In anaerobic digestion, microorganisms break down biodegradable material and produce methane. In an industrial setting, the process can be used to manage waste and to produce biogas fuel. A properly managed commercial anaerobic digester system can reduce the environmental impact of manure and food waste, while also producing energy that results in financial savings and revenues.

How It Works

An anaerobic digester is a vessel sized to grow and maintain a population of methane bacteria that feed on organic wastes placed in the unit. Manure consists of partially decomposed feed, waste feed and water. Manure alone or mixed with process water and flush water is generally too concentrated to be decomposed aerobically in a manure treatment or storage structure, because oxygen cannot diffuse into solution fast enough to support aerobic bacteria. Therefore, manure is broken down sequentially by groups of anaerobic bacteria. The bacteria grow without oxygen, decompose the waste, and produce methane as a useable fuel byproduct.

Anaerobic Digester System Components

An anaerobic digester system includes:

  • manure or food waste collection
  • pretreatment
  • an anaerobic digester
  • biogas recovery
  • biogas handling
  • biogas use

The digester may be heated or unheated, mixed or unmixed, a simple tank or a very complicated media packed column. The decision of which type of anaerobic digestion technology is used depends upon the characteristics and the collection technique of the manure and food waste.

How Fuel is Collected

Biogas formed in a digester bubbles to the surface and may be collected by a fixed rigid top, a flexible inflatable top or a floating cover depending upon the type of digester. The collection system directs biogas to gas handling components, where it may be filtered. Biogas is usually pumped or compressed to operating pressure and then metered to the gas use equipment.

How Fuel is Used

Biogas that is pressurized and metered can be used as fuel for heating, absorption cooling, electrical generation and cogeneration. Biogas can also be substituted for low pressure natural gas or propane in biogas fueled engines and direct combustion equipment.

Other Benefits

In addition to producing fuel, biodigesters reduce the odor from manure and food wastes. Anaerobic digestion is more extensively used outside of the U.S. where concern for treatment of animal waste has been a concern for a longer time. Particularly for dairies, a digester improves the handling capability of the manure and saves the farm money on materials handling. In addition, digester systems enable diaries to recover digested solids that can be used for bedding. The first pig manure digester systems in the U.S. were installed principally to control manure odors. It has been suggested that recent U.S. episodes of surface water contamination with E. coli, cryptosporidia, and pfiesteria blooms are linked with manure discharges. Anaerobic digestion systems eliminate those pathogens.

For more details on Anaerobic Digestion, read Farm Digesters and Digestion, an article by RCM Founder Mark Moser.