Anaerobic Digesters

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An anaerobic digester is any equipment or site that holds waste materials in an oxygen-free environment so that anaerobic bacteria can degrade the wastes. On a wastewater treatment plant’s property, the digester is a large tank that holds several thousand gallons of liquid and semiliquid material. Wastewater digestion also takes place in a similar way in lagoons or ponds even though these places are exposed to the air. Though the upper layers of the water contain some dissolved oxygen, the deeper layers become less aerobic and more anaerobic. If the contents of an anaerobic digester or pond receive little mixing, oxygen does not penetrate the depths and so anaerobic bacteria work at their best.

Anaerobic decomposition occurs naturally in swamps, bogs, stagnant ponds, deep bodies of water, and waterlogged soils. The bacteria in these places degrade organic matter to the simplest of compounds, which allows the nutrients to be recycled. For example, amino acids degrade to carbon dioxide (CO2), methane (CH4), hydrogen (H2), and small amounts of nitrogen and sulfur compounds.

Digestion of organic matter—in nature as well as in a digester—takes place in three stages. First, aerobic bacteria degrade complex compounds such as starches, proteins, and fibers into smaller carbohydrates or peptides. A second group of bacteria use these compounds as food and produce organic acids as end products of their enzyme reactions. The organic acids all have in common a carboxylic group as part of their structure (COOH). This section of the molecule contains a carbon that is linked both to an oxygen molecule and to an oxygen-hydrogen complex, called a hydroxyl group. Examples of the organic acids produced in this step are acetic acid (two total carbons), propionic acid (three carbons), and butyric acid (four carbons). As a third and final step, anaerobic bacteria use the acids for energy and produce methane, carbon dioxide, and a small amount of other gases (hydrogen, carbon monoxide, hydrogen sulfide, and nitrogen).

Anaerobic digesters at wastewater treatment plants make fuel in the form of methane gas, which can be burned to release its energy. Many sustainable wastewater treatment plants use the methane to provide heat for the digester so that the entire process can continue. This is an example of a sustainable loop.

The methane and the other biogases produced by anaerobic digestion serve as an energy source that can be used to run the treatment plant or do other work. Burning one cubic foot of biogas yields 10 Btu of heat energy. The amount of methane in the biogas mixture affects the energy production because methane is the main energy source. Each percentage of methane results in 10 Btu, so that a biogas containing 65 percent methane produces 650 Btu per cubic foot. Very active anaerobic digesters can produce enough methane to run a treatment plant’s heating, refrigeration, and electricity. Anaerobic digesters therefore play a role as the main powerhouse when designing a sustainable wastewater treatment plant.

Methane cannot solve every environmental problem. For one thing, though methane can be used for making energy, it is also a predominant greenhouse gas that causes global warming. Sustainable wastewater treatment plants help reduce this problem by capturing all the methane emitted by a digester and using it, but methane come from additional sources in the world. Human activities that lead to methane production are waste treatment, biomass burning facilities, energy plants, and landfills. Methane also comes from natural sources: anaerobic soil and water bacteria, some vegetation, fossil fuel deposits such as coal mines and natural gas fields, and certain animals. Scientists have begun to realize in the past decade or so that the animal sources of methane contribute a significant amount of greenhouse gas to the atmosphere.