Biodigesters recover methane from animal manure through a process called anaerobic digestion. Here’s how it works.

Methane is a gas that contains molecules of methane with one atom of carbon and four atoms of hydrogen (CH4 ). It is the major component of the “natural” gas used in many homes for cooking and heating. It is odorless, colorless, and yields about 1,000 British Thermal Units (Btu) of heat energy per cubic foot when burned. Natural gas is a fossil fuel that was created eons ago by the anaerobic digestion or anaerobic decomposition of organic materials. It is often found in association with oil and coal.

The same types of anaerobic bacteria that produce natural gas also produce methane. Anaerobic bacteria are some of the oldest forms of life on earth. They evolved before the photosynthesis of green plants released large quantities of oxygen into the atmosphere. Anaerobic bacteria break down or “digest” organic material in the absence of oxygen and produce “biogas” as a waste product. Anaerobic processes can be managed in a “digester” (an airtight tank) or a covered lagoon, a pond used to store manure, or a waste treatment facility. The primary benefits of anaerobic digestion are nutrient recycling, waste treatment, and odor control. Biogas production is a highly useful secondary benefit. Biogas produced in anaerobic digesters consists of 50% – 80% methane, 20% – 50% carbon dioxide, and trace levels of other gases such as hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulfide.

A variety of factors in Anaerobic Digestion affect the rate of digestion and biogas production. The most important is temperature. Anaerobic bacteria communities can endure temperatures ranging from below freezing to above 135° Fahrenheit, but they thrive best at temperatures of about 98°F.

Basic technologies for the utilization of Anaerobic Digestion

Medium-Btu biogas can be used in a number of ways. Typically after condensate and particulate removal, the biogas is compressed, cooled, dehydrated and then can be transported by pipeline to a nearby location for use as fuel for boiler or burners. Another alternative for biogas applications is to generate steam using a boiler onsite, however the customer for this steam would need to be close to the site since high pressure steel insulated pipeline is expensive and heat is lost during transport.

Electricity generated on-site using a reciprocating engine, steam turbine, or gas turbine, is also actively being used.

Anaerobic Digestion – Biogas can be upgraded into high-Btu gas and injected into a natural gas pipeline. As compared with other power generation alternatives, the capital cost for sale of upgraded pipeline quality gas is high because treatment systems that are used to remove CO2 and impurities are required. Also, upgraded gas needs a significant amount of compression to conform to the pipelines pressure at the interconnect point. However, the advantage of pipeline quality gas technology is that all the biogas produced can be utilized.

And when the process is all over, the material drawn from the anaerobic digester is called sludge, or effluent. It is rich in nutrients like ammonia, phosphorus, potassium, and more than a dozen trace elements, and is an excellent soil conditioner.

Mankind and Anaerobic Digestion

One thing is for sure. Mankind will never have a shortage of organic waste. Why not utilize this anaerobic digestion technology to not only safely clean it up, but also to help solve some of our energy needs.