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Anaerobic Digestion Basics

August 14, 2013 - 1:07pm


Disposal and treatment of organic waste represents a major challenge for the waste industry. Anaerobic digestion is an alternative to composting for a wide range of organic substances including livestock manure, municipal wastewater solids, food waste, industrial wastewater and residuals, fats, oils and grease, and other organic waste streams. Diverting organic waste to facilities that contain anaerobic digestion technology could lower greenhouse gas emissions, specifically methane released from landfills as waste breaks down. In addition to waste management, anaerobic digestion provides a renewable source of biogas, which can be burnt to generate heat or electricity or upgraded to be used as a vehicle fuel.   

Anaerobic digestion is a biological process that occurs naturally when microorganisms break down organic matter in environments with little or no oxygen such as swamps, water-logged soils and rice fields, deep bodies of water, and in the digestive systems of termites and large animals. This process produces biogas, a mixture of mostly methane and carbon dioxide with trace amounts of other gases. The natural activity or these organisms can be harnessed industrially to convert a range organic biomass materials into a biogas. This biogas can be combusted to generate electricity and heat, processed to remove non-methane compounds as a direct replacement for natural gas, or converted into renewable hydrocarbon liquid transportation fuels.

Anaerobic digestion of biomass is a complex process. Typically, organic materials are placed into an oxygen sealed tank called a digester, where it is broken down by a community of microorganisms in three basic stages. First, one group of bacteria breaks down the biomass into its component sugars, which a second group of organisms converts into carbon dioxide, hydrogen, ammonia, and organic acids. Finally, methane-producing (methanogenic) anaerobic bacteria convert these products into methane and carbon dioxide. In addition to biogas, the process also produces a solid and liquid residue called digestate which is rich in nutrients such as nitrogen, phosphorus, and potassium and can be processed into products like fertilizer and compost.