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Anaerobic Digestion

Through anaerobic digestion (leaving DEC's website) organic materials are processed in an airtight container by microorganisms which break down the material into biogas and a digestate. More than half of biogas is methane which can be used as a renewable energy source. The digestate can be land applied, composted and used as a soil amendment or processed into fertilizer pellets. While startup costs for anaerobic digestion facilities are significant, they may be eligible to receive grant funding to help offset these costs (See funding section below).

What can go into a digester?

Food scraps, manure, biosolids, fats, oils, grease, manure, animal slurry, fruit and vegetable waste, slaughterhouse waste, meat packing waste, dairy factory waste, brewery and distillery waste, etc. can all be put into a digester. Manure is a common base feedstock as it is produced continuously and contains a healthy population of anaerobic bacteria that is part of the anaerobic digestion process. Not all organic matter is suitable for a digester. Wood, leaves, and other fibrous materials contain lignin, which is difficult to digest, making them challenging feedstocks.

Types of Digesters

There are two basic types of digesters categorized by the percentage of water in the feedstock. A wet digester accepts material that contains less than 15 percent solids. The feed stock is pumpable. A dry digester, otherwise known as high solids anaerobic digestion, generally takes material that is more than 15% but less than 45% solids. The feed stock is stackable; it is too dry to be pumped. The residence time needed to complete the process in the dry digester is faster than in a wet digester.

For additional information on the types of digesters visit (leaving DEC's website)

Biogas for Electricity and Heat

Biogas can be used in place of natural gas. It consists of 60-70% methane gas, compared to natural gas which contains 99% methane. Each cubic meter of biogas is the equivalent of approximately 7 kWh, which is enough energy to power a 100 watt light bulb for 70 hours. Combined heat and power (CHP) systems employ a generator which converts the biogas to electricity. The system also captures the heat from the combustion of biogas and conversion to electricity. The heat can be used to heat nearby buildings as well as the water in the digester. However, neither the heat nor electricity can be transported easily. Therefore, the most efficient use of biogas is onsite. While selling excess electricity back to the grid is an option, the infrastructure required to do this can be expensive and the reimbursement rate in most of the United States is limited.

Biogas for transportation

Processed to remove carbon dioxide and impurities (leaving DEC's website), biogas can be used to power vehicles in the same way that natural gas can. Using biogas instead of gasoline or compressed natural gas reduces greenhouse emissions. It is beginning to be used more in the United States, with even more success in Europe.

Use of Solid and Liquid Byproducts

The solids produced can be land applied as a soil amendment, used for animal bedding, made into fertilizer pellets or composted and then used as a soil amendment. The liquid byproduct can also be land applied. Using products derived from anaerobic digestion instead of from fossil fuels reduces greenhouse gas emissions in two ways. First extracting and transporting coal, oil and natural gas requires energy. Since most of the energy used in New York State is fossil fuels, burning them will create greenhouse gases, mostly carbon dioxide. Second, the organic matter that goes in a digester would otherwise be landfilled. As organic matter degrades in a landfill, it produces methane, a greenhouse gas many times more potent than carbon dioxide. Many of New York State's landfills have gas collection systems to capture and either use or flare the methane produced. However, some of the gas cannot be collected and is released into the atmosphere.