Methane Digesters

What are Methane Digesters?

Methane digesters are oxygen-free tanks or containers that use microorganisms (i.e., different types of bacteria) to transform biomass (e.g., cow manure, waste-water) into ‘biogas’ (e.g., methane and carbon dioxide).  This biogas can then be fed to a gas engine to generate electricity, or to a boiler to generate heat. As explained by the Vermont Agency of Agriculture, Food and Markets, there are three common types of digester systems used in the United States:

  1. Covered lagoons:  with this type of system, manure lagoons are covered with a tarp that funnel biogas to a generator. These systems are not considered practical in Vermont, since low ambient temperatures inhibit bacteria activity.
  2. Plug flow systems:  these systems, which consist of concrete tanks that hold manure for about 20 days, are common in Vermont.
  3. Mixed tanks:  these systems work the same as plug flow systems but may contain lower solids and the effluent may be stirred.

According to GMP Cow Power, 1 cow can produce about 30 gallons of manure a day which, in turn, can generate enough electricity to power two 100-watt lightbulbs for 24 hours. The waste from 4-6 cows can generate about 1 kw of electricity.

There are several points worth noting that are not featured or modeled in the Atlas:

  • The bedding type used on the farm makes a difference (e.g., farmers that use sand would not run that bedding through a digester).
  • The amount of time cows spend outside grazing makes a difference (e.g., the manure that hits the ground is not collected).