An official website of the United States government.

We've made some changes to EPA.gov. If the information you are looking for is not here, you may be able to find it on the EPA Web Archive or the January 19, 2017 Web Snapshot.

Frequent Questions about Anaerobic Digestion

Frequent Questions about Anaerobic Digestion


What is anaerobic digestion?

Anaerobic digestion is the natural process in which microorganisms break down organic matter in the absence of air (an anaerobic environment).  Anaerobic digestion creates usable products such as biogas and digested material.

Top of Page


What is an anaerobic digester?

Anaerobic digesters are built systems (lagoons or tanks) where anaerobic digestion takes place. Anaerobic digesters manage organic wastes, produce gas and digested materials, minimize odors, reduce pathogens, and reduce solid wastes. Anaerobic digesters are also called “anaerobic digestion systems”, “biodigesters” or simply “digesters”.

Top of Page


What is co-digestion?

Co-digestion happens when more than one type of organic material is digested at the same time.  Digesters are often built for a single purpose.  For example, a farmer may build an anaerobic digester to handle cow manure.   If the farmer also takes food waste from a local grocery store and puts the food waste in the digester along with the cow manure, it is called co-digestion.   

Top of Page


What is biogas?

Biogas is the gas produced when bacteria break down organic matter in the absence of oxygen.  It is made up of mainly methane (CH4) and carbon dioxide (CO2), with small amounts of water vapor, particulates, and other gasses, such as hydrogen sulfide (H2S).  Biogas can be processed and used for a variety of energy needs, such as the generation of heat, power and fuel.

Top of Page


How is biogas generated?

Inside an anaerobic digester, naturally occurring microorganisms grow in the tank’s oxygen-free environment and break down (digest) the organic matter. As the organic matter decomposes, biogas is created. Once established in a digester, microorganisms will continue to break down organic materials and release biogas in the right conditions. The microorganisms need a steady supply of feedstock and a comfortable environment - warm temperatures, neutral acidity and no oxygen.

Top of Page


What is digested material and what is it used for?

Digested material is the solid and liquid material that remains at the end of the anaerobic digestion process. Digested material contains valuable nutrients (nitrogen, phosphorus and potassium) and organic carbon.  Typically, raw digested material is processed into a wide variety of products like fertilizer, compost, soil amendments or animal bedding. Factors influencing what products are made include the makeup of the initial feedstocks and local markets.  These co-products can be sold to agricultural, commercial and residential customers. 

Top of Page


What types of organic materials produce biogas?

Many types of organic material can be used as feedstock to produce biogas. Animal manures, wastewater solids, food scraps, restaurant fats, oils, and greases, and by-products from food and beverage production are some commonly digested materials. An anaerobic digester may be built for a single material or a combination of them. However, the feedstocks must be properly controlled to ensure that the system remains healthy and functioning.

Top of Page


Is biogas a renewable energy source?

Yes, biogas is a renewable energy source. It is produced from natural resources that are replenished in short periods of time.

Top of Page


Can biogas replace fossil fuels?

Yes, biogas can replace fossil fuels for the production of heat, power and fuel. With additional processing, biogas becomes renewable natural gas that can be used in the same place as fossil fuels.

Top of Page


How does biogas help reduce effects of climate change?

Biogas is made up of methane and carbon dioxide, which are powerful greenhouse gases. Anaerobic digesters are designed to capture these gases so they do not escape to the atmosphere. In most cases, the feedstocks used in digesters would have released methane directly as they decomposed in lagoons or landfills. In addition, using biogas for heat or electricity means that less energy needs to be produced by power plants. This reduces the amount of carbon dioxide emitted to the atmosphere by burning fossil fuels.

Top of Page


What types of facilities have anaerobic digesters?

Anaerobic digesters are usually built on sites that have a steady supply of organic materials and need energy or heat. Common examples include farms, water resource recovery facilities, food production facilities, and landfills. Also, stand-alone digesters can be built in a central location to accept organics from multiple businesses.

Top of Page


Is there a difference between landfill gas and biogas?

Landfill gas is a type of biogas.  Both can be converted to renewable energy (electricity or fuel).

Top of Page


Is there a difference between natural gas and biogas?

Biogas and natural gas have essentially the same components. But, the two gasses are obtained in different ways. 

Biogas is produced when readily available organic materials (e.g., manure or food waste) break down.  Natural gas is produced when ancient plants, tiny sea animals, and other organic materials break down in hard to access locations.  Usually, natural gas has to be extracted from underground reservoirs.  Natural gas can also be derived from petroleum refining.  

Top of Page


If I build a biogas system will it stink?

Biogas contains a small amount of hydrogen sulfide, which has a rotten-egg odor.  However, anaerobic digesters are completely enclosed and biogas is not released directly to the air.  Digesters are commonly installed at farms to reduce odors.  What comes out of a digester after processing is much less odorous than the feedstocks that go into digesters.

Top of Page


Are anaerobic digesters safe?

A properly designed and operated system is very safe. Anaerobic digesters are designed to meet local and national codes for safety. However, they do produce methane and hydrogen sulfide. These gases both burn easily and are harmful to inhale, so it is essential to use proper gas-handling precautions.  It is also important for plant operators to be well trained and follow established operational procedures.