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Lesson 2.


    Anaerobic digesters are biological treatment processes that turn waste into biogas in the absence of oxygen. This biogas is then used to create energy. When waste - either animal or vegetable wastes are put into the digester, anaerobic microbes digest the waste and produce methane as a by-product. The process is able to reduce odors and pathogens in manure and is to make farms more viable.

Anaerobic digesters use anaerobic metablolism to do its work. When microbes digest food without the presence of oxygen they can produce methane and other by-products as a result. In anaerobic digesters, the target is the methane gas. It is important to note that this process must take place in the absence of oxygen otherwise, depending on the type of microbes used, the microbes will start producing carbon dioxide and water or will die.

When manure or other waste is treated by anaerobic digesters and methane is produced, the biogas is taken to a turbine where it is burned. The burning of methane releases the energy stored within the methane molecules. This energy is used to turn the turbines in the plant and produce electricity.


   The methanogenic processesinvolve critical balances between electron acceptors and donors. To function most efficiently, the hydrogen concentration must be maintained ata alow level. If hydrogen and organic acids accumulate, methane production can be inhibited, resulting in a stuck digester.

Anaerobic digestion has many advantages. Most of the microbial biomass produced in aerobic growth is used for methane production in the anaerobic digester. Also, because the process of methane production is energetically very insufficient, the microbes must consume about twice the nutrients to produce an equivalent biomass as that of arrobic systems.

The anaerobic digestion process involves three steps:

  1. The formation of the sludge components to form organic acids, including acetate;
  2. Production of the methanogenic substrate: acetate, carbon dioxide and hydrogen
  3. Methane production by methane producers.

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    Under certain conditions, methane can be ideal microbial waste product: once it is produced under anoxic condition it leaves the microorganism's environment by diffusing up in the water column where it can be oxidized by methanotrophs or release to the atmosphere.

This eliminates the problem of toxic waste accumulation that occurs with many microbial metabolic products, such as organic acids and ammonium ion.


Methane is a greenhouse gas of increasing concern that can be derived from a variety of sources - in the presence of oxygen. These include ruminants, rice paddies and landfills. Landfills especially can release methane to the atmosphere over longer terms. Another interesting sources of atmospheric methane is microorganisms that inhibit the gut of wood-eating termites.

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A small-scale biogas reactor or anaerobic digester is an anaerobic treatment technology that produces

(a) a digested slurry (digestate) that can be used as a fertilizer and

(b) biogas that can be used for energy.

Biogas is a mix of methane, carbon dioxide and other trace gases which can be converted to heat, electricity or light. Small-scale biogas reactors are typically designed to produce biogas at the household or community level in rural areas. The airtight reactors are filled with animal manure from the farm. Kitchen and garden wastes can also be added and toilets can directly be linked to the reactor for co-treatment of excreta.

Schematic of a biogas reactor

The benefits of agricultural small-scale biogas plants

A biogas reactor is an airtight chamber that facilitates the anaerobic degradation of blackwater, sludge, and/or biodegradable waste (e.g. animal manure, kitchen and garden wastes). It also facilitates the collection of the biogas, a mixture of methane (CH4) and carbon dioxide (CO2) produced in the fermentation processes in the reactor. The gas forms in the slurry and collects at the top of the chamber, mixing the slurry as it rises. The pressure exerted by the rising gas can be used to transport the gas to the collection vessel or directly to where it is going to be used. The digestate is rich in organics and nutrients, almost odourless and pathogens are partly inactivated.

Biogas reactors are often installed at household or community level in rural areas for the co-digestion of animal manure and toilet products.

The produced gas can be recovered and used either directly for cooking and lighting or it can be transformed into heat in a gas heater system or into combined heat and power (CHP) in a cogeneration unit. The digestate (nutrient-rich sludge) can be used as fertilising soil amendment in agriculture.

Animal manure and kitchen waste contain a lot of organic matter and generally, the process produces enough biogas for the family to cover at least cooking requirements. Humans produce less excreta, which contains less material that can be converted to biogas than animal dung (e.g. cows). However, toilets, if available can directly be linked to the biogas plant where human faeces are digested together with the other wastes. This option provides a safe treatment of human excreta and thus improves the hygienic situation of the family. The availability of a renewable green energy source reduces the use of firewood for cooking and indoor air pollution. Thus biogas digesters have the potential to minimize health risks and environmental pollution by using human excreta as a resource for producing energy and fertiliser

Suitable digesting temperature20 to 35 °C

Retention time40 to 100 days

Biogas energy6kWh/m3 = 0.61 L diesel fuel

Biogas generation0.3 – 0.5 m3 gas/m3 digester volume per day

Human yields0.02 m3/person per day

Cow yields0.4 m3/Kg dung


Gas requirement for cooking0.3 to 0.9 m3/person per day


Gas requirement for one lamp0.1 to 0.15m3/h

Design Considerations

Biogas reactors can be brick-constructed domes or prefabricated tanks, installed above or below ground, depending on space, soil characteristics, available resources and the volume of waste generated. They can be built as fixed dome or floating dome digesters. In the fixed dome, the volume of the reactor is constant. As gas is generated it exerts a pressure and displaces the slurry upward into an expansion chamber. When the gas is removed, the slurry flows back into the reactor. The pressure can be used to transport the biogas through pipes. In a floating dome reactor, the dome rises and falls with the production and withdrawal of gas. Alternatively, it can expand (like a balloon). Rubber-balloon biogas plants, are the most simple and cheapest ones to construct. To minimize distribution losses, the reactors should be installed close to where the gas can be used.

Anaerobic digestion is a biological process, which is carried out by a special mix of bacteria. When the reactors first are installed, it may take some time until the specific biogas producing bacterial community has installed. It can help to seed the reactor with anaerobic sludge form a septic tank or another anaerobic digester.
The hydraulic retention time (HRT) in the reactor should be at least 15 days in hot climates and 25 days in temperate climates. For highly pathogenic inputs, a HRT of 60 days should be considered. Normally, biogas reactors are operated in the mesophilic temperature range of 30 to 38°C. A thermophilic temperature of 50 to 57°C would ensure the pathogens destruction, but can only be achieved by heating the reactor (although in practice, this is only found in industrialized countries).If the temperature of the biomass is below 15°C, gas production will be so low that the biogas plant is no longer interesting from an economic point of view. At higher temperature, not only methane production can be increased but also free ammonia, which can have an inhibitory effect on the digestion performance.

Small-scale biogas digesters receiving animal waste show higher biogas production rates then bioreactors using only human wastewater as substrate

 A typical plastic bag / rubber balloon biogas plant (left) and balloon plant under construction (right).

Often, biogas reactors are directly connected to private or public toilets with an additional access point for organic materials. At the household level, reactors can be made out of plastic containers or bricks. Sizes can vary from 1,000 L for a single family up to 100,000 L for institutional or public toilet applications. Because the digestate production is continuous, there must be provisions made for its storage, use and /or transport away from the site.

The design size of the reactor depends on the HRT (depending on the temperature) and the volume of fermentation slurry (i.e. the feed material). The required volume is calculated by multiplying the daily amount of fermentation slurry by the HRT.

  • Cattle    ~    Daily manure, % of life-weight Dung = 5    ~    Total Solid = 16    ~    Life-weight = 135-900
  • Buffalo ~ Daily manure, % of life-weight Dung = 5    ~ Total Solid = 14    ~ Life-weight = 340-420
  • Pigs ~ Daily manure, % of life-weight Dung = 2    ~ Total Solid = 16    ~ Life-weight = 30-75
  • Sheep/goats ~ Daily manure, % of life-weight Dung = 3    ~ Total Solid = 30    ~ Life-weight = 30-100
  • Chickens    ~ Daily manure, % of life-weight Dung = 4.5    ~ Total Solid = 25    ~ Life-weight = 1.5-2
  • Human    ~ Daily manure, % of life-weight Dung = 1    ~ Total Solid = 20    ~ Life-weight = 50-80


To predict how much biogas will be produced with the wastes added to the reactor, one needs to know the chemical oxygen demand (COD) of the sludge or the biodegradability constant (total methane produced during a retention time of at least 50 days). Biogas, in general, can be obtained from any organic material, but the COD or biodegradability constant depends on the type of substrate. Animal dung has much higher methane producing potential than human excreta for example. The digestion of human faecal matter alone would not be economically interesting as the produced organic waste from a typical average family would not produced sufficient biogas to cover its needs. Considering a production of 0.12 Kg to 0.6 Kg faeces per day and person and 20 to 150 L Biogas per Kg, the production of biogas would range from some few L to maximal 90 L (as a rule of thumb, 20 to 30 L Biogas per person per day is realistic). This is far less than the amount of biogas required to prepare the meal of one person, which is about 300 to 900 L biogas per day.

Besides kitchen waste, garden wastes and plants can be added to the reactor to increase the biogas generation. Green plants are well suited for anaerobic digestion and their gas yields are high, usually above that of manure. Feed material containing lignin, such as straw or wood resist anaerobic fermentation and should therefore not be used in biogas plants or at least be pre-composted and preferably chopped before digestion.

Small-scale biogas digesters generally follow a wet anaerobic digestion process with an optimal total solid (TS) content of 5 to 10%. The fluid properties of the slurry are important for the operation, as it is easier for the methane bacteria to come into contact with feed material accelerating the digestion process. Animals dung has generally higher TS content then required. To obtain an optimum TS content, substrates can be diluted (with greywater or toilet wastes for instance). Cow dung, for instance, which contains 18% of TS, is diluted with water in the ration of 1:1 (by weight) to obtain the optimum concentration of 9% TS.

Types of Biogas Reactors

The main design elements of small-scale biogas reactors are: an inlet, an airtight reactor chamber, a vessel for biogas collection (e.g. upper part of the reactor, floating drum, plastic balloons, see above picture), and an expansions chamber. Optionally, there are connections from the toilet(s) and a grinder for the kitchen and garden wastes. There are three general types of biogas reactors designs:

  1. rubber-balloon biogas plants,
  2. floating-drum plants and
  3. fixed-dome plants.

1. Rubber-balloon biogas plants

The balloon plant consists of a huge common plastic bag (e.g. PVC) in which the sludge settles on the bottom (there is no expansion chamber) and the gas is collected in the upper part from where it is withdrawn. The inlet and outlet are attached directly to the plastic skin of the balloon and there is now expansion chamber. To transport the gas from the balloon to where it will be used, the needed pressure is achieved through the elasticity of the balloon and can be enhanced by weights placed on the balloon. Besides its extremely low-cost and simplicity, this type of small-scale biogas digester has also the advantage of higher temperatures under sunny sky and its ease for cleaning and emptying. However, its life span can be relatively short as it is susceptible to damage and construction is so easy, that the creation of local employment is minimal. Another option, which is also easy and cheap to build is the construction of a fixed-dome reactor, but to replace the fixed dome by plastic.

2. Fixed-dome reactors

The fixed-dome plants consist of a digester with a fixed, non-movable gasholder, which sits on top of the digester. When gas production starts, the slurry is displaced into the compensation tank. Gas pressure increases with the volume of gas stored and the height difference between the slurry level in the digester and the slurry level in the compensation tank. Fixed-dome biogas plants have relatively low construction costs and do not contain rusting steel (as the floating-drum reactors). Thus, if they are well constructed, they have a long life span. The underground construction saves space and protects the digester from temperature changes. However, temperatures are generally relatively low. The construction provides opportunities for skilled local employment. However, problems of gas-tightness of the brickwork gasholder (or cracks) can cause heavy losses of biogas. Fixed-dome plants are, therefore, recommended only where experienced biogas technicians can supervise construction. The gas pressure fluctuates substantially depending on the volume of the stored gas.

3. Floating-drum reactors

Floating-drum plants consist of an underground digester and a moving gasholder. 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. If the drum floats in a water jacket, it cannot get stuck, even in substrate with high solid content. Floating dome reactors have the advantage that the gas pressure remains constant as it depends on the weight of the gasholder. The construction is relatively easy and construction mistakes do not lead to major problems in functioning and gas yield. However, the material costs of the steel drum are very high and all the steel parts are susceptible to corrosion. Because of this, floating drum plants have a shorter life span than fixed-dome plants, and regular maintenance costs for the painting of the drum arise.

Three types of fixed-dome biogas reactor. Each biogas plant consists of a digester and a gasholder.

Locally made digester setup stocked with cow dungs in Aje Okeagbe Akoko Ondo State Nigeria, January 2012.

Health Aspects/Acceptance

The digestate is partially sanitized but still carries a risk of infection. However, the state of hygienisation of the effluent slurry of biogas digesters strongly depends on the influent concentration in pathogenic microorganism, the retention time and the temperature. High temperatures and long retention times are more hygienic. If more than 55°C are achieved for one to a few days, inactivation can be considered as efficient. Depending on its end-use, further treatment might be required. There are also dangers associated with the flammable gases that, if mismanaged, could be harmful to human health.

Anaerobic digestion only removes organics, and the main mineral material and almost all nutrients remain in the bottom sludge. Almost 100% of the phosphorus and about 50 to 70 % of the nitrogen as ammonium is still found in the digested sludge. Therefore, the secondary product compost from biogas reactors is a valuable resource for food production. Generally, in a well-functioning and designed biogas digester, the pathogen removal in the slurry is sufficient so the treated sludge can be reused for soil fertilisation. To increase the safety, it may be aerobically composted (or processed in a sludge drying or humification bed) before reuse.

Costs Considerations

Investment costs of anaerobic digesters are moderate and the potential of self-help is relatively high (even though planning requires skilled labour and expert design). Both biogas and fertilising sludge create value added, thus making biogas digesters interesting from an economic point of view.

Operation & Maintenance

If the reactor is properly designed and built, repairs should be minimal. To start the reactor, it should be inoculated with anaerobic bacteria, e.g., by adding cow dung or septic tank sludge. Biogas reactors need a start-up phase during which the microorganism responsible for anaerobic digestion instal and stabilise. Organic waste used as substrate should be shredded and mixed with water or digestate prior to feeding.

Gas equipment should be carefully and regularly cleaned so that corrosion and leaks are prevented. Grit and sand that have settled to the bottom should be removed. Depending on the design and the inputs, the reactor should be emptied once every 5 to 10 years. The reactors should also regularly be checked for foaming, scum formation or gas tightness (and rusting in the case of floating drum reactors using a steel drum). No skilled operator is required, but households should be trained to understand the system. 

Working Principle

Animal manure is mixed with human wastes and crop residues in an airtight reactor, where it is decomposed in absence of oxygen and with a relatively high moisture content. Organic wastes are transformed into biogas, which can be reused for cooking, lightening or as another energy source. Nutrients (nitrogen and phosphorus) remain in the digested slurry which ca be used as a fertiliser in agriculture.



This type of reactor is generally constructed in rural areas where animal dung and space is available and the demand for biogas (as alternative energy) and fertiliser is high. But they can also be adapted to urban areas in the case of a high demand for biogas.



80 to 85 % BOD; Relatively high pathogen removal; N and P remain in the sludge; HRT from one week up to several month depending on T



Low capital and low operating costs

Self-help Compatibility

Design needs to be done by expert and construction should be supervised; operation staff needs to receive training to understand the functioning. Can be constructed with locally available material.


De-sludging should not really be required but cleaning (removal of stones and other indigestible material) may be useful; Checking for gas-tightness should be done regularly as well.


Resistant to shock loading. Reliable if operated and maintained well.


Main strength

High removal of organic pollutants without any requirement for energy; Generation of biogas and fertiliser (compost).


Main weakness

Expert design and supervision of construction is required; The organic and solid content in the influent needs to be monitored.

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This technology can be applied at the household level, in small neighbourhoods or for the stabilization of sludge at large wastewater treatment plants. Biogas reactors provide energy for cooking, lighting and heating as well as fertilising sludge for soil improvement. In rural areas, they are often used for the digestion of animal manure, kitchen waste (and only optionally toilet wastes).

Often, a biogas reactor is used as an alternative to a septic tank, since it offers a similar level of treatment, but with the added benefit of biogas. However, significant gas production cannot be achieved if blackwater is the only input. The highest levels of biogas production are obtained with concentrated substrates, which are rich in organic material, such as animal manure and organic market or household waste. It can be efficient to co-digest blackwater from a single household with manure if the latter is the main source of feedstock. Greywater should not be added as it substantially reduces the HRT. Wood material and straw are difficult to degrade and should be avoided in the substrate.

Biogas reactors are less appropriate for colder climates as the rate of organic matter conversion into biogas is very low below 15°C. Consequently, the HRT needs to be longer and the design volume substantially increased.

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