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Anaerobic Digestion – An Introduction and Commercial Status in the US – As of 2006

(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.

The process of anaerobic digestion consists of three steps.
The initial step is the lysis (breaking open of the original cells) and then decomposition (hydrolysis) of plant or animal matter. This step breaks down the organic content of the feedstock (known as substrate) to usable-sized molecules such as sugars.ᅠ
The second step is the conversion of this disintegrated matter to organic acids. The acids go through further complex biological reactions and are transformed to methane gas.
The process temperature critically affects the rate of digestion and must be held within the mesophillic range (95 to 105 degrees Fahrenheit) with an optimum of around 100 degrees F. It is possible to run digesters in the thermophillic range (135 to 145 degrees Fahrenheit), however the more rapid digestion procedure that takes place, needs to be more highly monitored and to avoid instability and to be reliable, have to be more carefully kept track of.
Numerous anaerobic digestion innovations have been developed over the past 10 to 15 years and are now commercially proven. This is leading to an increasing number of these plants being installed, especially on farms.
Back in 2006 when an equivalent to this article was originally published by the California Energy Commission ( a number of experimental flagship projects had been underway for some time, and this lead the author to state that:
“anaerobic processes are readily available and have actually been shown for use with agricultural wastes and for dealing with municipal and industrial wastewater”.
The following projects were described, as follows:ᅠ
“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.
The 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.
Given 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].”
Most anaerobic digestion technologies are commercially available. Where unprocessed wastes cause odor and water pollution such as in large dairies, anaerobic digestion reduces the odor and liquid waste disposal problems and produces a biogas fuel that can be used for process heating and/or electricity generation.
This article has been used as an updated citation for Wikipedia at:

Read more about anaerobic digestion.

One Response to Anaerobic Digestion – An Introduction and Commercial Status in the US – As of 2006

  1. Eugene Warren March 3, 2015 at 6:14 pm #

    I value you sharing your interest in AD. Really looking forward to read more.

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