The Breakdown on Anaerobic Digestion

Anaerobic digestion is an old technology that has recently generated renewed interest because of increasing energy costs and efforts to reduce greenhouse-gas emissions. Historical records indicate that biogas derived from anaerobic digestion was used to heat bath water in Assyria during the 10th century B.C. More recently, anaerobic digestion has been used for more than 100 years for the treatment of municipal wastewater treatment sludge and industrial wastes and wastewaters. It is also the process that converts municipal landfill solid waste to biogas.

The Energy and Environmental Research Center has conducted numerous projects utilizing anaerobic treatment, ranging from anaerobic digestion of waste potatoes and animal manure, enhancing the digestion of municipal wastewater sludge, to anaerobic treatment of gasification condensates and agricultural processing wastewaters. The key point to understand is that anaerobic processes are carried out by living organisms in the absence of molecular oxygen. Anaerobic digestion to produce a methane-rich biogas involves a symbiotic relationship between two different types of bacteria. Acid-forming bacteria convert complex organic matter (carbohydrates, proteins and fats) into low molecular weight compounds including acetic acid, hydrogen and carbon dioxide. Methane-producing bacteria then convert acetic acid, hydrogen and carbon dioxide into methane.

A careful balance must be maintained between the acid-forming and methane-producing bacteria to ensure stable biogas production. This requires careful control of temperature, pH and substrate (digester feed material) loading rate. Because acid formers have a much higher specific growth rate and are more tolerant of changes in temperature, pH and loading rates, their intermediate products (organic acids) can rapidly accumulate, resulting in inhibition of the slower-growing methane-producing bacteria, which ultimately results in process upset and loss of methane production. A properly maintained anaerobic digester, however, is capable of producing a methane-rich biogas that can be burned to produce heat, used in a boiler to produce steam, used in an internal combustion engine or turbine to produce electricity, or cleaned up and sold to a gas pipeline company.

A key niche for anaerobic digestion is the conversion of organic waste materials to biogas. Waste materials often have high water content that reduces their potential for combustion processes because the energy required to dry the materials exceeds the value of the energy recoverable through combustion. Anaerobic digestion reduces both the volume and mass of the waste materials and typically produces a product that is readily dewatered. The character of the waste material (chemical and physical properties) must be considered when the suitability of anaerobic digestion is evaluated. Substrate composition is a key factor in determining the methane yield and methane production rates from the digestion of biomass.

In an upcoming issue, we'll discuss a new EERC project, funded through the third funding cycle of the Xcel Energy Renewable Development Fund, which will test and demonstrate a novel biotechnology at a dairy in Minnesota. The technology is aimed at enhancing anaerobic digestion of dairy manure to generate a biogas having increased methane content and significantly reduced hydrogen sulfide to produce heat, steam or power.

Dan Stepan is a senior research manager at the EERC in Grand Forks, N.D. He can be reached at or (701) 777-5247.