The science of a biogas digester may seem complicated, but it is actually quite simple.
Archaea, a microorganism, is found in manure. These microorganisms live in environments where there is heat and no oxygen. Archaea, specifically Methanogens, are the chief creators of the biogas output: 70% Methane, 30% Carbon Dioxide, and Less than 1% of Hydrogen Sulfide.
A digester is a system that utilizes these microorganisms to create biogas by replicating the environment that they are commonly found, a stomach.
Other factors in the digester are the Carbon to Nitrogen ratio, maintaining a 25:1 ratio of carbon to nitrogen mass in which cow manure is naturally prone to be; a 40:50:10 ratio of biomass to liquid to gas space, as this provides an environment for Archaea to flourish and enough biomass to maintain a system; and maintaining a pH of 7 at the output valve, where the digestate comes out.
Digestate is the term used to describe the “sludge” that exits the system. This “sludge” or grey water is comprised of nutrients leftover by the digestion process and the water in the system. In our system, we are planning on incorporating the digestate with our school’s hydroponic garden.
Our digester also has a scrubber attached to it. A “scrubber” is a something used to “scrub” out the Carbon Dioxide in the gas mixture. The scrubber is full of water and gas is injected into it; the Carbon Dioxide is dissolvable in water, thus the CO2 is separated from the mixture. For our system, the CO2 dissolved water will be used in our school’s Algae Farm.