Biogas Production Plants
Biogas production plants use anaerobic digestion to produce methane gas from animal, human, and food waste.
Biogas is the product of a biological process called anaerobic digestion. Bacteria decompose organic matter, in the absence of oxygen, and produce a gas that is approximately 60% methane and 40% carbon dioxide. The carbon dioxide is not removed before the biogas is used. Biomass sources for biogas include animal, human, and food waste; cellulosic agricultural products; and landfills.
Biogas Production Plant Components – On-Farm Digester
An on-farm biogas production plant includes the following major components:
Manure from farm animals (dairy, swine, and poultry) is collected and stored as a liquid, slurry, or semisolid. It is then transported to an anaerobic digester. Three types of anaerobic digesters are in common use:
The gas is collected from the digester and piped to a combustion device to be used for space or water heating, or to generate electricity. The biogas may require additional treatment or scrubbing to refine it for use as a fuel. The remaining liquid and solids are separated. The nutrient-rich liquid is returned to the farm for reuse or for land application as a fertilizer. The solid fiber can be used for composting as a soil conditioner.
Biogas Production Plant Components – Municipal Wastewater Sludge Digester
If the facility was located at the wastewater treatment plant, the sludge would be delivered directly to receiving bins. Transport vehicles would unload sludge generated elsewhere into the bins. The sludge would pass through grinders and shredders to decrease the particle size of the organic material and remove stones and other solids. The sludge would then enter a mixer that adds water to reduce the solids concentration and would next be pumped to a buffer container to be heated, conditioned, and possibly stored for several days. The fluidized sludge would then be injected into the digester in small doses approximately 10 to 15 times a day. The sludge would be heated to about 95 degrees F and remain in the digester from 10 to 30 days. The produced biogas would be collected and passed through a gas conditioning system to remove moisture and other contaminants such as hydrogen sulfide. The conditioned biogas would then be delivered to the end user, which is most often a generating system that would produce both electricity and hot water or steam.
Once digested, the sludge would be pumped to a heated post-fermentation vessel where it would continue to off-gas for approximately 5 days. The wastewater sludge would next enter equipment that separates the solids from the liquid. The solids would be transferred to a transport container and transported off-site for further processing or disposal. The liquid would be returned to the waste treatment plant.
Biogas Production Plant Size
A biogas production plant typically requires 10 to 20 acres for all the facilities. The total area of the site is usually larger, landscaped, and serves as an exclusion area to the public.