Encyclopedia of Renewable Energy. James G. Speight

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      Aerobic Digestion

Aerobic digestion is a bacterial process occurring in the presence of oxygen. Under aerobic conditions, bacteria rapidly consume organic matter and convert it into carbon dioxide. Once there is a lack of organic matter, bacteria die and are used as food by other bacteria. This stage of the process is known as endogenous respiration. Solids reduction occurs in this phase.

      Composting is also an aerobic process that involves mixing the wastewater solids with sources of carbon such as sawdust, straw, or wood chips. In the presence of oxygen, bacteria digest both the wastewater solids and the added carbon source and, in doing so, produce a large amount of heat.

      In an aerobic system, such as composting, the microorganisms access free, gaseous oxygen directly from the surrounding atmosphere. The end products of an aerobic process are primarily carbon dioxide and water which are the stable, oxidized forms of carbon and hydrogen. If the biodegradable starting material contains nitrogen, phosphorus, and sulfur, the end products may also include their oxidized forms: nitrate (NO₃–) phosphate (PO₄2–), and sulfate (SO₄2–). In an aerobic system, the majority of the energy in the starting material is released as heat by their oxidation into carbon dioxide and water.

      Composting systems typically include organisms such as fungi that are able to break down lignin and cellulose to a greater extent than anaerobic bacteria. It is possible, following anaerobic digestion, to compost the anaerobic digestate, allowing further volume reduction and stabilization. When considering an overall system energy and carbon balance, anaerobic digestion performs better than the main alternative, composting.

      The aerobic digestion process was initially used in designs for new plants that normally treated waste activated sludge from treatment systems that did not contain a primary settling process; only waste activated or trickling filter sludge, or mixtures of waste activated or trickling filter sludge. Typically, if a primary settling process was incorporated to the design, anaerobic digestion was the process of choice because reliable techniques to thicken and aerobically digest higher than 4% solids were not established at the time.

      Because of tighter effluent standards for both nitrogen and phosphorus being enforced in the United States in the late 1990s, primary clarifiers have slowly been eliminated from the process train to preserve a good carbon-to-nitrogen ratio typically required to achieve successful biological nitrogen removal.

      As a result of the combination of the new effluent limits and new techniques that provided the capabilities to control aerobic digestion processes and accurately predict the performance of the system, aerobic digestion has become attractive once again. A number of anaerobic digesters have been converted to aerobic digesters because of their relatively easy operation, lower equipment cost, and because they can produce a better quality supernatant with lower nitrates and phosphorus, therefore, protecting the liquid side upstream. An additional benefit of aerobic digestion is the fact that they can achieve comparable volatile solids reduction with shorter retention periods, they have less hazardous cleaning and repairing tasks, and, typically, do not produce an explosive digester gas.

      Anaerobic digestion of animal waste is the primary cause of odors, solids buildup and many diseases in swine, dairy and poultry facilities, processing plants, municipal waste systems, and septic systems. Animal waste concentrated in pits under slatted floors or collected in holding tanks or lagoons has the natural tendency to “go anaerobic.” Anaerobic digestion occurs when the anaerobic microbes are dominant over the aerobic microbes. Anaerobic microbes will naturally become dominant in pits or lagoons because of the lack of oxygen in solutions containing heavy concentrations of animal waste, which results in a high biological oxygen demand (BOD). These microbes feed on the animal waste at the bottom of the pits and lagoons. As they digest waste, large amounts of toxic gases are released due to the digestion processes common to the anaerobic microbes.

      The anaerobic digestion of animal waste can be changed to aerobic digestion by proper applications of beneficial aerobic microbes in a highly concentrated form. If aerobic microbes are introduced into an environment that is lacking oxygen, they will begin to build oxygen into this environment as long as they survive and reproduce. Aerobes have the ability to do this in a liquid media or in the soil as long as there is an adequate moisture and food source for them to feed on and reproduce.

      Once the aerobic microbes become dominant, the aerobic digestion of waste begins. Aerobic digestion, unlike anaerobic digestion, does not produce the pungent gases. The aerobic process results in a more complete digestion of waste solids, reducing the buildup in pits and lagoons by more than 50% in most cases. The aerobic process also improves the environment of the workers and the animals and helps to keep pathogens in check. With proper application, the anaerobic process is converted to aerobic and overall production cost will be reduced.

      See also: Anaerobic Digestion, Digester, Digestion.