Waste Advantage Magazine

Anaerobic Digestion—What Are the Economics?

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Although solid waste AD facilities are just starting to emerge, haulers and their customers may see some benefits if the development trend continues. A Pro Forma Model objective can be constructed for strategic planning of an anaerobic digestion facility.

Anaerobic digestion (AD) of food wastes in the U.S. is a developing market that is poised for continued growth in many regions. Although solid waste anaerobic digestion facilities are just starting to emerge, haulers and their customers may see some benefits if the development trend continues. This article discusses what an anaerobic digestion plant does, and what the authors see as the economics where they recently have been installed and are operating.

Anaerobic Digestion
Anaerobic digestion is the bacterial breakdown of organic materials in the absence of oxygen. This biological process produces a gas, sometimes called biogas, principally composed of methane (CH4) and carbon dioxide (CO2). The anaerobic process is most often used to treat organic wastes other than unsegregated MSW, such as sewage sludge (biosolids produced from treated sanitary sewage at a wastewater treatment plant), yard vegetation, agricultural wastes (both animal and plant) and some industrial waste sludge. Depending on the waste feedstock and the system design, biogas is typically composed of 55 to 75 percent methane. The anaerobic digestion process occurs in three steps:

  1. Decomposition of plant or animal matter by bacteria into molecules such as sugar.
  2. Conversion of decomposed matter to organic acids.
  3. Organic acid conversion to methane gas.

AD can occur naturally in a landfill or in a controlled environment such as a manufactured chamber or vessel. In an AD facility, the incoming waste stream is initially processed for removal of potential contaminants, reduced in size and delivered to an air-tight reactor vessel, typically called a “digester”. AD facilities are usually classified based on whether the system uses a single reactor or multi-reactor system, and if water is added for processing (wet systems) or absent during processing (dry systems). There are more than 200 AD facilities around the world with operating capacities greater than 2,500 tons per year (TPY). These plants process not only the organic fraction of the MSW waste stream, but also organic waste from food industries and animal manure. Europe leads in the number of AD plants and total installed capacity principally due to the European Union Directive that requires member states to reduce the amount of landfilled organics by 65 percent by 2020.

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