Biodigesters, also known as anaerobic digesters (AD), are anaerobic treatment systems used to convert animal waste, human excreta and other organic waste to a nutrient-rich slurry suitable for fertilizer and burnable biogas (methane). The waste is digested through anaerobic fermentation by microorganisms naturally present in the waste. The fertilizer is pathogen-free, rich in nitrogen, phosphorus and potassium providing a sustainable means of enhancing agricultural soils through recovery and reuse. Waste digestion is usually accomplished via an airtight container in which the organic waste matter is diluted with water and the application of various mixing methods. There are several basic types of biodigesters for human waste:

• Stand-alone - fee-based biodigesters accepting organic waste, organics recycling, and on-site industry based typically accommodating food waste but can also include other wastes
• Water Resource Recovery Facilities (WRRF) - municipal or publicly owned plants focused on resource recovery
• Septic tank and Latrines (Bio Latrines) - variations on the conventional septic tank and pit latrines that facilitate bio digestion; Typically residential and commercial single site (PODTANKS Non-Electric Sewage Treatment Plant System).

Federal and state governments in the U.S., as well as in industry, are now referring to Wastewater Treatment Facilities as Water Resources Recovery Facilities (WRRF), in which the management of waste includes anaerobic digesters in the production of clean water, recovery of nutrients, and renewable energy; clearly a progressive turn toward mainstreaming resource recovery in environmentally sustainable ways. There are over 1,200 water resource recovery facilities in the United States, with over half using the biogas they produce as an energy resource for electricity or heating (Water Environment Federation). Thirty percent generate electricity from biogas to run the facility, ten percent are able to sell electricity back to the grid, and two percent generate biogas of high enough quality for natural gas pipelines. On a smaller scale, conventional septic tanks include both aerobic and anaerobic digestion to degrade the biomass that settles in the tank, leaving sludge that needs to be periodically pumped out for disposal. However, septic tank conversions to biodigesters (Septic Tank Conversion) are commercially available for single residences, as are biodigester wastewater treatment systems based on the same technology. Another promising emerging biodigester technology is the Omni Processor, which produces clean drinkable water, energy and ash that can be used in construction. The Omni Processor is currently processing about one third of Dakar’s sludge.

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