Anaerobic Digestion (Small-scale)

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 (MES et al. 2003; JENSSEN et al. 2004; WRAPAI 2009). Find more information on the use of biogas in the factsheets about direct use of biogas and small-scale conversion of biogas to electricity. 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 (GTZ 2007).

Biogas guideline data. Adapted from WERNER et al. (1989); ISAT/GTZ (1999), Vol. I; MANG (2005)

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. For more information on the different types of biogas reactors read the section “Types of Biogas Reactors”.

Anaerobic digestion is a biological process, which is carried out by a special mix of bacteria (see also the factsheet on anaerobic treatment of waste and wastewaters). 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 (ISAT/GTZ 1999, Vol. I). At higher temperature, not only methane production can be increased but also free ammonia, which can have an inhibitory effect on the digestion performance (ISAT/GTZ 1999, Vol. I).

Temperature ranges for anaerobic fermentation and respective required retention times. Source: WERNER et al. (1989) and MANG (2005)

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. For more information on different small-scale biogas reactors see the section below.

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.

Standard life-weight values of animal husbandry and average manure yields (dung and urine) as percentages of life-weight (adapted from WERNER et al. 1989)

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 MES et al. 2003). Biogas, in general, can be obtained from any organic material (SASSE 1988), 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 (JOENNSSEN 2004; YONGFU 1992) and 20 to 150 L Biogas per Kg (FAO 1996; SASSE 1996; GTZ 2009), 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 (ISAT/GTZ 1999, Vol. II).

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 (WERNER et al. 1989). Feed material containing lignin, such as straw or wood resist anaerobic fermentation and should therefore not be used in biogas plants (WERNER et al. 1989) or at least be pre-composted and preferably chopped before digestion (SASSE 1988).

Biogas production per category of animal and human faeces.

Small-scale biogas digesters generally follow a wet anaerobic digestion process with an optimal total solid (TS) content of 5 to 10% (SASSE 1988; NIJAGUNA 2002). 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 (MAZUMDAR 1982; NIJAGUNA 2002).

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 below), 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: rubber-balloon biogas plants, floating-drum plants and fixed-dome plants.

Rubber-balloon biogas plants

(adapted from ISAT/GTZ 1999, Vol. I)

Rubber-balloon biogas plants are the most simple and cheapest ones to construct. 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 (see below), but to replace the fixed dome by plastic.

Fixed-dome reactors

(adapted from ISAT/GTZ 1999, Vol. I)

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.

Floating-drum reactors

(adapted from ISAT/GTZ 1999, Vol. I)

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.

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 (SASSE 1988). If more than 55°C are achieved for one to a few days, inactivation can be considered as efficient (SCHOENNING & STENSTROEM 2004). 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 (JOENSSEN et al 2004). 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.

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.

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.