Anaerobic Digestion (Small-acale)
Dorothee Spuhler, seecon gmbh
Anaerobic Digestion (Small-scale)
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Depending on the initial situations and respective local circumstances, there is no guarantee that single measures described in the toolbox
will make the local water and sanitation system more sustainable. The main aim of the SSWM Toolbox is to be a reference tool to provide
ideas for improving the local water and sanitation situation in a sustainable manner. Results depend largely on the respective situation
and the implementation and combination of the measures described. An in-depth analysis of respective advantages and disadvantages and
the suitability of the measure is necessary in every single case. We do not assume any responsibility for and make no warranty with
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Anaerobic Digestion (Small-scale)
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Contents
1. Concept
2. How can it optimise SSWM
3. Design principals
4. Treatment efficiency
5. Operation and maintenance
6. Applicability
7. Advantages and disadvantages
8. References
Anaerobic Digestion (Small-scale)
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1. Concept
Background
Small-scale anaerobic biogas reactors are very common in agricultural
regions in industrialised as well as developing countries.
Because this plants not only allow the treatment of wastes (manure, green
waste, toilet products) but also result in the on-site production of a
renewable energy source, such plants have been widely disseminated by
many rural developing programmes in the past 30 years.
In Nepal for instance more than 200’000 such plants have been constructed
in the past 20 years.
The main features of small-scale anaerobic biogas reactors are:
• Requires animal dung (rich in organic matter and high productions
yields) to produced sufficient energy for the household
• Can co-treat toilet products and kitchen or garden waste (green waste)
• Depend on relatively high daily mean temperature as anaerobic
digestion, the process which produces biogas slows down drastically
with decreasing temperatures.
Anaerobic Digestion (Small-scale)
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1. Concept
Background
On-site recycling of nutrients and energy
Toilet, Kitchen and Garden
Waste
+ Manure
Adapted from: http://www.seco.cpa.state.tx.us/energy-sources/biomass/images/manurebiogas.gif [Accessed: 30.05.2010]
Anaerobic Digestion (Small-scale)
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1. Concept
What are Small-Scale Anaerobic Digesters?
… Airtight reactors, typically designed to produce biogas at the household or
community level.
Biogas gas is produced by the conversion of green waste by a process called
anaerobic digestion.
During anaerobic digestion, microorganism transform organic matter contained
in the wastes into biogas
The produced biogas can be used either directly for coocking, heating
or lightening
or be transformed into combined heat and power (CHP) in
small cogeneration plants.
With time the reactors fill up and digested sludge (sludge which organic
fraction was already converted to biogas) accumulates in the bottom.
Nutrients remain in the sludge, which is a well-balanced soil amendment.
Toilets can be linked to the reactors and co-digested with the animal dung, but
biogas production from human manure is only low and therefore animal dung
and green wastes are required to cover a familiy’s needs.
Anaerobic Digestion (Small-scale)
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1. Concept
“The Ecocylce of biogas”
What are Small-Scale Anaerobic Digesters?
Coocking
Cattle Dung /
Manure
Lightning
Toilet Products
(Excreta,
Faeces)
Biogas
Heating
Kitchen / Garden
Organic Waste
(Green Waste)
Electricit
y
Fuel
Fertiliser
Anaerobic Digestion (Small-scale)
D. Spuhler (2010), Adapted from: www.kristianstad.se/; http://www.newseedadvisors.com/2009/09/10/invest/;
http://www.hydroharrys.com/hydroharrys_about_fertilizer.php and www.clker.com [Accessed: 02.06.2010]
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1. Concept
Possible Benefits for Users:
Under the right conditions a biogas plant yields several benefits to end-users
• Social:
◦ Improved sanitation: reduction of pathogens, worm eggs and flies
◦ Reduction of workload: less firewood collection, better cooking
performance
◦ Improved indoor air quality: less smoke and harmful particle emission
of biogas stove compared to wood or dung fuels;
• Environmental
◦ Production of green energy
◦ Reduction of greenhouse gas emission
◦ Organic fertilizer production
• Economical:
◦ Better Health more work capacity
◦ Fertilizer, better crop yields, better Health
◦ Fuel substitution
Anaerobic Digestion (Small-scale)
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1. Concept
Possible Benefits for Users: Reduced indoor pollution
Source: M. WAFLER
Coocking with fuelwood
Anaerobic Digestion (Small-scale)
Source: M. WAFLER
Biogas stove
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1. Concept
What is Anaerobic Digeastion ? (1/3)
Degradation of organic material by bacteria. In the absence of air
(anaerobic). Four stages:
•Hydrolisis
◦ Cleavage of a chemical compound through the reaction with water.
◦ Insoluble complex molecules are bracken down to short sugars, fatty
acids and amino acids.
•Fermentation (Acidogenesis)
◦ Products from hydrolysis are transformed into organic acids, alcohols,
carbon dioxide (CO2), hydrogen (H) and ammonia (NH3).
•Acetogenesis
◦ Organic acids and alcohols are converted into hydrogen (H2), carbon
dioxide (CO2) and acetic acid (CH3COOH). Therefore, oxygen is
consumed and anaerobic conditions are created
•Methanogenesis
◦ Methanogenic bacteria (methanogenesis), transform the acetic acid,
carbon dioxide and hydrogen into biogas.
Anaerobic Digestion (Small-scale)
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1. Concept
What is Anaerobic Digestion ? (2/3)
D. SPUHLER (2010)
Anaerobic Digestion (Small-scale)
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1. Concept
What is Anaerobic Digestion ? (3/3)
Source:
http://water.me.vccs.edu/courses/ENV149/changes/Feat11_pi
cII-1.jpg [Accessed: 02.06.2010]
Anaerobic Digestion (Small-scale)
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1. Concept
What is Biogas ?
Biogas is a mixture of methane and carbon dioxide.
The properties of biogas are similar to the ones of natural gas.
Biogas is the common name for the mixture of gases released from
anaerobic digestion.
Typically biogas is composed of:
Methane (CH4)
Carbon Dioxide (CO2)
Hydrogen (H)
Nitrogen (N2)
Hydrogen sulphide (H2S)
50 to 75 %
25 to 50 %
5 to 10 %
1 to 2 %
Traces
Sources: YADAV & HESSE (1981); FAO (1996); PIPOLI (2005); GTZ (2009
Source: MUENCH (2008)
Methane is the valuable part of the biogas. Biogas that contains about
60 to 70 % of CH4 has a calorific value of about 6 kWh/m3 what
corresponds to about half an L of diesel oil. (ISAT/GTZ 1999, Vol. I)
Anaerobic Digestion (Small-scale)
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1. Concept
Examples:
Small-scale Biogas plants
Source: M. WRIGHT, Ashden Awards
Source: M. WRIGHT, Ashden Awards
Biogas plant for cow dung,
Padli village (India)
Anaerobic Digestion (Small-scale)
Source: M. WRIGHT, Ashden Awards
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1. Concept
Examples: Small-scale Biogas Plants
Biogas
lamp
Source: M. WRIGHT, Ashden Awards
Source: M. WRIGHT, Ashden Awards
Anaerobic Digestion (Small-scale)
Adding greywater to the
biogas reactor to optimise
moisture conditionss
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1. Concept
Examples Small-scale Biogas Plants
The
“Mudbooster”
Plant
Source: UNKNOWN
Anaerobic Digestion (Small-scale)
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1. Concept
Examples Small-scale Biogas Plants
Source: C. RIECK (2009)
Wet clay is used to fit the concrete lid
of the manhole gas-tight.
Source: SuSanA
Biogas outlet and manhole with
remouvable cover from a underground
biogas plant Installed by the NGO TED
in Maseru, Lesotho (Susana)
Anaerobic Digestion (Small-scale)
Source: C. RIECK (2009)
The manhole is filled with water to keep
the clay sealing wet and gas tight. Gas
leackage would be indicated by bubbles.
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2. How it can optimize SSWM
Biogas plants can contribute to
sustainable sanitation
Biogas plants transform
traditional manure management;
reducing CH4 and CO2 emission
Anaerobic Digestion (Small-scale)
Biogas substitutes
conventional energy
sources, reducing
reliance on fossil fuel
and firewood (CO2)
Digested sludge
can substitute
chemical
fertiliser
D. SPUHLER (2010), adapted from: http://www.terranet.or.id/mitra/dewats/photo/masukan1256.jpg;
http://www.borda-sea.org/modules/cjaycontent/index.php?id=6; http://whrefresh.com/wpcontent/uploads/2010/01/potato_field.jpg;
http://www.greenspec.co.uk/images/energy/CHP/chp2.gif]; http://peda.gov.in/eng/images/ruralbiogas-plant_179.jpg; [Accessed: 30.05.2010], BPO (2006) and BUNNY (n.y.)
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1. Concept
Examples: Biogas Appliances
Biogas lamps
K.P. Pravinjith
Biogas
cooking
stoves
Krämer (TBW)
PBO (2006)
M. Wafler
Chang Mai
Biogas generator
Anaerobic Digestion (Small-scale)
Biogas rice
cooker
Biogas boiler
Source: UNKNOWN
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3. Design Principals
Basics: Process Parameters
Anaerobic digestion = Biological system of bacteria
Optimal conditions required that bacteria feel wealthy…
•Temperature
◦ Performance
◦ Retention time
•pH: Wide range,but methanogenesis requires neutrality (6.5-7.5). (MES et al.
2003)
•Total solid (TS)
◦ Solids for digestion (organics) - Liquid for fluidity of slurry.
◦ Optimal TScontent: 5 to 10%. (SASSE 1988; NIJAGUNA 2002)
•COD: Chemical oxygen demand: Methane production potential
Anaerobic Digestion (Small-scale)
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3. Design Principals
Basics: Daily manure yield for different cattle
Sources: OEKOTOP; WERNER et al. (1998)
Anaerobic Digestion (Small-scale)
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3. Design Principals
Basics: Gas yields for different feedstocks
Sources: OEKOTOP; WERNER et al. (1998)
Anaerobic Digestion (Small-scale)
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3. Design Principals
Basics: Biogas Guideline data
Suitable digesting temperature
20 to 35 °C
Retention time
40 to 100 days
Biogas energy
6kWh/m3 = 0.61 L diesel fuel
Biogas generation
0.3 – 0.5 m3 gas/m3 digester volume
per day
Human yields
0.02 m3/person per day
Cow yields
0.4 m3/Kg dung
Gas requirement for cooking
0.3 to 0.9 m3/person per day
Gas requirement for one lamp
0.1 to 0.15m3/h
Adapted from WERNER et al. (1998); ISAT/GTZ (1999), Vol. I; MANG (2005)
Anaerobic Digestion (Small-scale)
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3. Design Principals
Types of Digester: Bag or Rubber Balloon Biogas Plants (1/2)
Huge common plastic bag (e.g. PVC): sludge settles on the bottom and biogas
is collected in the top. Gas is transported by the pressure from the elasticity of
the balloon (can be enhanced by placing weights on the balloon).
• Most simple design, easy and low-cost ( if material locally available)
• Temperature enhanced when exposed to sun
• Simple to clean but lifespan generally limited
Plastic bag
Gas pipe
Biogas accumulates in the top of the bag
Leveled
surface
Inlet
Layer of
compacted backfill
Anaerobic Digestion (Small-scale)
Source: adapted from FAO (1996)
To reuse or
further
treatment
(e.g. drying
bed)
Batch mode: emptying
once every few years
Plug-flow reactor: the
slurry moves through
continuously much like
a train a tunnel
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3. Design Principals
Types of Digester: Bag or Rubber Balloon Biogas Plants (2/2)
Underground plugflow reactor bag
biogas plant () and
balloon biogas
collection chamber
(). (Philippines,
Garry Baron)
Source: http://www.habmigern2003.info/biogas/Baron-digester/Baron-digester.htm [Accessed: 02.06.2010]
Anaerobic Digestion (Small-scale)
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3. Design Principals
Types of Digester: Fixed-dome Biogas Plants (1/3)
Airtight underground reactor out of concrete or brick work (most often round),
with a fixed (also airtight) dome in which gas is collected. Gas pressure is
absorbed by the slurry which is displaced into a compensation tank.
• Most widely disseminated
• Long life-spam
• Underground: safes space and protect from temperature changes
• Construction must be supervised
Anaerobic Digestion (Small-scale)
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3. Design Principals
Types of Digester: Fixed-dome Biogas Plants (2/3)
Biogas
collection
Inlet
Fixeddome
Seal
Removable cover
Biogas accumulates in
the dome
Slurry
Overflow tank /
compensation
chamber
Source: adapted from http://peda.gov.in/eng/images/rural-biogas-plant_179.jpg [Accessed: 02.06.2010]
Anaerobic Digestion (Small-scale)
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3. Design Principals
Types of Digester: Fixed-dome Biogas Plants (3/4)
Source: K.P. PRAVVIJITH
Source: K.P. PRAVVIJITH
Anaerobic Digestion (Small-scale)
Source: K.P. PRAVVIJITH
Source: K.P. PRAVVIJITH
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3. Design Principals
Plastic dome
Any pit can be filled with
organic waste and covered
airtight with a plastic sheet
in order to collect biogas
Source: ISAT/GTZ (1999, Vol. I)
Anaerobic Digestion (Small-scale)
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3. Design Principals
Floating-drum
Floating-drum Biogas Plants
Floating-drum plants consist of an
underground digester and a moving
gasholder (mostly of made out of
steel).
The gasholder floats either directly
on the fermentation slurry or in a
water jacket of its own. The gas is
collected in the gas drum, which
rises or moves down, according to
the amount of gas stored. The gas
drum is prevented from tilting by a
guiding frame.
• Easy to and to control operation
• Material costs are high
• High risk of corrosion and
rusting (short lifespam).
Anaerobic Digestion (Small-scale)
Inlet
Biogas
Outle
t
Slurry
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3. Design Principals
Floating-drum Biogas Plants
Different design of
floating drum plants 
 Open
gasholder
MUELLER (2007)
MUELLER (2007)
Anaerobic Digestion (Small-scale)
 Floating drum plant
with inlet from the the
NGO BIOTECH (India)
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3. Design Principals
Toilet linked Biogas Reactors
Co-digestion of toilet products (nightsoil or blackwater) is a sustainable
solutions for
• Hygienically safe on-site treatment of toilet excreta
• Production of fertiliser
• Production of renewable energy
The mixing of animal dung with blackwater increases its fluidity and
results in optimal moisture conditions for the anaerobic digestion.
Human manure has a lower content in organic matter and thus a
limited biogas yield.
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3. Design Principals
Toilet linked Biogas Reactors
Source: ???
Anaerobic Digestion (Small-scale)
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3. Design Principals
Toilet linked Biogas Reactors
Inlet for
animal
waste
Pourflush
toilet
Source: adapted from WELL (n.a.)
Gas outlet pipe
Link of toilet
Baffle to mix
influent with tank
contents
Anaerobic Digestion (Small-scale)
Biogas
reactor
Removable
cover annual
desludging
Collection and
expansions
chamber
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3. Design Principals
Toilet linked Biogas Reactors
Manure and green
waste mixing
chamber
http://www.ashdenawards.org/files/imagecache/large/fi
les/images/biogasnepal05a.jpg [Accessed: 02.06.2010]
Source: M. WAFLER
Pour-flush
toilet
Sludge drying bed
Expansions chamber
Biogas reactor
Anaerobic Digestion (Small-scale)
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4. Treatment Efficiency
Health aspects
Anaerobic digested sludge are generally pathogen free. Pathogen removal
depends temperature and retention time. Generally , at more than 55°C
pathogens are killed after a few days. At normal temperatures (mesophilic
digestion), longer time is required.
Source: SASSE (1988)
In reality, fresh sludge
is always mixed with
new sludge and it is
very difficult to control
retention times.
Therefore, caution
needs to be taken when
emptying and handling
sludge manually.
Source: WERNER et al. (1998)
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4. Treatment Efficiency
Nutrients
Anaerobic digestion only removes organics, and the main mineral material and
almost all nutrients remain in the bottom sludge.
Biogas slurry
• Phosphorus: almost 100 %
=
Fertilisers
• Nitrogen (ammonium): and 50 to 70 % (JOENSSEN et al. 2004)
Biogas Slurry = Fertiliser
Further treaments to increase the safety (pathogen removal)
• Composting
• Drying beds / Humification
Anaerobic Digestion (Small-scale)
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5. Operation and Maintenance (O&M)
Start-up
Seeding with living sludge form other anaerobic reactor required. The
establishment of the complex biological conditions for anaerobic
digestion and biogas production may takes some weeks to months.
Operation
No skilled operator is required but households should be trained to
understand the system.
Regular maintenance includes
• Checking for foaming or scum formation
• Checking for air/gas- tightness
• Checking for rusting (e.g. floating-drum reactor)
Anaerobic Digestion (Small-scale)
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6. Applicability
Small-scale biogas digesters can transform almost any biodegradable
waste into biogas.
Household or community scale.
Most often used for biogas production in rural areas from animal dung.
Green wastes (kitchen, garden, etc.) can be added.
If toilets are linked: safe and sustainable sanitation solution.
Underground construction provided: can also be constructed in urban
areas.
As anaerobic digestion is limited to moderate to high temperature, only
in areas where temperature does not fall short of for any substantial
length of time.
Anaerobic Digestion (Small-scale)
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7. Pros’ and Cons’
Disvantages:
• Experts are required for the
design of the reactor and skilled
labour is required for the
construction of a gastight tank
• Substrates need to contain high
amounts of organic matter for
biogas production
• Slurry may has to be further
treated before reuse (e.g.
composting)
Advantages:
• Low-cost
• Generation of biogas and
fertilizer
• Combined treatment of animal,
human and solid organic waste
• Low operation and maintenance
• Underground construction (low
space requirement and high
acceptance)
• Low risk of odours
• Resistance against shock loads
• Below temperatures of 15°C,
• Long life span if maintained and
biogas production is economically
operated correctly
not interesting (heating required)
• Reduces the amount of wood fuel • Requires seeding (start-up can be
and improves indoor air quality
long due to the low growth yield
of anaerobic bacteria)
Anaerobic Digestion (Small-scale)
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Thank you for your attention!
Source: ???
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8. References
BPO (2006): Support Project to the Biogas Programme for the Animal Husbandry Sector in some Provinces of Vietnam. BP I Final report. Hanoi: Biogas Project
Office (BPO) Hanoi
BUNNY, H., BESSELINK, I. (n.y.): The National Biodigester Programme in Cambodia. In Relation to the Clean Development Mechanism. National Biogidgester
PRobramme and NV Netherlands Development Organisation
FAO (1996): Biogas Technology - A Training Manual for Extension. Consolidated Management Services Nepal (P) Ltd. and Food and Agriculture Organization of
the United Nations (FAO) Available at: http://www.fao.org/docrep/008/ae897e/ae897e00.HTM [Accessed: 19.04.2010]
GTZ (2009): Biogas sanitation for black water or brown water, or excreta treatment and reuse in developing countries. Draft Version.(=Technology review).
Eschborn: German Agency for Technical Cooperation GmbH (GTZ) and Sustainable Sanitation Alliance (SuSanA) Available at:
http://www.gtz.de/en/themen/umwelt-infrastruktur/wasser/9397.htm [Accessed: 11.03.2010]
ISAT/GTZ (1999): Biogas Basics. (=Biogas Digest, Volume I). Information and Advisory Services on Appropriate Technology (ISAT) and German Agency for
Technical Cooperation GmbH (GTZ). Available at: http://www2.gtz.de/dokumente/bib/04-5364.pdf [Accessed: 19.04.2010]
JOENSSON, H., RICHERT A., VINNERAAS, B., SALOMON, E. (2004): Guidelines on the Use of Urine and Faeces in Crop Production. (= EcoSanRes Publication
Series, Report No. 2004-2). Stockholm: Stockholm Environment Institute (SEI)
MANG, H.-P., (2005): Biogas Sanitation Systems. (=Ecological sanitation course, Norway, 15.-20. August 2005). Beijing: Chinese Academy of Agricultural
Engineering
MES, T.Z.D. de, STAMS, A.J.M, REITH, J.H., ZEEMAN, G. (2003): Chapter 4. Methane production by anaerobic digestion of wastewater and solid wastes. In:
REITH, J.H., WIJFFELS, R.H., BARTEN, H.(Eds.) (2003): Biomethane and Biohydrogen. Status and perspectives of biological methane and hydrogen production.
Dutch Biological Hydrogen Foundation and the Netherlands Agency for Energy and the Environment (Novem). Available at:
http://gasunie.eldoc.ub.rug.nl/FILES/root/2003/3339875/3339875.pdf [Accessed: 25.04.2010]
MUELLER, C. (2007): Anaerobic Digestion of Biodegradable Solid Waste in Low- and Middle-Income Countries. Swiss Federal Institute of Aquatic Science
(EAWAG), Department of Water and Sanitation in Developing Countries (SANDEC)
http://www.eawag.ch/organisation/abteilungen/sandec/publikationen/publications_swm/downloads_swm/Anaerobic_Digestion_high_resolution.pdf
[Accessed: 27.04.2010]
MUENCH, E. (2008): Overview of anaerobic treatment options for sustainable sanitation systems. In: BGR Symposium „Coupling Sustainable Sanitation and
Groundwater Protection“ 14 – 17 Oct 2008, Hannover, Germany. Eschborn: German Agency for Technical Cooperation (GTZ) GmbH
Available at:
http://www.gtz.de/en/dokumente/en-bgr-conference-biogas-ecosan-muench-2008.pdf [Accessed: 23.04.2010]
NIJAGUNA, B. T. (2002): Biogas Technology. New Delhi: New Age International (P) Ltd.
PIPOLI, T. (2005): Feasibility of Biomass-based Fuel Cells for Manned Space Exploration. In: Proceedings of the Seventh Eurpean Space Power Conference,
Stresa, Italy. 9 to 13 May 2005.
SASSE, L. (1988): Biogas Plants. German Appropriate Technology Exchange (GATE) and German Agency for Technical Cooperation (GTZ) GmbH Available at:
http://www.borda-net.org/modules/wfdownloads/viewcat.php?cid=5 [Accessed: 25.04.2010]
WELL (n.y.): Using Human Waste. (=WELL Technical Briefs, No. 63) Loughborough: Water and Environmental health at London and Loughborough (WELL)
Available at: http://www.lboro.ac.uk/well/resources/technical-briefs/technical-briefs.htm [Accessed: 26.04.2010]
WERNER, U. STOEHR, U., HEES, N. (1998): Biogas Plants in Animal Husbandry. German Appropriate Technology Exchange (GATE) and German Agency for
Technical Cooperation (GTZ) GmbH Available at: http://www.scribd.com/doc/27434211/Biogas-Plant-in-Animal-Husbandry [Accessed: 25.04.2010]
YADAVA, L. S., HESSE, P. R. (1981): The Development and Use of Biogas Technology in Rural Areas of Asia (A Status Report 1981). Improving Soil Fertility
through Organic Recycling. (=Project Field Document No. 10.). Food and Agriculture Organization (FAO) and United Nations Development Programme (UNEP)
Anaerobic Digestion (Small-scale)
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SSWM is an initiative
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