(Published: 7 December 2014 – Replaces a Wikipedia broken link citation which had been broken for about 5 years. Blue text represents the most important text for citation purposes.)
Anaerobic digestion is a biological process that produces a gas mainly composed of methane (CH4) and (CO2) otherwise known as biogas. These gases are produced from organic wastes such as livestock manure, food processing waste, energy crops (e.g. maize), and so on.
Anaerobic processes either occur naturally or take place in a controlled environment such as a biogas plant. Organic waste such as livestock manure and numerous types of bacteria, are put in an airtight container called a digester so the fermentation process might take place.
Depending upon the properties of the waste feedstock and the system design, the resulting biogas is typically 55 to 75 percent pure methane. Cutting edge systems which subsequently purify this gas report producing an upgraded quality of biogas that is equivalent in composition to natural gas.
This “upgraded biogas” is more than 95 percent pure methane, and is referred to as biomethane. When biomethane is compressed it can be used as CNG to fuel transport vehicles, and many gas networks will accept it (subject to detailed quality controls) for injection into the regional natural gas grid/ supply system.
“anaerobic processes are readily available and have actually been shown for use with agricultural wastes and for dealing with municipal and industrial wastewater”.
“At Royal Farms No. 1 in Tulare, California, hog manure is slurried and sent to a Hypalon-covered lagoon for biogas generation. The collected biogas fuels a 70 kilowatt (kW) engine-generator and a 100 kW engine-generator. The electricity generated on the farm is able to satisfy regular monthly electric and heat need.ﾠThe biogas produce is burned in a boiler. The biogas produced is used to fire an 85 kW gas engine. The engine operates at 35 kW availability level and drives a generator to produce electrical energy. Most anaerobic food digestion innovations are commercially available. Where unprocessed wastes trigger odor and water pollution such as in big dairy product establishments, anaerobic digestion decreases the odor and liquid waste disposal problems, and also produces a biogas fuel. This fuel is an alternative energy source that can be made use of for on-farm heating and/or electrical power generation. If enough spare energy is available output can be sent to the local electricity supply, and there is also potential for waste heat in the form of hot water to be used for heating etc.
ﾠGiven the success of this project, three other swine farms (Sharp Ranch, Fresno and Prison Farm) have also installed floating covers on lagoons. The Knudsen and Sons project in Chico, California, treated wastewater which contained organic matter from fruit crushing and wash-down in a covered and lined lagoon. The biogas produce is burned in a boiler. And at Langerwerf Dairy in Durham, California, cow manure is scraped and fed into a plug flow digester. The biogas produced is used to fire an 85 kW gas engine. The engine operates at 35 kW capacity level and drives a generator to produce electricity. Electricity and heat generated is able to offest all dairy energy demand. The system has been in operation since 1982 [current 2014 operational status not known].”
Read more about anaerobic digestion.