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[[Image:Anaerobic_digestion.PNG|thumb|right|150px|Biogas reactor in Vietnam (for credits, click the picture)]]
[[Image:Biogas_as_source_of_energy.PNG|thumb|right|150px|[[Biogas as source of energy |Biogas as source of energy]], in Bangladesh (for credits, click the picture)]]
'''An Anaerobic Biogas Reactor is an anaerobic treatment technology that produces (a) a digested slurry to be used as a soil amendment and (b) biogas which can be used for energy. Biogas is a mix of methane, carbon dioxide and other trace gasses that can be easily converted to electricity, light and heat.'''
An Anaerobic Biogas Reactor is a chamber or vault that facilitates the anaerobic degradation of blackwater, sludge, and/or biodegradable waste. It also facilitates the separation and collection of the biogas that is produced. The tanks can be built above or below ground. Prefabricated tanks or brick-constructed chambers can be built depending on space, resources and the volume of waste generated.
The hydraulic retention residence time (HRT) of the fluid in the reactor should a minimum of 15 days in hot climates and 25 days in temperate climates. For highly pathogenic inputs, a HRT residence time of 60 days should be considered. Normally, Anaerobic Biogas Reactors are not heated, but to ensure pathogen destruction (i.e. a sustained temperature over 50°C) the reactor should be heated (although in practice, this is only found in the most industrialized countries).
Once waste products enter the digestion chamber, gases are formed through fermentation. The gas forms in the sludge but collects at the top of the reactor, mixing the slurry as it rises. Biogas reactors can be built as fixed dome or floating dome reactors. In the fixed dome reactor 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 will flow back down into the digestion chamber. The pressure generated can be used to transport the biogas through pipes. In a floating dome reactor, the dome will rise and fall with the production and withdrawal of gas. Alternatively, the dome can expand (like a balloon). Most often biogas reactors are directly connected to indoor (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 and can be built behind the house or buried underground. Sizes can vary from 1,000L for a single family up to 100,000L for institutional or public toilet applications.
The slurry that is produced is rich in organics and nutrients, but almost odourless and partly disinfected (complete pathogen destruction would require thermophilic conditions). Often, a biogas reactor is used as an alternative to a conventional septic tank, since it offers a similar level of treatment, but with the added benefit of biogas. Depending on the design and the inputs, the reactor should be emptied once every 6 months to 10 years.
Depending on the soil, location, and size required, the reactor can be built above or below ground (even below roads). For more urban applications, small biogas reactors can be installed on the rooftops or in a courtyard. To minimize distribution losses, the reactors should be installed close to where the gas can be used. Biogas reactors are less appropriate for colder climates as gas production is not economically feasible below 15°C.
Biogas is a mixture of methane (60%) and carbon dioxide (40%), produced by the anaerobic digestion of organic material, usually animal dung, human excreta and crop residue. Small-scale biogas digesters provide fuel for domestic lighting, cooling and cooking. Large-scale biogas plants are able to produce sufficient gas to fuel engines to generate electricity. The (thermal) energy available from biogas is about 6 kWh/m³. This corresponds to half a litre of diesel oil and 5.5 kg of firewood. 1 kg of human faeces generates about 50 litres of biogas: 1 kg of cattle dung delivers 40 litres of biogas, and 1 kg of chicken droppings generates about 70 litres of biogas.
==Health Aspects/Acceptance==
