Anaerobic lagoon

Anaerobic lagoons are used to dispose of animal waste, particularly that of cows and pigs. The waste is washed into the lagoon by flushing the animal pens with water. Solid waste, particularly the fibrous type of cows, is sometimes separated before the wastewater enters the lagoon to prevent the buildup of solid material. Anaerobic organisms naturally present in the manure and the environment decompose the waste in the anaerobic conditions of the lagoon.

Areas with cold winters are inappropriate for anaerobic lagoons because the activity of the microorganisms is highly dependent on temperature. It is critical to have the proper size for the lagoon, with volume being more important than surface area. A minimum of two meters is necessary for anaerobic conditions, but the depth should not exceed 6 meters. Sometimes a secondary lagoon is used to accept wastes while the primary lagoon is undergoing maintenance or for other purposes.

If the anaerobic lagoon system is being used for energy production, the primary lagoon has a cover floating on the surface of the water. The cover captures the biogas produced by anaerobic bacteria. The biogas produced by anaerobic lagoons is 50 to 75% methane, with carbon dioxide making up most of the rest. The gas is usually used to produce electricity using a microturbine or reciprocating engine, but it can also be used for water or space heating. The gas usually undergoes pretreatment, particularly dehydration, prior to combustion. Sometimes the carbon dioxide, which is incombustible, is also removed.