A methane digester is more properly known as an anaerobic digester because it decomposes organic waste by limiting oxygen intake and produces methane gas as a by-product. The anaerobic decomposition of organic matter such as animal feces, waste food, and vegetable matter is a natural process that produces methane gas, but, when it occurs in a methane digester, the gas can be collected and used as a fuel. Large farming operations and waste treatment facilities frequently recycle animal waste and sewage into a methane digester and use the gas, which is similar to natural gas, for heating and to produce electricity.
Anaerobic decomposition proceeds in two stages. In the first, the complex organic molecules in the waste are broken down by acidic microbes into peptides, alcohol, and simple sugars. These molecules are then broken down again by a different set of microbes. As a byproduct, the process produces gasses, including methane, hydrogen, and carbon dioxide, and solids that are high in nitrogen. This process differs from aerobic decomposition, or decomposition in the presence of oxygen, which produces mostly carbon dioxide and ammonia as by-products. In addition, it also reduces the raw material into a substance that is lower in nitrogen and that can be used as fertilizer.
The construction of a methane digester includes an airtight container with a port for the introduction of waste matter and a pipe for collecting the gas. Digestion proceeds best at temperatures in the range of 32°C to 35°C (89°F to 95°F) and will drop off sharply if the temperature falls below 16°C (61°F). The ratio of solids to water should be about 8%, which means that animal waste should be mixed with an equal amount of water. While a methane digester will function well with just animal waste, its efficiency can be improved by adding vegetable matter.
The dual benefits of fuel for electricity and heat, and the reduction of waste to a recyclable, odor-free sludge make the methane digester a cost-effective and environmentally-friendly option for waste treatment. A large digester can be constructed as a covered lagoon with manifolds to collect the gas, while a smaller one may consist of a tank that can either sit on the ground or be buried. Tank digesters can be heated, which makes them better candidates for colder climates. Although a methane digester reduces waste to an odor-free material, the process is not odor-free. Moreover, digesters emit harmful gases like carbon monoxide, sulfur oxides, and ammonia, so the immediate area around a large digester is not an ideal residential zone.