A gas produced by the breakdown of organic matter in the absence of oxygen.
- Biogas is carbon neutral - 
- Poop to biogas car, Fast Company - 
- Industry growing at 12% per year. 
- Used for cooking, heating, electrical generation
- Compressed and cleaned biogas for transportation with internal combustion engines
- Liquid slurry from digester tank can be used to fertilize duckweed pond
- Solid contents from digester can be composted (vermicompost, BSF compost, biochar compost)
- The CO2 contained in biogas can be piped into a greenhouse to give a boost to plant growth (CO2 concentrations up to 1500ppm may be beneficial). This is effect widely used by commercial growers, who are using mostly fossil fuel gases (heating/CO2 Enrichment).
- Production of Direct reduced iron from rust or high-grade iron ore.
- Various other uses in metallurgy and glassworks
- Oxy-fuel torch cutting
To think about
- Biogas production may need insulation or earth sheltering when used in cold climates witch could otherwise lead to breakdown of microbial populations. With CEB-blocks and insulation and option of heating, this is a design issue.
- Biogas can contain some sulfur (that smells like rotting eggs) and it is then not recommended to be used in engines for a prolonged time. The sulfuric acid can accumulate in the engine oil and it will need replacing and or cleaning after a while. To avoid some of this do not feed the digester with objects that generate this gas like plasterwalls.. There exists some additive that can limit this but should not be needed in this case. When used for direct burning like with a gas-nozzle for cooking it can be used directly. But some don't like that smell when cooking.
- Compressing into gas cylinders if needed for mobile transportation and more easy handling.
- Create smaller pieces of bulky biomass before it go into the digester to make it more effective, Could be using the hammer mill
A sealed container that carries out the anaerobic digestion process. It needs to be able to separate and remove the liquid and gas phases and have an entrance/exit for the substrate. Sometimes heated and stir the content to produce even more. Some new digesters have a step before the liquid is exiting the digester there it is heated up to 70 degree Celsius to remove some more of possible pathogens that could have survived.
- Guastavino masonry or plain CEBs to build biogas digester and holding tank for stored biogas. One holding tank could be used for the storage of biogas and Compressed fuel gas.
Cleaning or upgrading
Biogas needs cleaning or upgrading to remove as much as possible of the contained sulfur and or carbon dioxide and reach a higher energy value (see Appropedia: Biogas hydrogen sulfide scrubbing project and Biogas CO2 scrubbing project). The scrubbed carbon dioxide can be fed into a greenhouse for CO2 Enrichment. Upgrading is also necessary to be able to use biogas in engines for mobile gas-car transportation or electrical generation and limit corrosion and wear in motors.
See Biogas Upgrader
Simple Open Source Biogas System
Supergas - In 1996-97 Superflex collaborated with biogas engineer Jan Mallan to construct a simple, portable biogas unit that can produce sufficient gas for the cooking and lighting needs of an African family. The system has been adapted to meet the efficiency and style demands of a modern African consumer. It is intended to match the needs and economic resources that we believe exist in small-scale economies. The orange biogas plant produces biogas from organic materials, such as human and animal stools. For a modest sum, a family will be able to buy such a biogas system and achieve self-sufficiency in energy. The plant produces approx. 3-4 cubic meters of gas per day of the dung from 2-3 cattle. This is enough for a family of 8-10 members for cooking purposes and to run one gas lamp in the evening.
- UF thesis project - 
- 'Environmentalists' do not like CAFO-produced biogas 
- 20-800 cubic meters of methane per ton of biomass 
- 22 MJ/m3  - half the energy of a liter of standard fuel. Thus, at best 1/2 the per mass energy of standard fuels. Not bad, considering we can use waste such as grass clippings.
- Open Source Biodigester
- Biogas Upgrader
- Biogas start up
- Silage - can be used as feedstock for anaerobic digestion
- Gas Holder