Anaerobic digesters are biological treatment processes that turn waste into biogas. This biogas is then used to create electricity. When waste—either animal waste or vegetable waste—is put into the digester, anaerobic microbes digest the waste and produce methane as a by-product. This biogas is then burned to make electricity. The process is able to reduce odors and pathogens in manure and is able to make farms more viable.
Anaerobic digesters use anaerobic metabolism to do its work. When microbes digest food without the presence of oxygen they can produce methane and other by-products as a result. In anaerobic digesters, the target is the methane gas. It is important to note that this process must take place in the absence of oxygen. Otherwise, depending on the type of microbes used, the microbes will start producing carbon dioxide and water or will die.
When manure or other waste is treated by anaerobic digesters and methane is produced, the biogas is taken to a turbine where it is burned. The burning of methane releases the energy stored within the methane molecules. This energy is used to turn the turbines in the plant and produce electricity. Any heat that is wasted can also be used. Many times it is used as heating for the building or for hot water on farms.
Proponents of anaerobic digesters site several benefits to its use. They state that the digesters are able to produce energy by burning methane from organic waste rather than fossil fuels. They also say these digesters can improve manure storage and handling methods. Locally, they are able to reduce odors and the seeds of weeds that find their way into manure.
Proponents also site regional and global reasons for using them. Regionally, they claim that anaerobic digesters can reduce pathogens in manure without reducing nutrients. They also say that this treatment process can provide electrical grid support for cities. Globally, anaerobic digesters can reduce methane emissions. It can also be used as a method to reduce dependency on fossil fuels.
There are those, however, who do not agree that anaerobic digesters are the best option. Opponents say that anaerobic digesters only minimally reduce odors from manure. They also note that the digesters are not able to reduce chemical waste. The digesters, they say, can also release nitrogen and sulfur oxides as well as particulate matter, carbon monoxide, and ammonia, into the air. There are some that also argue that anaerobic digesters can help factory farms take over small farm communities.