1. Anaerobic Digestion (Small-acale)
Dorothee Spuhler, seecon gmbh

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3. Contents
Concept
How can it optimise SSWM
Design principals
Treatment efficiency
Operation and maintenance
Applicability
Advantages and disadvantages
References

4. 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.

5. On-site recycling of nutrients and energy
1. Concept
On-site recycling of nutrients and energy
Background
Toilet, Kitchen and Garden Waste + Manure
Adapted from: [Accessed: ]

6. What are Small-Scale Anaerobic Digesters?
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.

7. What are Small-Scale Anaerobic Digesters?
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)
Electricity
Fuel
Fertiliser
D. Spuhler (2010), Adapted from: and [Accessed: ]

8. Possible Benefits for Users:
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

9. Possible Benefits for Users: Reduced indoor pollution
1. Concept
Possible Benefits for Users: Reduced indoor pollution
Source: M. WAFLER
Source: M. WAFLER
Coocking with fuelwood
Biogas stove

10. What is Anaerobic Digeastion ? (1/3)
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.

11. What is Anaerobic Digestion ? (2/3)
1. Concept
What is Anaerobic Digestion ? (2/3)
D. SPUHLER (2010)

12. What is Anaerobic Digestion ? (3/3)
1. Concept
What is Anaerobic Digestion ? (3/3)
Source: [Accessed: ]

13. 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 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)
Methane (CH4)
50 to 75 %
Carbon Dioxide (CO2)
25 to 50 %
Hydrogen (H)
5 to 10 %
Nitrogen (N2)
1 to 2 %
Hydrogen sulphide (H2S)
Traces
Sources: YADAV & HESSE (1981); FAO (1996); PIPOLI (2005); GTZ (2009
Source: MUENCH (2008)

14. Examples: Small-scale Biogas plants
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)
Source: M. WRIGHT, Ashden Awards

15. Examples: Small-scale Biogas Plants
1. Concept
Source: M. WRIGHT, Ashden Awards
Examples: Small-scale Biogas Plants
Source: M. WRIGHT, Ashden Awards
Biogas lamp
Adding greywater to the biogas reactor to optimise moisture conditionss

16. Examples Small-scale Biogas Plants
1. Concept
Examples Small-scale Biogas Plants
The “Mudbooster” Plant
Source: UNKNOWN

17. Examples Small-scale Biogas Plants
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: C. RIECK (2009)
Source: SuSanA
Biogas outlet and manhole with remouvable cover from a underground biogas plant Installed by the NGO TED in Maseru, Lesotho (Susana) 
The manhole is filled with water to keep the clay sealing wet and gas tight. Gas leackage would be indicated by bubbles.

18. 2. How it can optimize SSWM
Biogas plants transform traditional manure management; reducing CH4 and CO2 emission
Biogas substitutes conventional energy sources, reducing reliance on fossil fuel and firewood (CO2)
Digested sludge can substitute chemical fertiliser
Biogas plants can contribute to sustainable sanitation
D. SPUHLER (2010), adapted from: [Accessed: ], BPO (2006) and BUNNY (n.y.)

19. Examples: Biogas Appliances
1. Concept
Examples: Biogas Appliances
Krämer (TBW)
K.P. Pravinjith
Biogas lamps
Biogas cooking stoves
Chang Mai
PBO (2006)
M. Wafler
Biogas generator
Biogas rice cooker
Biogas boiler
Source: UNKNOWN

20. Basics: Process Parameters
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 ( ). (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

21. Basics: Daily manure yield for different cattle
3. Design Principals
Basics: Daily manure yield for different cattle
Sources: OEKOTOP; WERNER et al. (1998)

22. Basics: Gas yields for different feedstocks
3. Design Principals
Basics: Gas yields for different feedstocks
Sources: OEKOTOP; WERNER et al. (1998)

23. Basics: Biogas Guideline data
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)

24. Types of Digester: Bag or Rubber Balloon Biogas Plants (1/2)
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
To reuse or further treatment (e.g. drying bed)
Source: adapted from FAO (1996)
Gas pipe
Inlet
Leveled surface
Biogas accumulates in the top of the bag
Plastic bag
Batch mode: emptying once every few years
Plug-flow reactor: the slurry moves through continuously much like a train a tunnel
Layer of compacted backfill

25. Types of Digester: Bag or Rubber Balloon Biogas Plants (2/2)
3. Design Principals
Types of Digester: Bag or Rubber Balloon Biogas Plants (2/2)
Underground plug-flow reactor bag biogas plant () and balloon biogas collection chamber (). (Philippines, Garry Baron)
Source: [Accessed: ]

26. Types of Digester: Fixed-dome Biogas Plants (1/3)
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
Source: Jan Lam, SNV at NBP Dhaka, March 2007 (left) and Mantopi Lebofa, Lesotho, 2006, both in MUENCH (2008)

27. Types of Digester: Fixed-dome Biogas Plants (2/3)
3. Design Principals
Types of Digester: Fixed-dome Biogas Plants (2/3)
Removable cover
Overflow tank / compensation chamber
Seal
Slurry
Biogas accumulates in the dome
Biogas collection
Inlet
Fixed-dome
Source: adapted from [Accessed: ]

28. Types of Digester: Fixed-dome Biogas Plants (3/4)
3. Design Principals
Types of Digester: Fixed-dome Biogas Plants (3/4)
Source: K.P. PRAVVIJITH
Source: K.P. PRAVVIJITH
Source: K.P. PRAVVIJITH
Source: K.P. PRAVVIJITH

29. 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)

30. Floating-drum Biogas Plants
3. Design Principals
Floating-drum
Biogas
Slurry
Inlet
Outlet
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).

31. Floating-drum Biogas Plants
3. Design Principals
Floating-drum Biogas Plants
Different design of floating drum plants 
 Open gasholder
MUELLER (2007)
MUELLER (2007)
 Floating drum plant with inlet from the the NGO BIOTECH (India)

32. Toilet linked Biogas Reactors
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.

33. Toilet linked Biogas Reactors
3. Design Principals
Toilet linked Biogas Reactors
Source: ??? 33

34. Toilet linked Biogas Reactors
3. Design Principals
Toilet linked Biogas Reactors
Collection and expansions chamber
Gas outlet pipe
Pour-flush toilet
Link of toilet
Inlet for animal waste
Baffle to mix influent with tank contents
Removable cover annual desludging
Biogas reactor
Source: adapted from WELL (n.a.)

35. Toilet linked Biogas Reactors
3. Design Principals
Toilet linked Biogas Reactors
Source: M. WAFLER
Sludge drying bed
Expansions chamber
Biogas reactor
Pour-flush toilet
Manure and green waste mixing chamber
[Accessed: ]

36. 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)

37. 4. Treatment Efficiency Nutrients Biogas slurry = Fertilisers
Anaerobic digestion only removes organics, and the main mineral material and almost all nutrients remain in the bottom sludge.
Phosphorus: almost 100 %
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
Biogas slurry = Fertilisers

38. 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)

39. 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.

40. 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)
Below temperatures of 15°C, biogas production is economically not interesting (heating required)
Requires seeding (start-up can be long due to the low growth yield of anaerobic bacteria)
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
Long life span if maintained and operated correctly
Reduces the amount of wood fuel and improves indoor air quality

41. Thank you for your attention!
Source: ??? 41

42. 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: [Accessed: ]
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: [Accessed: ]
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: [Accessed: ]
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 ). Stockholm: Stockholm Environment Institute (SEI)
MANG, H.-P., (2005): Biogas Sanitation Systems. (=Ecological sanitation course, Norway, 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: [Accessed: ]
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) [Accessed: ]
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: [Accessed: ]
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: [Accessed: ]
WELL (n.y.): Using Human Waste. (=WELL Technical Briefs, No. 63) Loughborough: Water and Environmental health at London and Loughborough (WELL) Available at: [Accessed: ]
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: [Accessed: ]
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)

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