Types Of Anaerobic Digesters Part 2

Anaerobic Digesters

The process of anaerobic digestion occurs in a sequence of stages involving distinct types of bacteria.  Hydrolytic and fermentative bacteria first break down the carbohydrates, proteins and fats present in biomass feedstock into fatty acids, alcohol, carbon dioxide, hydrogen, ammonia and sulfides.  This stage is called “hydrolysis” (or “liquefaction”).

Next, acetogenic (acid-forming) bacteria further digest the products of hydrolysis into acetic acid, hydrogen and carbon dioxide.  Methanogenic (methane-forming) bacteria then convert these products into biogas.

The combustion of digester gas can supply useful energy in the form of hot air, hot water or steam.  After filtering and drying, digester gas is suitable as fuel for an internal combustion engine, which, combined with a generator, can produce electricity.  Future applications of digester gas may include electric power production from gas turbines or fuel cells.  Digester gas can substitute for natural gas or propane in space heaters, refrigeration equipment, cooking stoves or other equipment.  Compressed digester gas can be used as an alternative transportation fuel.

Manure Digesters

Anaerobic digestion and power generation at the farm level began in the United States in the early 1970s. Several universities conducted basic digester research.  In 1978, Cornell University built an early plug-flow digester designed with a capacity to digest the manure from 60 cows.

In the 1980s, new federal tax credits spurred the construction of about 120 plug-flow digesters in the United States. However, many of these systems failed because of poor design or faulty construction.  Adverse publicity about system failures and operational problems meant that fewer anaerobic digesters were being built by the end of the decade.  High digester cost and declining farm land values reduced the digester industry to a small number of suppliers.


Municipal sewage contains organic biomass solids, and many wastewater treatment plants use anaerobic digestion to reduce the volume of these solids.  Anaerobic digestion stabilizes sewage sludge and destroys pathogens.  Sludge digestion produces biogas containing 60-percent to 70-percent methane, with an energy content of about 600 Btu per cubic foot.

Most wastewater treatment plants that use anaerobic digesters burn the gas for heat to maintain digester temperatures and to heat building space.  Unused gas is burned off as waste but could be used for fuel in an engine-generator or fuel cell to produce electric power.

Landfill Gas

The same anaerobic digestion process that produces biogas from animal manure and wastewater occurs naturally underground in landfills.  Most landfill gas results from the decomposition of cellulose contained in municipal and industrial solid waste.  Unlike animal manure digesters, which control the anaerobic digestion process, the digestion occurring in landfills is an uncontrolled process of biomass decay.

The efficiency of the process depends on the waste composition and moisture content of the landfill, cover material, temperature and other factors.  The biogas released from landfills, commonly called “landfill gas,” is typically 50-percent methane, 45-percent carbon dioxide and 5-percent other gases.  The energy content of landfill gas is 400 to 550 Btu per cubic foot.

Capturing landfill gas before it escapes to the atmosphere allows for conversion to useful energy.  A landfill must be at least 40 feet deep and have at least one million tons of waste in place for landfill gas collection and power production to be technically feasible.

A landfill gas-to-energy system consists of a series of wells drilled into the landfill.  A piping system connects the wells and collects the gas.  Dryers remove moisture from the gas, and filters remove impurities.  The gas typically fuels an engine-generator set or gas turbine to produce electricity.

The gas also can fuel a boiler to produce heat or steam. Further gas cleanup improves biogas to pipeline quality, the equivalent of natural gas.  Reforming the gas to hydrogen would make possible the production of electricity using fuel cell technology.


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