogen (N).
The metabolic characteristics of the photosynthetic bacteria allow them
to use these nutrients for metabolic activities, thereby aiding in water
treatment and biomass production (Blankenship et al., 1995). The application of the photosynthetic bacteria for the treatment of wastewater
dates back to the 1960s. The photosynthetic bacteria treatment also
yields certain high value-added products during the treatment of wastewater containing volatile fatty acids (VFA) of small molecular weight
(Blankenship et al., 1995). This marked the development of photosynthetic bacteria wastewater treatment and resource recovery technologies. Photosynthetic bacteria (PSB) wastewater treatment and resource recovery technologies have been under continuous development
for around the past 60 years. The study of the co-existence of photosynthetic bacterias along with Azobacter to improve soil quality and
plant yield has been applied since 1961 (Okuda and Kobayashi, 1961).
PSB-MBR (membrane bioreactor) technology was the first largest pilotscale (600 L) of its kind applied to retain biomass and eventually purify
wastewater (Kaewsuk et al., 2010). PSB-MBR system was also operated
for a long time to evaluate the stability of the system on a long-term
basis (Chitapornpan et al., 2013). Ultrasonic stimulation and microbiological degradation of CECs have emerged as new techniques for
promoting PSB biomass (Wu et al., 2012; Zhou et al., 2014a; Wu et al.,
2015a, 2015b; Liu et al., 2015). Hülsen et al. (2014) reported that
application of PSB-MBR system for treatment of low organic load
wastewater (municipal wastewater) has proved a vital treatment system
to purify wastewaters significantly. Purple non‑sulfur bacteria, purple
sulfur bacteria, green non‑sulfur bacteria and green sulfur bacteria can
be considered as four basic categories of PSBs (Lu et al., 2019c).
However, the most common PSB in wastewater is the purple non‑sulfur
bacteria (Rhodopseudomonas, Rhodomicrobium, Rhodospirillum). PSBs
due to their great potential in the biotechnology area have attracted
several researcher's attention in recent decades. The value-added substance production ability of PSBs have made them a center of attraction
for the research workers Fig. 3 and Fig. 4. Coenzyme Q10, single-cell
protein, carotenoids, bacteriochlorin, polyhydroxyalkanoates (PHA)
and 5-aminolevulinic acid (5-ALA) are the main value-added substances
produced from PSBs. Microorganism protein is a generic term for
single-cell protein (SCP) produced from microorganisms, such as algae,
bacteria, and fungus. The SCP production could relieve the stress of
molecular weight are usually less than 1 nm. The small size of CECs
makes UF and MF membranes impractical for filtration, although a
minor removal may result due to the adsorption process on fouling
layers or membrane surfaces. Therefore, RO and NF membranes are the
prime focus of this section. The literature identifies the use of three
rejection mechanisms for organic compound removal using RO and NF
membranes which include adsorption, Donnan-exclusion and size exclusion (Van der Bruggen et al., 1999). Fig. 2 shows a visual representation of the rejection mechanisms. Membrane and solute properties along with the system design, feed water quality, and operational
conditions govern the solute removal by these rejection mechanisms.
The altering of the membrane surface and its intrinsic properties due to
membrane fouling have also been found to have a profound impact on
solute rejection (Zularisam et al., 2006). Solute rejection results due to
the pore size of the polyamide layer of the membrane being smaller
than the solute size (Rizzo et al., 2019). This restricts the passage of
solute through the membrane, thereby leaving them in the retentate
(Agenson and Urase, 2007). Large molecules with a molecular weight in
the range of 200 g/mol are effectively removed by size exclusion mechanism (De la Rubia et al., 2008). On the other hand, NF has been
found to be effective for the removal of organic compounds of molecular weight greater than 200 g/mol. The rate of removal of CECs can,
however, differ considerably depending upon the type of NF membrane
utilized for filtration (Bourgin et al., 2018).
RO filtration, on the other hand, is effective for solutes of mass
range between 100 and 150 g/mol (Bellona et al., 2004; Rizzo et al.,
2019). RO and NF films are designed in a way to develop a negative
charge at the surface. The existence of negative charge results in the
development of zeta potential and so-called Donnan potential which
results from the development of Helmholtz electric double layers. It
causes the incoming ions to enter into the membrane and thereby aids
in overall ion rejection by the membrane (Ong et al., 2004; Rizzo et al.,
2019). Furthermore, Donnan potential plays a very important role in
removal of charged organic solutes and salts through the membrane.
The positively charged compounds have been reported to have a less
Fig. 2. Contaminant removal using high pressure membranes (Verliefde, 2008).
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Fig. 3. PSB technology (Adapted from Cao et al., 2019).
element present in wastewater (Nakajima et al., 1997). PSB can directly
absorb carbon dioxide from the air for biomass production, thus acting
as a carbon sink (Swingley et al., 2007). These benefits further enhance
the application prospects of PSB technology for the wastewater treatment process, value-added substance production as well as reducing the
carbon concentrations in the environment.
biological resources and can be considered an economical replacement
for traditional agriculture and livestock husbandry (Matassa et al.,
2015). The protein content in photosynthetic bacteria can be as high as
90% with the average protein content of 40–60%. The requirement for
feed is met by amino acids, 60% of which are essential amino acids
(Saejung and Thammaratana, 2016).
PSBs have been found to proliferate and assimilate nitrogen and
organic compounds rapidly and efficiently. PSB cells have an abundance of proteins, 5-aminolevulinic acid (5-ALA), coenzyme Q10, carotenoids, polyhydroxyalkanoates (PHA) and bacteriochlorins. These
substances have very high market value as compared to the conventional resources derived from wastes (i.e. biofuels), especially the
market value of 5-ALA and CoQ10 are very high. PSB multiplication
and accumulation of value-added substances takes place during the
process of treating wastewater. The whole procedure of PSB technology
has been illustrated in Fig. 3. Hence, a technology (hereafter referred to
as PSB technology) can be developed which can accomplish the goals of
value-added substance production as well as PSB wastewater treatment.
Using PSB technology, value-added products can be derived from lowgrade material along with the treatment of wastewater which is turned
into clean water. The goal of reduction of carbon emission can also be
achieved with the adoption of PSB, therefore helping in climate change
adaptation. PSBs during the wastewater treatment process use a carbon
source to produce cellular molecules and thus capture the carbon
3.4. Photosynthetic bacteria as photo bioreactors for neutralization of CECs
Photosynthetic bacteria are photosynthetic in nature and hence
require light to survive (Lu et al., 2019a, 2019b, 2019c). Light is very
essential for bio-resources production and for guaranteeing high treatment efficiency during PSB wastewater treatment process (Zhou et al.,
2014b; Qi et al., 2017; Lu et al., 2019a, 2019b, 2019c). To achieve the
desired bio-resource and effluent production, photosynthetic bioreactors (PBRs) are as important as microalgae to realize desirable effluent and bio-resource production. Most of the studies in the recent
past concerning the design of PBRs focused mainly on improving PSB
hydrogen generation (Chen et al., 2011; Zhang et al., 2017a, 2017b).
There has been however no studies on PBRs design aimed at improving
the PSB wastewater treatment process. Designs used in earlier PBRs
studies were mainly based on the principles of wastewater treatment
using bioreactors (Jahren et al., 2002).
Fig. 4. PSB treatment products and application (Lu et al., 2019a, 2019b, 2019c).
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limitations in integrating the biofuel production and wastewater
treatment, which include a high energy input requirement, the low lipid
accumulation in algae cultivation, the associated low nutrient recycle
efficiency and the high growth inhibition effect due to contaminated
post hydrothermal liquefaction of wastewater. There is emerging need
to develop other technologies in order to alleviate the disadvantages of
the environment-enhancing energy technologies. High ash content is
found in wastewater-grown microalgae (Roberts et al., 2013; Chen
et al., 2017). A very little contribution of ash content of biomass has
been reported for the accumulation of bio-crude oil (Cheng et al.,
2017). Mineral compounds are primary components of clay along-with
certain amounts of heavy metal salts. Of late, heavy metals due to their
negative impact on the environment and public health have drawn the
attraction of several researcher workers. A combined environment-enhancing energy paradigm should also have heavy metal issues been
taken into consideration. A novel system aimed at achieving detoxification of heavy metal wastewater, valuable metal recovery, biocrude
oil/ hydrochar production and obtaining valuable bio-products was
proposed (Li et al., 2018). Such integrated system not only aids in removing contaminants related with heavy metal accumulation but also
provides an efficient environment-enhancing energy system. Recovery
of heavy metals from polluted wastewaters is achieved via two pathways: chemical/physical absorption via hydrochar and biological adsorption through microalgae. The presence of oxygen containing functional groups on the hydrochar surface from microalgae provides an
excellent heavy metal removing capability to it (Saber et al., 2018). It
was also reported that modification of hydrochar with KOH enhances
the heavy metal absorption capability by improving the oxygen availability and aromatic groups, like the carboxyl groups (Sun et al., 2015).
The combined system converts the unstable fractions into stable ones
which reduce the environmental risk and improves the bioremediation
efficiency of toxic metals.
3.5. Role of Algae for degradation of CECs
A recently emerged technique called “phycoremediation” for heavy
metals (HMs) removal from wastewater which includes bioremediation
by algal species has become quite appealing (Babu et al., 2013; Oyetibo
et al., 2016; Ahmad, 2016; Poo et al., 2018; Salama et al., 2019). While
comparing bioremediation and phycoremediation for wastewater
treatment, the later has been found to have several advantages over the
former. The advantages of phycoremediation over bioremediation include (1) round the year use of algal biomass (Darda et al., 2019) (2)
cost effective (Kotrba, 2011; Salama et al., 2019) (3) application of
algal biomass in discontinuous and continuous regimes (4) no toxic
chemical or sludge production (5) Macro-algal biomass immobilization
is not essential (6) High HMs removal efficiency and uptake capacity
(Ajayan et al., 2011) (7) No oxygen or nutrient supply needed for dead
biomass (8) appropriate for aerobic and anaerobic effluent treatment
units (Salama et al., 2019) (9) algal biomass synthesis not needed (10)
algal biomass application possible for higher metal concentration in
wastewater as compared with membrane processes (Brinza et al., 2007;
Salama et al., 2019) (11) Regeneration and reuse of biomass in various
adsorption/desorption series (12) HMs sequestration with the potential
use of phytoplankton from aqueous media has been reported by several
studies (Jan and Parray, 2016; Lahiri et al., 2017; Salama et al., 2019).
Bio-sorption and bio-accumulation mechanisms are involved in the
removal of HM ions from wastewater with microalgae Table 6. Biosorption occurs in both dead and living cells and is considered as an
independent metabolic process (Fig. 5). In this process, micro-precipitation, ion exchange, chelation, and complexation result in the attachment of HM ions to the functional groups (Kumar et al., 2015; Park
et al., 2016). Several studies have suggested that algal cell wall components such as fucoidan and alginate having key functional groups are
responsible for biosorption of HM ions (Anastopoulos and Kyzas, 2015,
b; Zeraatkar et al., 2016; Salama et al., 2019). The ions such as Ca2+,
Na+, and K+ on the cell surface of algae are exchanged with the HM
ions present in the wastewater through ion-exchange (Salama et al.,
2019). Metal regeneration potential and metal selectivity are two important factors determining the viability of this process for HM removal
from wastewater. The binding of HM ions to the cell surface takes place
through physicochemical interactions which results in low selectivity in
the biosorption process. However, chemical modification of the biomass, such as oxidation by potassium permanganate or cross-linking
with epichlorohydrin can be used to increase selectivity (Luo et al.,
2006). Fig. 5 and Fig. 6 represent the bioaccumulation and biosorption
processes for HM ions removal (Salama et al., 2019). The cells have
numerous intracellular sites for sequestration and binding of metal ions
to its surface, and the uptake of metal ions involves a variety of
transporters.
Production of biofuels and valuable bio-products from biowaste is a
primary aim of the environment improving energy paradigm besides
maximizing environmental sustainability and achieving nutrient recovery simultaneously (Zhou et al., 2013). However, there are several
3.6. Micro-plastics detection in WWTPs
The microfibers present in domestic greywater usually end up in
terrestrial and aquatic environments, if left untreated (Browne et al.,
2011). The various sources of microfibers in WWTPs include cleaning
agents, air blasting media containing granulated polyethylene (PE),
polystyrene (PS) and polypropylene (PP) particles. It has been reported
that for the manufacture of commercial PCPs around 270 t of microbeads were used in the United States (Gouin et al., 2011). The size of
microbeads in facial scrubs manufactured in the United States are in the
range between 8 and 10 g per 100 mL, which ultimately are discharged
into wastewater drainage systems at a rate of 15.2 mg/ person/ day
(Chang, 2015; Kalcikova et al., 2017). Around 4500 to 94,500 PE microbeads are expected to be released from a single wash with a single
application of exfoliant scrubs (Napper et al., 2015). It has been reported that around up to 4000 PE fragments are present in a single
application of toothpaste (1.6 g) (Carr et al., 2016, b). These particles
can easily pass the initial screening in WWTPs given their sizes (Mason
Table 6
Removal efficiency of HMs by Algae.
Microalgae strain
Media
Reactor type
Metal
Removal efficiency
%
Mechanism
References
Spirulinaplatensis
“Wastewater”
Batch
Cu2+
91
“Biosorption”
Pterocladia-capillacea
Chlamydomonas reinhardtii
“Wastewater”
“Wastewater”
Batch
Batch
3+
Cr
La
20–100
30–100
Spirulinaplatensis
Chlorella sp.
Cystoseirastricta
Chitosan algal Biomass
“Wastewater”
“Wastewater”
“Aqueous solutions”
“Microbeads”
Batch
Batch
Batch
Batch
Ca2+
Ca2+
Pb2+
Cd2+
98
56
10
37
“Sorption”
“Adsorption/
Desorption”
“Adsorption”
“Biosorption”
“Biosorption”
“Adsorbent”
“Anastopoulos and Kyzas, 2015, b; Salama et al.,
2019”
“El Nemr et al., 2015; Salama et al., 2019”
“Birungi and Chirwa, 2014; Salama et al., 2019”
8
“Al-Homaidan et al., 2015; Salama et al., 2019”
“Raikova et al., 2016; Salama et al., 2019”
“Iddou et al., 2011; Salama et al., 2019”
“Sargın et al., 2016; Salama et al., 2019”
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Fig. 5. Binding groups (OH−, COO−, PO43−, NO3−, SH−, RO−, RS− and RNH2−) stimulating metal ion biosorption uptake, intracellular accumulation of metal and
surface binding by a living algae cell as shown through schematic representation (Salama et al., 2019).
the quantification of MPs is quite challenging. The particle count estimates are usually compromised while attempting to separate the MPs
without causing the breakdown of the particles. The organic matter
removal and the subsequent underestimation of the plastic particle
must be considered while estimating the mass of MPs in sludge and
wastewater products. Several studies have attempted to extract MPs
using different methods such as density-based solutions and application
of bleach using small quantities of sewage sludge (Carr et al., 2016, b;
Murphy et al., 2016, b; Ziajahromi et al., 2017). The mass and the high
concentration of organic matter in wastewater especially in case of
larger sample volumes affect the identification and quantification of MP
particles (Carr et al., 2016, b; Murphy et al., 2016, b; Ziajahromi et al.,
2017).
et al., 2016). The MPs particles released in biosolids or effluent provides
an entrance into terrestrial and aquatic systems (Talvitie et al., 2017a,
b). Microplastics detection in WWTPs is a three-step process that involves collecting the samples, pretreating the samples and finally
quantifying the microplastics (Carr et al., 2016, b). The methods,
however, adopted in each step are not standardized yet and may vary
according to sample characteristics (i.e. whether microplastics are
present in sewage sludge or wastewater). The dimensions of final
analysis outputs may also vary depending upon the technique adopted
for microplastic identification. The fate and interaction of microplastics
in wastewater is as shown in Figs. 7 and 8. In the influent of WWTPs, a
substantial variation in the concentration of microplastics ranging from
103 to as high as 108 microplastic particle/ m3 has been reported (Carr
et al., 2016, b; Murphy et al., 2016, b; Hidayaturrahman and Lee,
2019). However high removal rates of 80–95% has been reported for
microplastic removal from wastewater in WWTPs (Magnusson and
Wahlberg, 2014; Carr et al., 2016, b; Leslie et al., 2017). Despite such
high microplastic removal efficiencies, the number of discharged microplastics in effluent could still be very significant.
The factors such as the capacity, level, region, location, as well as
urban waste, season rainfall, etc., of WWTPs define the amount of unretained microplastics from WWTPs (Conley et al., 2019). While taking
seasonal variation into consideration, the WWTPs must operate at full
capacities due to increased seasonal populations in summer, which may
reduce the microplastic load retention capacities. There is a requirement of sampling from different seasons and areas for calculation of
total load from WWTPs to a specific marine area. It, therefore, becomes
imperative to assess the spatiotemporal variation in microplastic composition. There is a significant variation in the methods adopted for
detection, sampling, quantification, fate, and transport of MPs in studies related to the wastewater treatment processes (Sun et al. 2019).
This results in several uncertainties and a need for the adoption of a
standard approach (Michielssen et al., 2016; Ziajahromi et al., 2017).
While a developing practical management approach for wastewater
management, there are significant factors including the particle characteristics such as the origin, polymer type, shape and size of the MPs
both primary and secondary that must be taken into consideration. In
organic-rich environmental samples such as wastewater, sludge, etc.,
3.7. Treatment technologies for the removal of microplastics from
wastewater
Melding and degradation of MPs may occur as a result of treatment
with nitric acid (HNO3) and hydrochloric acid (HCl) treatment
(Claessens et al., 2013; Cole et al., 2014). Degradation of polyester
through the saponification of ester linkages may result due to the application of alkaline solutions such as sodium hydroxide (NaOH) (Dave
et al., 1987). Followed by treatment with 10% potassium hydroxide, a
16% reduction in the weight of polycarbonate (PC) was reported for
biological and sludge samples (Dehaut et al., 2016; Hurley et al., 2018).
Without compromising the integrity of plastic materials, the enzymes
have been found to be successful in digesting biogenic material in small
volumes of samples. For instance, 0.2 g (d.w) marine plankton samples
were applied with a 500 μg/mL concentration of proteinase-K for determining MP ingestion and incidence rates in seawater. Although the
effectiveness of this technique was > 97% for removal of biological
matter, applying this technique for large samples with a high content of
organic matter such as wastewater would turn out to be vastly expensive (Cole et al., 2014). A successful application of enzymatic oxidative procedure (cellulase, protease, and lipase) has been reported for
treating wastewater, however, the total digestion time reported was
more than 13 days (Mintenig et al., 2017a, b).
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Fig. 6. A). Phycoremediation approaches for HMs removal B). toxic mechanisms and effect on algal cell by HMs (Salama et al., 2019).
4. Advanced methods of wastewater treatment for CEC removal
et al., 2016) are employed for hydrogel synthesis. Hydrogels are sensitive to both chemical stimuli such as ionic strength (Zhu et al., 2017),
chemical agents (Yoshida and Okano, 2010) and pH (Rogina et al.,
2017). These are also sensitive to physical stimulus such as magnetic
field (Rao et al., 2018), shear rate (Liu et al., 2017; Xu et al., 2017),
temperature (Wang et al., 2017a, b), light (Tomatsu et al., 2011) and
electric field (Zhang et al., 2015a, b). Polysaccharides such as cellulose,
chitosan, hyaluronic acid, and alginate have gained a lot of popularity
in the recent past due to their certain special properties such as being
cheap, biodegradable, biocompatible and non-toxic (Qi et al., 2018).
The most commonly used polymer for hydrogel synthesis is Chitosan
(CS), which is produced from chitin via N-deacetylation. The chitin is
4.1. Chitosan-based hydrogels application for CECs removal
Wastewater treatment with the application of hydrogels is a topic of
growing interest and various studies are underway in this field.
Hydrogels are porous, 3-dimensional and flexible networks and the
presence of hydrophilic groups such as amide, hydroxyl and carboxyl
cause them to swell in water and other biological fluids (Hoffman,
2012). A range of synthetic methods such as phase separation (Omidian
et al., 2005), microemulsion formation (Ghayempour and Montazer,
2018), porogenation (Badiger et al., 1993) and freeze-drying (Butylina
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Fig. 7. Schematic diagram of Karaduvar WWTP a) tertiary treatment), and Silifke & Tarsus WWTPs b) secondary treatment)(Akarsu et al., 2020).
composed of randomly distributed N-acetyl-ɒ-glucosamine and β-(1 →
4)-ɒ-glucosamine units and is a constituent of the exoskeletons of insects, cell walls of algae, fungi, and crustacean (Abdul Khalil et al.,
2016; Shi et al., 2016). After cellulose, chitin is the second most
abundant biopolymer existing in nature (Hamed et al., 2016). There are
some drawbacks of chitosan hydrogels such as being less stable in
thermal, acidic medium besides having low mechanical strength. Physical and chemical networking methods have been proposed to overcome these disadvantages (Salehi et al., 2016). Chitosan hydrogels in
the physical networking methods are reversibly stabilized via hydrogen
bonding, molecular entanglements and ionic interactions (Hennink and
van Nostrum, 2012). Ionic networking is the most commonly used
physical networking method which involves the preparation of chitosan
hydrogels utilizing anionic cross-linkers like sulfosuccinic acid, sodium
citrate, and sodium tri-polyphosphate (Yadollahi et al., 2016). The
cross-linked hydrogels, however, have higher stability compared to
chitosan hydrogels due to the former being stabilized by covalent bonds
(Jóźwiak et al., 2017). Chemical networking can be termed as a chemical reaction resulting in the development of covalent bonds of irreversible nature between polymer chains and the cross-linker. Some of
the most popular chemical networking agents proposed for chitosan
hydrogel synthesis are epichlorohydrin (ECH) (Jóźwiak et al., 2017),
ethylene glycol diglycidyl ether (EGDGE), formaldehyde (Sadeghi et al.,
2016), glutaraldehyde (GLA) (Gonçalves et al., 2017), N,N-methylene
bis-acrylamide (MBA) (Milosavljević et al., 2011) and genipin (Delmar
and Bianco-Peled, 2016). Care must be taken in biomedical applications
of these materials and hence should be removed from the gels due to
their toxic nature (Hennink and van Nostrum, 2002). Most of the studies on hydrogels are based on investigating the removal of heavy metal
ions and dyes. However, recently hydrogels have been adopted for removing emergent pollutants which include industrial chemicals, pharmaceuticals, pesticides and wood preservatives (Geissen et al., 2015). In
fact, chitosan hydrogels on the basis of recent studies have been found
to exhibit high potential for emergent pollutant removal. Emergent
pollutants can be dangerous to ecosystems and human health even at
low concentrations (Basheer, 2018). The excellent intrinsic properties
such as reusability, adsorption capacity, and fast kinetics have brought
chitosan hydrogels in special attention in the area of wastewater
treatment. The only major disadvantages of chitosan hydrogels include
poor mechanical properties and low stability, which however, can be
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Fig. 8. Interaction of microplastics with wastewater (Raju et al., 2018).
combination has been found to give moderate results for mineralization
of RB5 dye contained in wastewater and the aqueous solution. The
combination is very effective for removing colour of dyes (Bilińska
et al., 2019). The two-step process (EC → O3) was also found to have a
clear advantage over one-step (EC + O3) process when analyzed on cost
basis. The filtration cost of two-step process was almost less than half as
that of one-step process. A general recommendation on utilizing ozone
treatment and electro-coagulation coupling treatment for industrial
waste water is by applying EC for a short time followed by ozonation as
a second step treatment. The application of this approach also reduces
the ozonation time to 10 min as compared to 60 min in the case of
ozonation only. A colour removal as efficient as 98% can be achieved
using this technique (Bilińska et al., 2019).
solved by following a better preparation method.
4.2. Ozone treatment and electrocoagulation coupling
An efficient method for the treatment of textile wastewater is ozonation (O3) Fig. 9. The strong oxidant nature of ozone makes it highly
effective for textile wastewater treatment and can decompose even
complex dyes. The efficiency of the ozonation process among available
physical networking methods can be understood from the fact that it is
least affected by textile auxiliaries which form a part of the wastewater
matrix, besides the presence of saline condition has no effect on ozonation process (Mustapha, 2015). The chromophores of azo dyes responsible for colour are oxidized first during the oxidation process and
are usually considered to be the main targets of “ozone attack” (Bilińska
et al., 2017).
Application of electro-coagulation (EC) method for colour removal
is ineffective even after continuing treatment for an extended time.
Ozonation and EC combination have been used as two step (EC → O3)
as well as one-step (EC + O3) treatments as represented in Fig. 10. The
4.3. Nanotechnology for wastewater treatment
Pure water is essential for human consumption and various other
purposes. It is required as a feedstock for food processing, pharmacological, chemical and medical industries. Water purification methods
Fig. 9. Textile wastewater treatment (Bilińska et al., 2019).
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Fig. 10. Experimental setup for the combined EC + O3 process (Adapted from Bilińska et al., 2019).
commonly adopted can be broadly classified into two categories: physical methods and chemical methods. Sedimentation, filtration,
boiling, distillation, desalination, and reverse osmosis are considered as
physical methods of water purification. Flocculation, chlorination, and
coagulation are the most commonly used methods of chemical purification of water (Theron et al., 2008). Another kind of physicochemical process and a widely investigated method for wastewater
treatment is photocatalytic degradation of dissolved water pollutants
under irradiation. The traditional methods such as adsorption on activated carbon, sand filtration, chlorination, coagulation, flocculation,
and sedimentation are not effective for dissolved toxic metal ion and
organic compound removal. There are other techniques available for
removal of dissolved substances such as ozone treatment, incineration,
and irradiation with ultraviolet light, although they are expensive for
trace contaminant removal. Nanotechnology and nanoscience are undergoing a phase of rapid progress over the past few decades. This has
led to researchers exploring advance nanostructured materials with
advantageous and unique properties that could provide sustainable and
efficient solutions to present water-related problems (Lu et al., 2016).
The dimensions of nanomaterials usually range between 1 and 100 nm
(Stark et al., 2015). Nanomaterials have a lesser number of atoms due
to their small size which gives rise to different unique properties in
contrast to bulk materials. These substances have more surface dependent properties and greater surface area to volume ratio due to their
small size. Furthermore, these substances are excellent adsorbents of
different water pollutants due to their superior physicochemical properties (Das et al., 2014). Nanoparticles have been a hot topic of research
and development in the past few decades. These unique particles have
been efficiently applied in different fields such as medicine, biology,
catalytic chemistry, and sensing. The application of nanoparticles in the
field of wastewater treatment has also attracted widespread attention
Fig. 11 and Fig. 12 (Biju, 2014; Dauthal and Mukhopadhyay, 2016).
The small size of the nanomaterials along with large surface area results
in greater adsorption reactivity and higher capacity of the nanoparticles.
CECs removal in WWTPs includes trickling filters, activated sludge,
biofiltration, and soil aquifer treatment, all of which are regarded as
conventional biological treatment processes capable of removing CECs
to various degrees from 88 to 92%. The contaminant removal mechanisms for wastewater treatment including oxidation by O3
/chlorine, sludge adsorption, and anoxic/aerobic/anaerobic biodegradation have different removal efficiencies. The removal efficiencies
may differ based on the factors associated with the wastewater
Fig. 11. Metal oxide nanoparticles and their applications (Singh et al., 2019).
treatment technology used (e.g., rainfall, wastewater effluent dilution,
and temperature) and/or on the physicochemical properties (e.g., hydrophobicity, shape, size, and charge) of the compounds. Due to the
difference in removal efficiencies, the evaluation of these removal
mechanisms has become quite challenging. For instance, a removal rate
of 67% was reported for highly hydrophobic triclosan (log KOW ¼
4.76) which can be attributed to the adsorption of sludge in the WWTP.
The compound, however, may be poorly oxidized during the chlorination treatment. A high removal (71%) rate for the relatively less
hydrophobic sulfamethoxazole (log KOW ¼ 0.89) was reported in the
same study, which can be due to oxidation during chlorination (Kamaly
et al., 2016). Compounds having primary or secondary amine groups
such as trimethoprim, diclofenac, and sulfamethoxazole are highly reactive with chlorine particularly when heterocyclic ring structures are
formed from amines. WWTPs having different wastewater compositions
have been found to have varying removal rate performance for various
types of antibiotics. Using conventional sludge treatment technique, a
study reported a wide removal range of < 5%–20% for trimethoprim
and < 5%–60% for Sulfamethoxazole. The constituents of wastewater
consisting of analgesics such as naproxen (NPX) and ibuprofen (IBP) are
quickly biodegraded. However, due to moderate hydrophobicity of
13
Ecological Engineering 152 (2020) 105882
A.I. Shah, et al.
Fig. 12. Schematic illustration of up conversion nanoparticles-based fluorescence resonance energy transfer (FRET-apta) sensor for rapid and ultrasensitive bacteria
detection (Adapted from Birui et al., 2017).
membrane is called retentate or concentrates (Levesque et al., 2009),
while the portion that gets filtered and passes to the other side of the
membrane is termed as permeate. Organic materials are effectively
removed by the NF process Table 7. However, for the removal of microbial growth, disinfection by chlorine is very important. The water of
optimum quality can be produced with the application of NF membranes characterized by high removal of organic materials and low
inorganic material detention. These types of NF membranes also result
in reduced microbial growth. The energy requirement for water treatment has gone up by 60–150% with the usage of NF systems which
demands the development of a low energy system. Sombekke (1997)
suggested green energy as an alternate and efficient way to reduce
energy requirement. However, the cost of green energy is comparatively higher than conventional energy. Reducing the pressure requirement by providing a more permeable nanofilm may be one way to
reduce the energy requirements of NF. Therefore, the system has to be
designed in a way to have a balance of optimum design and operation at
low energy requirements. The reduction of the operational cost of NF
systems by reducing energy consumption is a major leap towards
commercialization NF systems. Nanotechnology not only provides
methods for purifying the contaminated water but also offers ample
scope for sensing and monitoring water pollutants (Baruah et al., 2019).
Commercial nano-based sensor kits have been widely used to detect the
levels of inorganic, organic and microbial contaminants in water
(Baruah et al., 2019). The nanomaterials hold immense potential for
solving global water problems, but they are equally likely to cause severe environmental toxicity (Baruah et al., 2019). However, to assess
the effect of nanomaterials on health and the environment, methods
these compounds (log KOW ¼ 3.97 for IBP, log KOW ¼ 3.18 for NPX)
certain degree of removal may also occur due to the compounds getting
adsorbed to the activated sludge surface. Iodinated contrast agents,
such as iohexol, iopromide (IPM), and iopamidol are poorly adsorbed,
oxidized and biodegraded. Poor partition coefficients with activated
sludge are a result of the contrast agents being very hydrophilic (log
KOW ¼ 2.10 to 3.05). An earlier study observed that while estimating
the rates of removal of selected 29 CECs during biological treatment
along with chlorination, the removal rate for different compounds
varied considerably: diclofenac, caffeine, NPX, propylparaben, and IBP
were removed significantly (> 90%), whereas the removal rates for
IPM, iopamidol, iohexol, N,N-diethyl meta-toluamide, and acesulfame
were < 5%. The CECs removal efficiency thus depends mainly on the
compound physicochemical properties and the adopted mechanism of
CECs removal (e.g., oxidation by chlorine, adsorption to sludge, biodegradation).
Nanoparticles have been reported to effectively remove contaminants such as organic pollutants, bacteria, inorganic anions and
other emerging pollutants (Méndez et al., 2017; Varjani et al., 2017).
These particles have proven quite effective in terms of their application
in managing pollutants in a variety of wastewater ecosystems including
metal oxide (MNPs), nanocomposites, carbon nanotubes (CNTs), and
zerovalent nanoparticles (Prasad and Thirugnanasanbandham, 2019).
4.4. Nano-filtration process
Nanofiltration (NF) involves the passage of feed through a semipermeable membrane. The portion of the ingoing stream rejected by the
14
Ecological Engineering 152 (2020) 105882
“Nadafi et al. (2011)
“Shirmardi et al. (2013)”
“Zhao et al. (2011)”
“Zhao et al. (2011)”
“Ren et al. (2011)”
“Maliyekkal et al. (2013)”
“Maliyekkal et al. (2013)”
Biochar
Chemical property
Unit
Value
pH
EC
TSS
Ash content
Volatile matter
Fixed carbon
Dimensionless
dS/m
meq/l
%
%
%
8.51
1.32
13.2
44.7
32.1
23.2
–
21.12
–
–
0.0567
–
–
and tools are not so well developed to date (Baruah et al., 2019).
Therefore, it is necessary to analyze the long-term effects of these nanomaterials on humans as well as environment before being adopted on
a large scale (Baruah et al., 2019). It is assured that novel nanomaterials
are going to play vital roles in providing safe drinking water in the near
future, so that the ever-increasing demand for potable water can be met
(Baruah et al., 2019).
based
based
based
based
based
based
based
496
166.67
106.3
68.2
3.00
1200
1100
Biochar is a pyrogenic carbon material rich in carbon, stable, low
cost and can reduce greenhouse gases by acting as a carbon sink
(Creamer and Gao, 2016; Wang et al., 2017a, b) Table 8. Hydrothermal
or thermal conversion of biomass results in the formation of biochar
Fig. 13. Biochar serves as a low-cost adsorbent and its use is widely
recognized for removal of CECs (organic compounds, heavy metals, and
other environmental pollutants) from soil and water (Wang et al.,
2017a, b; Zhang et al., 2017a, b; Wan et al., 2018; Yao et al., 2018;
Zhang et al., 2017a, b). Engineered biochar loaded with metal oxyhydroxides has been found to be very effective in phosphorous removal
from aqueous solutions under different conditions (Wan et al., 2017;
Yao et al., 2013; Zhang et al., 2012, 2013; Zhang and Gao, 2013). The
exhausted phosphorus-laden biochar can act as a slow-release fertilizer
in the soil and can be used in agriculture for improving soil fertility and
hence production (Wan et al., 2017; Yao et al., 2013). The complex
nature of wastewater can however not be reflected from the previous
evaluations since all the studies were carried out in laboratory prepared
solutions under optimal batch sorption conditions. The use of biochar
for adsorption of phosphorus in fixed bed filtration and its performance
as a filter medium in dynamic real wastewater flow conditions is still
undetermined.
Very high volume of wastewater is released from various sources
such as agricultural sectors, processing industries, residence areas, and
commercial buildings. Engineered activated biochar is being used since
few decades for removing wastewater-borne pollutants from industrial
and domestic wastewater through the process of adsorption, where the
pollutants are adsorbed on the adsorption sites of the biochar material
(Mohan et al., 2014; Vyrides et al., 2010; Moreno-Castilla, 2004).
However, activated biochar can last up to a maximum period of 1 year
after which it has to be discarded. The disposal of used activated biochar can lead to secondary environmental pollution. Despite the regeneration of used activated biochar (i.e. regeneration by steam,
thermal, chemical, and biological) for reuse is feasible, the cost of the
regeneration process is often higher than the acquisition of newly activated biochar. Thus, an efficient and cost-effective activated biochar is
needed. Meanwhile, engineered activated biochar production from a
renewable source such as biomass wastes is appealing. The performance
of engineered activated biochar could be chemically modified for
contaminant removal from the wastewater. For example, improved
adsorption capacity to heavy metals has been reported for activated
biochar engineered by polymer matrix (Ghaedi et al., 2008). The
modified activated biochar, when compared to pristine activated biochar, showed higher removal efficiency of cationic dyes (Zhang et al.,
Pesticides
Heavy metals
“Single walled carbon nanotubes”
“Single walled carbon nanotubes”
“Graphene”
“Graphene”
“GrapheneNanosheets”
“Reduced graphene oxide nanosheets”
“Reduced graphene oxide nanosheets”
“Reactive Blue 29(RB29)”
“Acid red 18″
Cd (II)
Co(II)
Ni(II)
Chlorpyrifos
Endosulfan
Dyes
Carbon
Carbon
Carbon
Carbon
Carbon
Carbon
Carbon
Adsorbent material
Contaminant
Nano-material Used
Table 8
Chemical properties of Biochar (Fiaz et al., 2014).
4.5. Efficacy of engineered biochar in CECs removal
Category
Table 7
CEC removal abilities, rate constants, and adsorption capacities of different nanomaterials (Singh et al., 2019).
Adsorption Capacity (mg/g)
Rate constant (k1,h-1)
Reference
A.I. Shah, et al.
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Ecological Engineering 152 (2020) 105882
A.I. Shah, et al.
Fig. 13. Phosphorus recovery using engineered biochar (Adapted from Zheng et al., 2019).
Table 9
Application of biomass-derived engineered activated biochar in catalysis.
Source of AB
Applications
References
“Biomass”
“Bulgarian peach stone”
“Castanea mollissima shell”
“Coconut Shell”
“Coconut Shell”
“Groundnut”
“Olive Stone”
“Palm Kernel Shell”
“Wood sawdust”
“Bamboo”
“Olive stone”
Photocatalytic degradation of Orange G dye
Methanol decomposition
Propane dehydrogenation
Guaiacol hydrodeoxygenation
Ozone decomposition
Reduction of organic dye
Bio-oil deoxygenation
Methane dry reforming
Isobutene dimerization
Oleic acid asterification
Methanol dehydration
“Vinayagam et al.(2018)”
“Tsoncheva et al. (2018)”
“Hu et al. (2018a, b)”
“Cai et al. (2017)”
“Zhang et al. (2011)”
“Vandarkuzhali et al.(2018)”
“Cordero-Lanzac et al.(2017)”
“Liew et al. (2018a, b)”
“Malaika et al.(2018)”
“Niu et al.(2018)”
“Moreno-Castilla et al.(2001)”
Table 10
Recent progress in wastewater treatment by biomass-derived engineered activated biochar (AB).
Source of AB
“Banana peel”
“Coconut shell”
“Coffee grounds”
“Date press cake”
“Laundry sewage sludge”
“Lignocellulosic wastes”
“Oil palm mesocarp fibre”
“Palm Kernel Shell”
“Palm kernel shell”
“Palm shell”
“Pecan nutshell”
“Pistachio wood waste”
“Plum stones”
“Salvadora persica”
Applications
References
2+
2+
2+
Adsorption of Cu , Ni , Pb
Removal of COD and polyphenol
Adsorption of methyl orange
Adsorption of Cr3+
Adsorption of dye (Remazol Brilliant Blue R)
Adsorption of Cd 2+ and Ni2+
Treatment of palm oil mill effluent
Treatment of palm oil mill effluent
Removal of Herbicides
Removal of dye (Procion Red MX-5B)
Adsorption of Zn2+, Cd2+, Ni2+, Cu2+
Removal of Hg2+
Adsorption of Cu2+ and Pb2+
Adsorption of Cu2+, Pb2+, Ni2+
“Van Thuan et al.(2017)”
“Ge et al.(2018)”
“Rattanapan et al.(2017)”
“Norouzi et al.(2018)”
“Silva et al.(2016)”
“Nayak et al.(2017)”
“Ibrahim et al. (2017)”
“Liew et al. (2018a, b)”
“Lam et al. (2018)”
“Hariani et al. (2018)”
“Aguayo-Villarreal et al. (2017)”
“Sajjadi et al. (2018)”
“Parlayıcı and Pehlivan (2017)”
“Wahid et al. (2017)”
activated biochar applications in wastewater treatment derived from
different biomass are illustrated in Table 9. The modification of the
surface of engineered activated biochar by chemicals (e.g. phosphoric
acid, sulfuric acid) can be achieved for specific catalytic reactions by
introducing certain functional groups. Some researchers used xerogel
mesoporous carbon, olive stone derived activated biochar and
2015a, b). A higher adsorption capacity for phosphate was reported for
the hydroxyl‑iron‑lanthanum modified activated biochar (Liu et al.,
2013). It can be concluded that modified activated biochar is very effective in wastewater treatment. However, before commercialization
the production costs must be taken into account; otherwise, these
products will be unable to progress from the research stage. Engineered
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Ecological Engineering 152 (2020) 105882
A.I. Shah, et al.
commercial activated biochar for methoxylation of α-pinene catalyzation into α-terpinyl methyl ether (Matos et al., 2014). Olive stone activated biochar treated with phosphoric acid due to the addition of
phosphate groups exhibited a higher catalytic activity as compared to
the rest of the materials used. Cellobiose also referred to as glucose
dimer was hydrolyzed using activated biochar derived from sulfonated
palm kernel and then used as an acid catalyst (Fraga et al., 2016).
Catalyzation of hydrolysis process cellobiose was achieved using the
activated biochar by introducing sulfonated groups (-SO3H) on its
surface and treating it with concentrated sulfuric acid (Kobayashi et al.,
2011). The activated biochar thus has been found to exhibit a comparatively higher catalytic activity than catalysts available commercially. Hence it can be concluded that a novel research direction in
wastewater treatment can be provided by using biomass-derived engineered activated biochar for aiding catalyzation process. The application of biomass-derived engineered activated biochar for aiding catalyzation process has been illustrated in Table 10.
interaction of ENPs with plants has however been discussed in numerous papers after review by Auvinen et al. (2017). A range of effects
of ENPs on the growth of food plants have been observed with some
indicating inhibition, enhancement, or no effect (Ma et al., 2010).
However, no significant negative impact on biomass production in
wetland plants under exposure to ENPs has been observed (Jacob et al.,
2013). Some researchers have indicated a positive stimulating effect on
plant growth by ENPs although most of the ENPs are in essential plant
nutrients.Enhanced biological phosphorus removal (EBPR) (Chen et al.,
2013; Chen et al., 2012), membrane bioreactor (MBR) (Yuan et al.,
2015; Zhang et al., 2014), and sequencing batch reactor (SBR) (Alito
and Gunsch, 2014; Qiu et al., 2016), are some of the recently reported
silver nanoparticles (AgNPs) effects in the biological treatment processes. Qiu et al. (2016) reported distinct changes in structures of microbial communities as a result of the addition of 1.0 and 5.0 mg/L of
AgNPs to the influent without significantly affecting the performance of
the SBR system. An addition of 0.2 and 2 mg/L AgNPs spikes reportedly
showed a decrease of more than 30% for NH4+-N and COD removal,
however, the SBRs recovery for 24 h was ~95% (Alito et al., 2014). In a
biological treatment process, no significant adverse effect on EBPR removal was observed even by increasing AgNPs concentration from 1 to
5 mg/L both in chronic and acute experiments (Chen et al., 2013). Yuan
et al. (2015) reported that anaerobic–anoxic–oxic membrane bioreactor
(A2O-MBR) microbiome experienced minor effects due to a 285-day
exposure of AgNPs (0.1, 1, and 5 mg/L). The microbiome structure of
A2O-MBR was influenced by the addition of AgNPs., however, with
persistent exposure the structure showed stability. Zhang et al. (2014)
reported that no impact on the water quality of the effluents was observed due to a continuous 60-day loading of AgNPs (0.1 mg/L) and the
structure of nitrifying bacteria community was comparatively stable in
a MBR system. Under a 120 day, AgNPs exposure a decreased removal
efficiency was reported for phosphorus and nitrogen while evaluating
the performance of constructed wetlands (Huang et al., 2017).No remarkable inhibitory effects on nitrogen removal efficiency of CWs was
reported for short-term exposure to TiO2 NPs (Yang et al., 2020).
However, disruption of microbial physiological metabolism was observed due to long-term exposure to TiO2 NPs which resulted in deposition and penetration of NPs into cell membrane. Reduction in nitrogen conversion efficiency of CWs fed with TiO2 NPs could further be
explained as a result of decreased abundance of gene encoding enzymes
and functional genes associated with nitrogen transformation as well as
NADH and ATP production processes. Substrate adsorption can be
considered as a primary removal mechanism of AgNPs in CWs (Bao
4.6. Engineered nanoparticlesfor removal of CECs inconstructedwetlands
A novel and emerging natural technology for wastewater treatment
has been found in the form of constructed wetlands (CWs) (Auvinen
et al., 2016; Hu et al., 2018a, b; Huang et al., 2017). CWs as an extensively eco-technology can treat a wide variety of contaminants in the
wastewater, including the metal nanoparticles.CWs especially in rural
areas function as ecosystem stabilizers and serve as means for decentralized wastewater treatment. They have a low-maintenance, long life
and are robust in operation (Wu et al., 2014; Wu et al., 2018). The use
of CWs for domestic wastewater treatment is well established. A variety
of industrial effluents containing various engineered nanoparticles
(ENPs) can also be treated in wetlands constructed artificially (Wu
et al., 2015a, b). Auvinen et al. (2017) discussed the ENP transformation process and the environmental factors affecting these processes in
the constructed wetlands. The mobility of ENPs can significantly decrease due to aggregation and subsequent sedimentation with organic
compounds suspended in the wetlands and thereby restricting their
distribution in the initial segment of wetlands (Fig. 14). Other processes
which also have been shown to be important include sulfidation (defined as the reaction of ENPs with sulfide) and adsorption to the substrate (Auvinen et al., 2017). The evaluation of the interaction between
wetland plants and the ENPs can, however, be more complex. Plants
may absorb certain ENPs from the wetlands which may finally end up
being accumulated in root surface thus being toxic to plants. The
Fig. 14. ENPs flow in horizontal sub-surface artificial wetlands (Auvinen et al.,2017).
17
Ecological Engineering 152 (2020) 105882
A.I. Shah, et al.
et al., 2019).CWs can be concluded from the overall results as a tertiary
or independent systems and economical alternatives for treatment of
Engineered Nano Material (ENMs) from wastewaters.In addition, it is
impossible to arrive at the conclusion as to whether AgNPs should have
a negative effect on pollutant removal since most of these research
works are based on short dated exposure.
sulfonate) with plants growing in artificial wetlands and reported a
surfactant removal rate of over 95%. Artificial wetland technology,
based on these factors can be concluded to be an effective alternative
towards wastewater treatment (Ortiz et al., 2014).
4.7. CEC (detergent) removal and wastewater treatment with artificial
wetlands (biofilters)
Different advanced wastewater treatment methods were investigated and compared for their technical and economic feasibility on
pilot and full-scale in the last decade (Hollender et al., 2009; Abegglen
and Siegrist, 2012; Margot et al., 2013; Prieto-Rodríguez et al., 2013a,
2013b; Dela Cruz et al., 2013). The methods of filtration using tight
membranes were generally found to be cost-effective as in RO and NF
techniques. The main costs in all the available wastewater treatment
processes are the energy cost, excluding solar-powered or activated
carbon treatment. Solar-powered AOPs for CECs abatement from urban
wastewater may well compete with the available AOP treatment techniques with further advancement, especially in areas with high yearly
average solar irradiation (between latitude 40°N and 40°S). So far in
relation to cost efficiency, advanced urban wastewater treatment for
CECs abatement by ozonation and adsorption (with both GAC and PAC)
processes have been found to be highly successful. For upgrading of
municipal WWTPs, the state should also fund these feasibility studies
for CECs removal steps (Antakyali, 2016).
Worldwide wastewater discharges are increasing at tremendous
rates. In 2015 alone, China released 73.53 billion tons of wastewater
(Judd, 2017). These large volumes of wastewater discharges can be an
indispensable source of nutrients for PSBs. The establishment of fullscale PSB WWTP is possible after evaluating the economics of PSB
WWTPs. Due to the absence of industrial PSB WWTP, a rough qualitative analysis could be made while considering the sum of the operating expenditures (OPEX) and capital expenditure (CAPEX) viewed as
integral costs of a WWTP (Judd, 2017). While comparing with the
conventional activated sludge, it was found that PSB technology has
lower OPEX and a higher CAPEX. A lower OPEX can because the (1)
saving of disposal cost due to the absence of excess sludge production
(2) extraction of value-added substance from PSBs is attractive (3) PSB
WWTPs do not require aeration, thereby saving electricity costs. The
reasons for higher CAPEX can be that (1) PSB WWTPs requires additional facilities, like extraction column and dryer (2) when compared to
the traditional aeration tank, PSB wastewater treatment with photobioreactors are more complex and less developed than traditional
aeration tank. With this analysis that lower OPEX and higher CAPEX,
PSB WWTPs are considered more economical than traditional technologies when the service life exceeds a certain threshold. The integral
costs will continue to reduce with the wide-scale adoption of PSB
technology, thereby promising to be a better economic technology.The
studies investigate the possible designs and treatment steps for the
processes based on individual treatment plant and the contaminants.For
constituting the state-of-the-art CECs removal GAC reactors, PAC
treatment are assessed comparatively. Monetary costs, besides the
technical feasibility play an important role in these processes, and the
best option can be arrived at only after proper assessment.
5. Economic feasibility and cost evaluation
Detergents are made of bleaches, surfactants, adjuvants and different additives (Romero, 2006). They involve the breaking up of the
stain with the hydrophobic head dissolving in greases and oils and
hydrophilic head dissolving in the water. The ions which are hydrophobic in nature and hence can remove grease and stains. The problems
related to wastewater containing CECs (greases and detergents) are
crucial in the areas where priority is to have proper sewage coverage
and portable water (Lahera, 2010; Zurita et al., 2011). The problems
are aggravated due to the sheer volume of wastewater released into
water bodies. The ordinary wastewater treatment plants (WWTPs) in
most of the cases have been found ineffective for treating huge quantities of wastewaters (Table 11) (Zurita et al., 2011). The infrastructure
for wastewater treatment is also insufficient specifically in rural and
remote areas. Artificial wetlands are composed of aquatic plants, microorganisms, and a substrate and thus offer a more effective and longlasting solution towards the degradation of CECs from wastewater.
Wastewater flows into or under the surface layer of the support. The
plants in the artificial wetlands incorporate air for microorganism
growth, which disintegrates CECs in wastewater flows under or into the
top layer of the support (Shibao et al., 2015). Channels made of substrates (fibers, gravels) constitute artificial wetlands. These substrate
materials support low-level water flow and rooted vegetation (Reed
et al., 1988). Intensive bio-treatment systems constructed from emergent macrophytes function as intensive biological treatment systems
and promote improved sedimentation by reducing the resuspension of
particles on the surface of the sediment and mixing of the column
(Vymazal, 2013). The filtration and degradation process occurring
within artificial wetlands easily removes CECs. The oxygen requirement
is fulfilled by the photosynthetic activity of the plants or by oxygen
supplied to the water column by diffusion through the air-water interface (Kadlec et al., 2000). The macrophytes in these artificial wetlands also restrict the growth of algae by covering the entire water
surface, thereby blocking the light penetration into the water column
(Vymazal, 2014). The blocking of light eventually prevents eutrophication. The complex chemical, physical and biological processes
occurring in parallel between the microorganisms, plants, and substrates in a wetland determine the effectiveness of the artificial wetland
for use as a wastewater treatment system (Vymazal, 2014). A worldwide research based on the removal of contaminants such as nitrogen,
biochemical oxygen demand (BOD), phosphates and some metals from
natural and artificial wetlands was described by Thomas et al. (2017).
There are very few studies aimed at evaluating the applicability of
wetlands in the removal of surfactants from the wastewater. Thomas
et al., (2017) investigated the removal of LAS (Linear alkylbenzene
6. Conclusions
Table 11
Effects of constructed wetlands on wastewater treatment (Jianbo et al., 2008).
Indicator
Before treatment (mg/
L)
After treatment(mg/
L)
Removal rate (%)
COD
TP
TN
DO
Transparency
270
8.86
12.72
< d1
9
96
6.82
9.95
2.14
22.25
64.44
23.02
21.78
–
–
Wastewaters may be said to be untapped resources and with the
shortage of water and energy resources across the world, the focus has
shifted towards deriving valuable products from wastewaters. The
treatment of wastewater and the generation of clean water, however,
remains a primary focus. A high degree of water treatment and derivation of valuable resources can be achieved simultaneously through
PSB (photosynthetic bacteria) wastewater bioconversion. The technology can be very beneficial to cope up with water and food shortage
by providing the water and bio-resources for further utilization in food
production. The negative impact of wastewater on the environment can
It is too low to be detected; dl: below detection limits.
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Ecological Engineering 152 (2020) 105882
A.I. Shah, et al.
imperative to arrive at technologies that are environment friendly,
focus on long term wastewater management, and require minimal
maintenance expenditures.
be easily avoided. The industrial application promotion and development of the technology requires further intensive study. Urban
Wastewater treatment with advance treatment methods have been
adopted at various scales to overcome water scarcity in urban areas and
to provide water for drinking, agriculture and other activities.
Wastewater is being considered as a water resource and could be an
effective option to balance the water demand especially in arid and
semiarid regions of the world. A centralized management system would
be a need of the hour in the future for water conveyance and distribution using both conventionally available sources as well as unconventional sources such as wastewater.
CECs and their removal have become a growing concern which is
being investigated in different wastewater treatment processes. The
partial removal of CECs, such as EDCs, pharmaceutical, and personal
care products (PPCPs), and heavy metals by conventional WTPs (flocculation, sedimentation, filtration, and coagulation) and WWTPs (biological processes) have been discussed in this review. However, for the
removal of various CECs certain membranes such as UF, FO and RO
have been found to perform better with varying degrees of removal
depending on the membrane properties, membrane operating conditions, water quality, and compound properties. The overall CEC removal trends observed can be summed up as (i) less hydrophobic, more
polar, and less volatile hydrophobic organic CECs were found to have
less retention when compared with more hydrophobic, less polar and
more volatile organic CECs, thereby showing that CECs retention process is purely dependent on hydrophobic adsorption in UF membrane
(ii) With RO membranes a higher removal of CECs was achieved at the
expense of flux reduction and inductively coupled plasma mass spectrometry (ICP), however, both RO and forward osmosis (FO) were
found quite successful in CECs removal; (iii) while, RO and FO had a
significant metal rejection rate (> 95%) irrespective of operating conditions and water quality, however, at alkaline and neutral conditions
better metal rejection was found with UF membrane; (iv) Before RO and
FO, UF technique can be used as a pre-treatment step or in a hybrid
process such as powdered AC-UF as a separation stage. Many studies
focused on synthetic solutions or examined only a single ion at limited
solution pH and conductivity ranges. However, they were still limited
to a few membranes (e.g. FO, RO, or UF). Thus, it is necessary to investigate the rejection mechanisms for FO, RO, and UF membranes in
the presence of co- and counterions in natural source waters through a
systematic rejection assessment for various CECs. Even the best RO
concentrate treatment options produce hazardous products containing
high concentrations of CECs. In order to determine appropriate approaches and technologies to treat membrane concentrates containing
high concentrations of CECs a comprehensive study is needed. Besides,
more studies on larger-scale processes are needed because, as of now
too little information is currently known about FO, RO, and UF membrane processes for full-scale implementation.
With wastewater treatment, sanitation and economy are gaining
pace, wastewater treatment based on several advanced and new approaches have become new research subjects in the field of water
management. Wastewater treatment and its reuse are gaining momentum and several new and advance methods can be mentioned
which include approaches based on risk management and adoption of
‘decision support system and best available technologies’ concept. The
expenditure on energy requirements for wastewater treatment and
reuse is considered to be neutral given the advancement in the techniques and mitigation from potential negative impacts on the ecosystem. Although other options such as industrial or urban use are
gaining significance in terms of reuse and reclamation, usage of reclaimed water for agricultural purposes remains the most sought option. Increasing the levels of water table via recharge of groundwater
using treated wastewater remain other options that need to be explored.
The properties of wastewater may vary with its sources, region and
season. Given the numerous available methods and new technologies
coming into frame in the field of wastewater treatment, it has become
Interest statement
The authors whose names are listed certify that they have NO affiliations with or involvement in any organization or entity with any
financial interest (such as honoraria; educational grants; participation
in speakers bureaus; membership, employment, consultancies, stock
ownership, or other equity interest; and expert testimony or patentlicensing arrangements), or non-financial interest (such as personal or
professional relationships, affiliations, knowledge or beliefs) in the
subject matter or materials discussed in this manuscript.
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