Course 2 Unit 4
Introduction to anaerobic treatment
technologies
Lecturer: Mariska Ronteltap
m.ronteltap@unesco-ihe.org
In this file:
Part A – Fundamentals of anaerobic digestion
Part B – Anaerobic treatment technologies relevant
for ecosan concept
In a separate file:
Part C – Examples and case studies
This unit deals with which part of the sanitation
system?
Crop grown with ecosan products as fertiliser (closing the loop)
Part A
Household
toilet
Household
toilets, but can
also include
showers, bath
tubs, sinks
Part B
Part C
Part D
Treatment
& storage
Collection &
transport
Urine, faeces,
greywater
transport
(road-based
vehicles in
combination
with pipes)
Part E
Re-use in
Agriculture
Transport
Treatment
for faeces
and
greywater,
storage for
urine
Transport of
sanitised urine and
faeces by truck;
treated greywater
transport by pipes
Sale of fertiliser
(sanitised human
excreta); irrigation
with treated
greywater
 Anaerobic digestion can be used to treat faeces, greywater and other
organic waste with the aim to produce biogas and a fertiliser
 A certain degree of pathogen kill can be achieved through raised
temperatures and/or extended digestion times in the anaerobic digester
from Course 1 Unit 3
Reminder: Overview of ecosan technology components (where
does anaerobic treatment come in?)
organic solid waste
faeces
urine
collection
Vacuum toilets and vacuum sewerage
(see Course 2)
(see Course 3)
-
Rainwater harvesting
Gravity Sewerage (conv. or small-bore, central or decentral)
-
Greywater
separation
Dehydration Toilet
Composting toilet
treatment
rainwater
Waterless urinals, UD
toilets
UD toilets
utilisation
greywater
Constructed wetlands,
Prolonged storage
Anaerobic Digesters
Storage
Urine
processing
ponds, trickling filters,
septic tanks, soil
filters,…
Disinfection
(if required)
Composting
Wastewater treatment (centralised or decentr.)
Soil conditioning with treated
excreta and solid biowaste
Fertilizing with
urine
Reuse: irrigation,
toilet flushing
Reuse of wastewater e.g. in agriculture, aquaculture
Marked in red are those technologies covered in this course unit
Source: based on GTZ-ecosan project Resource Book
(UD = urine diversion)
Reuse: irrigation,
cleaning, toilet
flushing
from Course 1 Unit 3
Reminder: Important treatment technologies
often used as part of ecosan concepts
Process
Technical options
Reason for popularity in ecosan
Composting
Composting plants for secondary
treatment
Composting toilet
Suitable for faecal matter and organic solid waste
treatment
Produces valuable end product (compost)
Low energy demand
Pathogen destruction (if thermophilic)
Anaerobic
treatment
Septic tanks
UASB
Anaerobic ponds
Anaerobic digesters
Suitable for faecal sludge, blackwater, faeces (e.g.
together with manure), organic solid waste
Preserves nitrogen (unlike aerobic wastewater treatment)
Produced biogas for cooking, lighting, heating
“Natural
systems” (lowrate biological
systems)
Constructed wetlands
Aerobic or facultative
ponds/lagoons
Waste stabilisation ponds
Suitable for greywater treatment
Low energy use
Cheap if land available
Can have aesthetic and environmental benefits (e.g.
increased bird life)
High-rate
biological or
physical
systems
Package plants using attached
growth processes
Membrane bioreactor
Trickling filter
Suitable for greywater treatment in urban areas (limited
space)
High quality effluent is produced
Course 2 Unit 4
Course 2 Unit 4
Part A: Fundamentals of anaerobic digestion
Mantopi Lebofa (from NGO TED) lighting the
biogas flame (Lesotho, Dec 2006)
Overview about anaerobic treatment in general
(this is not specific to ecosan)
 Anaerobic treatment works with organic input materials, such as:
- liquid organic material
- solid organic material (provided it is has a water content of ~
50% or more), i.e.
- slurries/sludges
- organic kitchen waste
- greywater together with excreta
 Anaerobic treatment is not so suitable for:
- individual houses, unless animal excreta is available too
(farmers)
 Anaerobic digestion may be a direct alternative to UDD toilets, e.g.
for public toilets, institutions (schools, hospitals, prisons)
- biogas used for lighting, cooking
 The end product (digested material) is not pathogen-free but still fit
for reuse
Reminder: what is “organic”?
– An organic compound is any member of a large class of chemical
compounds whose molecules contain carbon and hydrogen; therefore,
carbides, carbonates, carbon oxides and elementary carbon are not
organic (see below for more on the definition controversy for this
word). The study of organic compounds is termed organic chemistry,
and since it is a vast collection of chemicals (over half of all known
chemical compounds), systems have been devised to classify organic
compounds.
– The name "organic" is a historical name, dating back to 19th century,
when it was believed that organic compounds could only be
synthesised in living organisms through vis vitalis - the "life-force".
The theory that organic compounds were fundamentally different from
those that were "inorganic", that is, not synthesized through a lifeforce, was disproved with the synthesis of urea, an "organic"
compound by definition of its known occurrence only in the urine of
living organisms, from potassium cyanate and ammonium sulfate by
Friedrich Wöhler in the Wöhler synthesis.
(Source: www.wikipedia.org)
Substrates (input materials) on which anaerobic
treatment processes are used in ecosan context
 High-strength greywater (as a pre-treatment step), rule of
thumb: BOD > 400 mg/L
 Blackwater with or without urine (blackwater: faeces, urine,
small amount of water – e.g. from vacuum toilets) – as a pretreatment step
 Human excreta together with animal excreta and greywater,
followed by reuse in agriculture
High-strength greywater (example from
Jordan, see Course 2 Unit 1 Part D)
“Blackwater” from vacuum toilets
in Sneek, the Netherlands (see
also Part C of this presentation)
Course 2 Unit 4
Basic anaerobic digestion (AD) terminology
Term
Description
Anaerobic
Without oxygen
Aerobic
With oxygen, e.g. in activated sludge
plants or in aerobic ponds
Anaerobic digestion /
degradation / treatment
These terms are all used
interchangeably, and mean “breaking
down of organic matter”
Digestate / digester
residue / digested organic
matter
The effluent from a digester; the liquid
product of the anaerobic digestion
process
Biogas
Gas produced by microorganisms in
anaerobic process (typically 66%
methane content)
Biogas digester /
anaerobic digester
A covered vessel (or reactor) in which
anaerobic digestion occurs
Just as an aside: Another note on terminology
In Germany (and perhaps other countries, too) there is currently
still an unwritten convention:
 Plants/processes where the input is mainly agricultural waste
are called biogas plants
 Plants/processes where the input is mainly municipal organic
solid waste (“green waste”) are called fermenters or anaerobic
digestion plants
Basic schematic representation of how dry
solids content is determined in the laboratory
Further drying
of solid residue
(at 105ºC), then
weighing of the
dried mass
Sample to be analysed for total
solids content
Sample is either filtered
(schematic above) or, if it is too
thick for filtering, it is dried (at
105ºC) without filtering
Final result: grams
of dry solids per L
of sample
Convention for the unit of dry solids (d.s.)
The measurement result is commonly expressed as % d.s.
Example: 1% d.s. is equal to 10,000 mg/L of solids in water
 This means that 99% of the sample consists of water
Another example: 100% d.s. = 1,000,000 mg/L = 1 kg/L = no
water in the sample
The total dry solids consist of two parts:
volatile solids and inert solids
Volatile solids (VS)
– Also called “organic solids”
– That fraction of the total
solids which can be burnt
(volatilised) in the muffle
oven at 520°C
– Only the volatile solids can
be broken down by
anaerobic digestion
Inorganic or inert solids (e.g. grit, sand)
Total solids (TS)
– = (organic solids +
inorganic solids)
– Measured after
drying at 105°C
– “Dry solids” is
another word for
total solids
Course 2 Unit 4
Anaerobic digestion process overview
 In the anaerobic digestion process, micro-organisms convert
complex organic matter to biogas, which consists of methane
(CH4) and carbon dioxide (CO2)
 Some organic matter remains even after the digestion step, and
this is called digestate or digester residue or digested organic
matter
 Anaerobic digestion is used to treat high-strength wastewater,
organic solid waste, sewage sludges, blackwater, faecal sludge,
agricultural waste, food industry waste (e.g. breweries, slaughter
houses, dairy), manure,....
 Anaerobic digestion with biogas production also occurs in landfills,
septic tanks, cows’ rumen, natural or constructed wetlands, dams
where vegetation was flooded  all these sites produce methane
gas!
As an aside: significant methane releases from
other human-influenced processes
 Rice production
 Thawing permafrost in Siberia (due to climate change)
 Bio-industry
Remember: Methane is a dangerous (potent)
greenhouse gas
Methane is a greenhouse gas with a global warming potential over
100 years of 23 i.e. when averaged over 100 years each kg of
CH4 warms the earth 23 times as much as the same mass of
CO2
Source: www.wikipedia.org
Some facts about methane
 Methane: CH4
 Methane is the major component of “natural gas”*, about 97% by
volume
 At room temperature and standard pressure, methane is a colorless,
odorless gas (the smell characteristic of natural gas is an artificial
safety measure caused by the addition of an odorant)
 Methane has a boiling point of −162°C at a pressure of one
atmosphere
 As a gas it is flammable** only over a narrow range of concentrations
(5–15%) in air
 Methane has a calorific value of 10 kWh/Nm3 or 35,900 kJ/Nm3
 Hence, biogas with 65% methane has a calorific value 6.5 kWh/m3
(23,300 kJ/m3)


* Natural
gas is a gaseous fossil fuel consisting primarily of methane but
including significant quantities of ethane, butane, propane, carbon dioxide,
nitrogen, helium and hydrogen sulfide. It is found in oil fields and natural gas
fields, and in coal beds.
** Flammability or Inflammability is the ease with which a substance will ignite,
causing fire or combustion. Materials that will ignite at temperatures commonly
encountered are considered flammable. Source: www.wikipedia.org
Anaerobic digestion process schematic
Biogas (methane): “Green energy”
Example: Gas flowrate: 665 Nm3/d *
Organic matter
(energy-rich)
Example:
Liquid flowrate: 10 m3/d
Mass flowrate: 1 ton VS/d
Anaerobic
digester
(biological
reactor)
* Calculated by using
0.95 Nm3/kg VS
destroyed - see next
slide
Digestate (energypoor; can be used as
fertiliser; includes
anaerobic biomass)
Liquid flowrate: 10 m3/d
Mass flowrate: 0.3 ton
VS/d
Nm3 stands for normal cubic metre, meaning a measurement at STP or
standard temperature and pressure (absolute pressure of 100 kPa (1 bar)
and a temperature of 273.15 K (0 °C))
Course 2 Unit 4
Some guidelines for amount of biogas produced
per amount of organic material digested
 Sewage sludge: 0.75 – 1.12 Nm3 per kg of volatile solids
destroyed (typical value: 0.95 Nm3/kg)
 Organic solid waste:
– 0.38 – 0.42 Nm3 per kg of volatile solids added (at a
retention time of 14 days) for single-stage processes
– Up to 0.6 Nm3 per kg of VS added for two-stage
processes (two-stage: a process whereby step 1 & 2 is
separated (in separate reactors) from step 3 & 4 as shown
in slide 24)
Example: Standard design of household
biogas plants in Nepal
Waste (water)
Digester residue
~ 1 million of these in Nepal (in 2006)
Note: At many landfill sites around the world, the biogas produced is now being
captured and used (can be with high-tech or low-tech methods)
Landfill on island of Maui - Source http://atdpweb.soe.berkeley.edu/pix/maui/landfill.jpg
Course 2 Unit 4
Anaerobic digestion (AD) microbiology
fundamentals
 Under anaerobic conditions, organic substances are not aerated
(oxidised), but are fermented (reduced)
(Reduction = assimilation of electrons)
 Energy-rich end products, like organic acids or alcohols are
electron acceptors
 It is quite a “slow” process (low growth rate of methanogens)
compared to aerobic processes  relatively long sludge
retention times are required
 Like all biological processes, it is temperature dependent (higher
conversion rates at higher temperatures)  digesters are
typically heated / insulated or below ground
 The process occurs as a four-step process (see next slide)
4 steps in anaerobic conversion
Remember: this is not a
complete conversion some organic matter will
remain (digestate)
Note that biogas is a mixture – not
only the useful CH4
Depending on the substrate there
24
can be other gases too (slide 27)
Additional explanations on the 4-step process
shown on previous slide
1. Volatile fatty acids (VFAs) are an intermediate product:
– They should not accumulate under normal operation
– VFAs (e.g. acetic acid) accumulate if step 4 is inhibited
 In that case, pH value will drop (e.g. to pH of 4.8) and the
digestion process will stop (no more gas production)
 This is also called a “sour” digester, and is usually very
smelly (a well operating digester produces almost no odours)
Some information on methanogens (they
belong to the group of microorganisms called
archaea)
 Methanogens are archaea that produce methane as a metabolic
byproduct. They are common in wetland, where they are
responsible for marsh gas, and in the guts of animals such as
ruminants and humans, where they are responsible for flatulence.
They are also common in soils in which the oxygen has been
depleted.
 Methanogens are anaerobic. All methanogens are rapidly killed by
the presence of oxygen.
 Archaea are a major division of microorganisms. Like bacteria,
Archaea are single-celled organisms lacking nuclei and are
therefore prokaryotes, classified as belonging to kingdom Monera
in the traditional five-kingdom taxonomy.
 Note: the methanogens are a type of microorganism, but do not
belong to the group of bacteria.
 Source: www.wikipedia.org
Course 2 Unit 4
Biogas composition
The methane fraction produced in the biogas varies with
the input material; as a rule of thumb:

carbohydrates:
approx. 50 vol.-% methane

fats:
approx. 70 vol.-% methane

proteins:
approx. 84 vol.-% methane
Compound
Vol %
Methane
50-75
Carbon dioxide
25-50
Nitrogen
<7
Oxygen
<2
Hydrogen sulfide
<1
Ammonia
<1
Biogas uses
1.
2.


Biogas can be burnt and used for
cooking or lighting
Biogas can also be converted to
electricity and heat (part of the heat is
often used to heat the digester) 
“Combined heat and power plants”
(CHP), or co-generation plants
If biogas is not used it should be flared*
because methane is a greenhouse gas
Biogas from individual septic tanks is
normally not flared (assumption is that
volume is negligible – but is that a fair
assumption?)
* see next slide for explanation of a flare
Top photo: hands of Mantopi Lebofa,
Lesotho, Dec. 2006
Explanation for previous slide:
What is a flare (for biogas) exactly? (slide 1 of 2)
What is a flare (for biogas) exactly? (slide 2 of 2)
 There are many companies who can provide the equipment for a
flare (e.g. for landfill gas flares)
- Just as an example, you can look at this website (photos from
the previous slide are from their website):
http://www.parnelbiogas.com/products.htm
This supplier states (for more information, see their website):
Our flare systems can also be equipped with:
• Knockout drums
• Single or multiple blower arrangements
• Paperless chart recorders
• Methane monitors
Rule of thumb for removal of different
compounds by anaerobic digestion (AD)
Compound
Removal
Organic matter
High level of removal (but not good enough for
direct discharge to surface waters; would need
aerobic post-treatment)
Nitrogen and
phosphorus
No removal
Pathogens
Not much removal unless operated at
thermophilic* temperatures and very long
retention times (see next slide)
Heavy metals
No removal
* Thermophilic (~55° C) anaerobic digestion will achieve
more pathogen removal than mesophilic (~ 35° C) anaerobic digestion
Pathogen removal in AD processes
 In small biogas digesters, the process is operating at ambient or
mesophilic temperatures, and is difficult to control
– Temperature and retention time therefore vary and sufficient pathogen
reduction is difficult to achieve even at long retention times
 Example research results for pathogen removal in AD (Heeb et al., 2007):
Pathogens
Termophilic
(53-55°C)
Mesophilic
(35-37°C)
Ambient
(8-25°C)
fatality
HRT
fatality
HRT
fatality
HRT
Salmonella
100 %
1-2
100 %
7
100 %
44
Shigella
100 %
1
100 %
5
100 %
30
Polivirus
-
-
100 %
9
-
-
Schistosoma
ova
100 %
<1
100 %
7
100 %
7-22
Hookworm
100 %
1
100 %
10
90 %
30
Ascaris ova
100 %
2
98.8 %
36
53 %
100
Course 2 Unit 4
Important design parameter: residence time
 The residence time in a digester is also called hydraulic
residence time (HRT), or retention time (t)
 It is the length of time that the liquid stays in the reactor
 Once you know the design residence time for your process,
you can calculate the required volume of the digester
V = Q · tdesign
With:
Q:
flowrate (m3/d), e.g. 0.5 m3/d
tdesign:
design residence time, e.g. 30 days
Then required volume is: 15 m3
 Examples (see also Part B):
– Anaerobic baffled reactor: HRT = 2-3 days
– Sewage sludge digestion: HRT = 15 – 20 days
Degradability of organic materials
Easy to degrade
Lots of biogas in
short time (short
residence time)
Examples:
Sugar
Vegetables
Fats
Faeces
Not so much
biogas and long
residence times
needed
Hard to degrade
Grass
Leaves
Wood chips
Yield of biogas from different sources (1/2)
Yield of biogas from different sources (2/2)
Advantages of “biogas toilets” (anaerobic
treatment of mixed toilet waste) compared to
UDD toilets
 No need to separate urine, hence
easier for the toilet user, no extra
piping, no extra tank
 Can receive toilet flushwater hence no need to abandon habit
of flushing with water
 Can receive greywater
 Biogas can be used for cooking
and lighting
 Can take animal manure and
organic solid waste
 Can have the image of a “hightech” solution
Household biogas digester (fixed dome) during
construction in Lesotho (note gas outlet at the
top) – Photo by Mantopi Lebofa
Typical applications for “biogas toilets”
 Public toilets in slums, e.g. in India; Kibera slum in Nairobi
 Toilets at schools, universities, prisons and other institutions
(e.g. India, Rwanda)
 Situations where animal waste is available and can be
combined with human waste (e.g. Nepal, India, China)
 Regions where pour-flush toilets are commonly used (also in
combination with anal washing with water)
  See Part C for examples
Disadvantages of “biogas toilets” compared to
UDD toilets
Biogas toilets...:
 Are not suitable for individual households unless the toilets can also
receive animal waste (e.g. from cows)
 Have higher capital cost – depending on the number of people served
 Require more know-how for construction (higher safety precautions)
 Produce digestate which can be relatively high in pathogens
– OK for use as fertiliser but needs further safety barriers for safe
reuse
 You don’t have your nutrients available in a high-concentrated stream
 You need to decide on a case-by-case basis which type of toilet is
better suited
Course 2 Unit 4
Advantages of anaerobic wastewater treatment
(for greywater) compared to aerobic* treatment
 Production of energy-rich methane
 No energy demand for aeration
 No removal of nitrogen and phosphorus (this is an advantage if
effluent is to be reused in agriculture)
 High organic loading rates can be applied
- Suitable for high-strength wastewater (high BOD)
 Low production of excess sludge; the digestate is highly
stabilised and can easily be dewatered
* Examples for aerobic wastewater treatment: activated sludge plants, trickling filter plants
(see Course 2 Unit 1 Part D)
Disadvantages of anaerobic wastewater
treatment (of greywater) compared to aerobic
treatment
 Effluent from anaerobic treatment has higher COD concentration than
from aerobic treatment
- If better effluent quality is required then a second (aerobic) treatment
step may be required
 Does not remove nutrients (this is a disadvantage if effluent is discharged
to receiving water body)
 Start-up of the process may take long time (slow growth of methanogens)
 Anaerobic microorganisms are sensitive to some toxic compounds
 Can cause odour problems if not operated properly
 Only limited pathogen removal
Course 2 Unit 4
Classification of anaerobic digestion processes
 By temperature:
- Mesophilic (35°C)
- hermophilic (55°C)
 By operation:
- Batch
- Continuous
- Fed-batch or semi-continuous
 By water content of input material:
- Wet systems: TS content < 15% d.s.
- “Dry” systems: TS content 25-50% d.s.
-  rule of thumb: AD does not work if all input material has
TS > 50% d.s (too dry)
Remember: 15% d.s. means 150,000 mg/L dry solids content
and TS stands for total solids (same as d.s. which stands for dry solids)
Example for anaerobic digestion operating
and performance parameters
Operating parameters
 Hydraulic retention time in
digesters (also called
treatment time): 15 – 20
days
 Operating temperature:
– Ambient
– Mesophilic (35°C)
– Thermophilic (55°C)
 Type and composition of
feed (input material)
– TS and VS content of
feed
– Degradability
Performance parameters
 VS loading rate: 1.6 – 4.8
kg/m3/d
 VS destroyed: 56 – 66%
 Methane content in biogas
(%) – expect 50 – 75%
 Gas production per kg VS
destroyed (m3 /kg VS
destroyed)
Values provided on this slide are for high-rate
complete-mix mesophilic anaerobic digestion
(Metcalf & Eddy, page 1513 and 1514)
How to detect a failing anaerobic treatment
process







Odour
Explosion (worst case !! – extremely rare) – see next slide
Foaming
Low pH value (step 4 of 4-step process on slide 24 is inhibited)
No or low biogas production
Low methane content in biogas
Volatile solids (VS) fraction in effluent close to the VS fraction in
the influent, indicating no VS removal
How could an explosion of an anaerobic
digester occur?
 If a vacuum develops in the digester (e.g. leaks of liquid): 
air is sucked in  if methane content is 5-15% in air, and
there is a spark, then there could be an explosion
 If digester is in an enclosed building and biogas leaks out: 
if there is a lack of ventilation and a spark, then there could
be an explosion
  Checking for liquid and gas leaks is an important
operational maintenance task
 Having said all this, I have never heard of such an explosion
actually having taken place (have you?)
Main possible causes of process failure
 Organic overload (too much BOD added per m3 and day)
- This applies particularly to easily degradable substrate, e.g.
brewery wastewater
 Insufficient alkalinity and therefore a drop in pH (could add
alkalinity, e.g. lime)
 Toxic substances in influent are inhibiting methanogens (this
applies only to industrial wastewater)
Course 2 Unit 4
Course 2 Unit 4
Part B: Anaerobic treatment technologies relevant for ecosan
concept
Household biogas plant (fixed dome) in Maseru, Lesotho (at the end of construction)
Two principal types of construction to deal with
gas development
 Fixed dome in which a pressure builds up (see Lesotho
example in Part C)
– Common for small-scale plants
– Needs skilled workers for construction but less attention
during operation (no moving parts)
 Floating or moveable dome/cover which allows an expansion
of the gas volume in the digester (see examples in Part C)
– A “gas bubble” can be used
– This type is more common for large-scale plants
Course 2 Unit 4
Overview of commonly used anaerobic
treatment technologies
#
Process name
Mechanical
mixing
Covered
reactor
Biogas
collection
Scale
1,
2
Septic tanks,
anaerobic baffled
reactors
No
Yes
No
Household or
neighbourhood
3
Household biogas
plants*
No
Yes
Yes
Household,
neighbour-hoods,
institutions
4
Anaerobic ponds
No
No
No
Community
5
Upflow anaerobic
sludge blanket
reactor (UASB)
Yes
Yes
Yes
Neighbourhood or
community
* Also called household biogas digesters or decentralised biogas plants (i.e. not just
limited to households)
1- Septic Tanks
 Very common on-site sanitation
system for excreta and
greywater
 Relatively common also in some
high-income countries: Australia,
USA
 In most cases, biogas is not
collected (amount is small unless
animal manure is digested as
well; in that case it is no longer
called a septic tank)
Maseru, Lesotho, Dec 2006
(septic* tanks are always
underground)
* Septic is a word used for sewage that has gone anaerobic, but it is not really
a scientific term
Septic tank process principles
Ground level
Ground level
 Combined settling, skimming and anaerobic digestion
 Commonly followed by filtration of effluent (e.g. sub-surface
soil disposal field)
 No mechanical equipment (no moving parts)
Reminder: How can septic tanks affect the
groundwater?
from Course 1
Unit 3
Ground level
Ground level
Effluent to
soil infiltration
(normal)
Wastewater
from house
Soil
Soil
Soil
Soil
Soil
(unsaturated
zone)
Faecal sludge (if “leaking
septic tank”)
Groundwater (aquifer)
The effluent from septic tanks is commonly infiltrated into the ground (on purpose).
But faecal sludge is NOT meant to leak out from the septic tank (but often does if
not designed properly).
Septic tank design and advantages and
disadvantages
 Design:
 Sedimentation tank
 Settled sludge partially
stabilised by anaerobic
digestion
 Almost no removal of
dissolved and suspended
matter
 1-3 compartments
 → look for national design
standards!
 Disadvantages:
 Low treatment efficiency (COD
removal approx. 50%; almost no
nitrogen removal)
 O&M often neglected
(desludging) or unknown!
 Relies on water for toilet
flushing
 Effluent quality is difficult to
monitor
 Requires periodical removal of
faecal sludge (every 3 - 10
years, depending on tank size)
 Faecal sludge management is
often not carried out properly
(often just dumped in
environment)
 Advantages:
 Simple technology for on-site
treatment
 Little space required
(underground)
 Institutional acceptance is
high
This slide and the next four slides were provided by Dr. Doulaye Koné
from SANDEC/Eawag, Switzerland
Course 2 Unit 4
Septic tank design schematic
(2 compartments)
Aim is to achieve some mixing and contact of influent with sludge layer
which contains the anaerobic digestion micro-organisms
Toilet
wastewater,
greywater
Wastewater
(solids settling)
Faecal sludge
Wastewater
effluent
(partially
treated)
Course 2 Unit 4
Septic tank design aspects




Mainly rectangular (some exceptions if prefabricated)
Length to width ratio: 3 to 1
Depth: 1 to 2.5 m
First chamber is at least 50% of the total volume (2 chambers
→ 1st chamb. = 2/3; 3 chambers → 1st chamb.= 1/2)
 Manholes in the cover slab: one each above inlet and outlet and
one at each partition wall
 Tank must be watertight and stable → construction material:
reinforced concrete (most common), steel (corrosion problems),
polyethylene, fibreglass or plastic. Cheap solution: bricks
Are there any national design standards in your country?
2- Anaerobic baffled reactor (baffled septic
tank)
Wastewater
influent
Effluent
Faecal sludge
 Improved septic tank with 2 to 3 chambers in series (up to 5)
 Intensive contact between resident sludge and fresh influent
 Treatment efficiency: 65 to 90% COD removal; HRT = 2-3 days





Advantages:
Higher treatment efficiency than septic tanks, hardly any blockages
High removal efficiencies, also for suspended and dissolved solids
Disadvantages:
Construction and maintenance more complicated than for
conventional septic tank
Anarobic baffled reactors during construction
3 – Household* biogas plants
 Household biogas plants produce a continuous flow of digested
material (liquid sludge), which is used as fertiliser (despite not being
free of pathogens)
 Desludging (removal of sludge) is only necessary if there is a buildup of inert material (e.g. sand; lack of mixing)
- Expectation is to “never” have to desludge them (> 15 years)
 These plants do not aim for solids settling but rather good mixing
- Therefore, they have their reactor outlet at the bottom (rather
than at the top like a septic tank)
 * The word “household” is a bit misleading  they can also be used
for institutions, businesses, hotels etc.
Course 2 Unit 4
Household biogas plants are common worldwide, particularly in Asia
 Millions of plants worldwide, particularly in China, India,
Nepal
– Example rural Nepal: about 1 million biogas plants in
2006
• Often users only apply manure but no human excreta
nor greywater  but this could change
 Work best in conjunction with animal manure
– Sufficient biogas for cooking and lighting needs of one
family (if they have one cow for example)
– Rule of thumb: 1 cow equals 17 people with respect to
biogas production from excreta (Ralf Otterpohl,
Ecosanres Discussion Forum on 4 July 2006)
Household biogas plant schematic (fixed dome)
Removable cover for
occasional desludging (rare)
Course 2 Unit 4
4 - Anaerobic ponds





Also called lagoons (in the US) or waste stabilisation ponds
Low-rate anaerobic process (e.g. 1 – 2 kgCOD/m3/d)
Solids settling and anaerobic decomposition
Depth: 5-10 m
Could be covered for odour control and gas collection (but most of
them are not covered)
 Usually several ponds in series (last pond: aerobic maturation pond
with algae; pathogen kill by sunlight)
Influent
(faecal
sludge,
greywater or
conventional
ww.)
Effluent
Sludge layer
(increasing over time)
This slide was provided by Peter van der Steen (UNESCO-IHE)
Anaerobic ponds
This pond is not covered
A sludge crust may form and act
as a cover
http://cff.wsu.edu/Project/galleryconstruction.htm
Limitations of anaerobic ponds
 Large land area required
- Potentially high costs for covers, if these are used
 Needs desludging after 10-15 years (this is often forgotten!)
- e.g. By stopping inflow, then settling and drying for 2 months then
manual emptying  reuse in agriculture?
 Feed flow distribution inefficiencies
 Poor contact between substrates and biomass (see schematicEffluent
Influent below)
Substrates
Sludge layer (biomass)
The issue of methane emissions and covering
of ponds
 Anaerobic ponds emit biogas which contains the greenhouse
gas methane
 It is possible to cover the ponds and to collect the biogas (for
energy generation or flaring)
 Floating cover systems: floating membrane made of lined
PVC or High Density Polyethylene
- Covers needs to be durable, UV protected, chemically
resistent to biogas; support foot traffic and rainwater loads
 New: emission reduction contracts can be signed based on
capturing the methane gas from anaerobic lagoons  sale of
biogas emission reduction possible
- First example in developing country: Santa Cruz, Bolivia in
2006 (Source: Menahem Libhaber in Huber’s Symposium
Water Supply and Sanitation for All, Sept. 2007, Berching,
Germany)
Are you aware of anaerobic ponds (waste stabilisation
ponds/lagoons) in your city? Could they be covered?
5 - Principles of UASB reactors
 UASB = Upflow anaerobic sludge blanket reactor
– Inflow flows in vertical direction (from bottom up – upflow)
 A high sludge concentration is maintained in the reactor,
which results in long solid retention times
 Short hydraulic retention times
 Good contact between substrates (COD) and the sludge
(bacteria)
 High-rate system (high organic loading rates, e.g. 2 -24
kgCOD/m3/d)
 UASBs can treat:
– Blackwater (faeces and urine), manure
– Conventional wastewater (high strength), greywater
– Industrial effluent
– Agricultural organic waste
The biogas contains sulphide,
which can be removed in iron
filters (FeS precipitation)
A UASB reactor for the
treatment of 6000 PE (person
equivalents) domestic
wastewater
Course 2 Unit 4
UASB reactor components – slide 1 of 3
Biogas
Effluent
Influent
In the sludge
bed biogas
bubbles are
produced that
rise through the
sludge bed and
mix it.
There is good
contact between
the dense sludge
bed and the
upflowing
substrate
UASB reactor components – slide 2 of 3
Biogas
Effluent
Influent
The biogas
bubbles are
directed into a
separator
UASB reactor components – slide 3 of 3
Biogas
In the settler
compartment
there is no
turbulence since
the bubbles have
been removed.
Ideal conditions
for settling.
Effluent
Influent
Solids settle onto
the settler and
periodically slide
back into the
sludge bed.
Concluding remarks regarding anaerobic
digestion
 Great potential: provides biogas for cooking, lighting and
heating; and provides (liquid) fertiliser
-Is increasing in importance in the light of climate change
(need for alternative energy sources)
 Most interesting for:
-Combination with animal waste
-Institutions with lots of people, e.g. prisons, public toilets,
schools, universities
 Anaerobic digestion as part of a sanitation system can help to
close the loop of nutrients otherwise wasted to the
environment, and ensure recycling of valuable wastes in a
sustainable manner
 Remaining issues:
- Quality of digestate not well documented
- Pathogen removal in mesophilic AD is quite low  but
digestate is widely used in agriculture anyway  use
multiple-barrier approach (see Course 3)
Course 2 Unit 4
References
 Butare, A and Kimaro, A (2002) Anaerobic technology for toilet
wastes management: the case study of the Cyangugu pilot project,
World Transactions on Engineering and Technology Education,
Vol.1, No.1.
http://www.eng.monash.edu.au/uicee/worldtransactions/WorldTrans
AbstractsVol1No1/Microsoft%20Word%20-%2032_Butare.pdf *
 Heeb, J., Jenssen, P., Gnanakan, K. & K. Conradin (2007): ecosan
curriculum 2.0. In cooperation with: Norwegian University of Life
Sciences, ACTS Bangalore, Swiss Agency for Development and
Cooperation, German Agency for Technical Cooperation and the
International Ecological Engineering Society. Partially available
from www.seecon.ch and
http://www2.gtz.de/dokumente/oe44/ecosan/cb/en-m23-ecosanhuman-dignity-lecture-2006.ppt
 Tchobanoglous, G., Burton, F.L., Stensel, H.D. (2003) Wastewater
Engineering, Treatment and Reuse, Metcalf & Eddy, Inc., McGrawHill, 4th edition. This is a good book on conventional wastewater
treatment
 Zhang Wudi et al. (2001): Comprehensive utilization of human and
animal wastes. Proceedings of the First International Conference
Ecological
Sanitation
in Nanning
2001,EcoSanRes, China
* on
Also
under Extra
Materials
on the I-LE