Encyclopedia of Renewable Energy. James G. Speight

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      Anaerobic Digestion

      Anaerobic digestion is the decomposition of biological wastes by microorganisms, usually under wet conditions, in the absence of air (oxygen), to produce a gas comprising mostly methane and carbon dioxide.

      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 involve an anaerobic process. 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 o f 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. The process is a multi-stage biological treatment process whereby bacteria, in the absence of oxygen, decompose organic matter to carbon dioxide, methane, and water (Table A-20).

Table A-20 Schematic of the anaerobic digestion process.

      In this way, the waste sludge is stabilized and the obnoxious odor is removed. The process can, however, be described adequately and simply as occurring in two stages, involving two different types of bacteria. The process occurs in the absence of air, the decomposition in this case is caused not by heat but by bacterial action. In the first stage, the organic material present in the feed sludge is converted into organic acids (also called volatile fatty acids) by acid forming bacteria. In the second stage, these organic acids serve as the substrate (food) for the strictly anaerobic methane-producing bacteria, which converts the acids into methane and carbon dioxide.

Any organic substance can become subject to anaerobic digestion so long as there are warm, wet, and airless conditions. For example, marsh gas is a product of the anaerobic digestion of vegetation at the bottom of ponds; this gas rises to the surface and bubbles, and the gas is also combustible. With the aid of human intervention, there are two products of this process, biogas and landfill gas. The chemical processes behind the production of these gases are complex. The end result of the process is a well-established sludge in which 40 to 60% of the volatile solids are destroyed. Finally, a combustible gas is produced consisting of 60 to 75% v/v methane and the remainder largely being carbon dioxide. (Table A-21).

Table A-21 Summary of most common chemical reactions in the anaerobic digestion process.

      Thus, simplified examples of the chemistry of anaerobic digestion process are as follows:

Organic acids formed in the first stage of the waste treatment process are converted to methane at the same rate at which they are formed. If not, they accumulate and ultimately lower the pH, leading to inhibition of the second stage of the digestion process and digester failure. Temperature must be maintained within certain ranges – heating increases the activity of the anaerobic bacteria, reducing the required digestion time. A pH of 7.0 to 7.5 is recommended to encourage the methane-producing stage.

      The organic content of the sludge is significantly reduced by conversion into gaseous end-products; the obnoxious odor of the sludge is removed, and the final digested sludge has a characteristic tarry odor; fats and greases are broken down by the process. The liquid fraction (supernatant) contains increased levels of ammonia as a result of the breakdown of organic nitrogen (proteins). This makes the digested sludge liquor potentially suitable for agricultural use; the biogas that is formed is a mixture of carbon dioxide (CO₂) and methane (CH₄) that can be used for digester heating or to generate power.

      The slow rate of bacterial growth usually requires long periods of time for start-up and limits the flexibility of the process to adjust to changing feed loads, temperatures, and other environmental conditions. In addition, the process is prone to upsets if not regularly monitored and if corrective action is not taken in time.

      See also: Acetogenesis, Acidogenesis, Aerobic Digestion, Digester, Digestion, Hydrolysis, Methanogenesis.

      Anaerobic Digestion – Gas Production

      A typical gas system for the production of gas by anaerobic digestion comprises the digester cover, pressure and vacuum relief devices, water trap, flame trap, pressure regulator, gas meter, check valve, pressure gauges, waste gas burner, and a gas holder. The continuous stirred tank reactor (CSTR) is an example of a type of anaerobic digester.

      The digester is covered to contain odors, maintain temperature, keep air out, and collect the gas. Fixed covers are more usual than floating covers. During normal operation, there is a space for gas collection between the cover and the liquid surface of the digester contents. The cover of a digester has certain unique features that the operating staff must be aware of, for example, how the variation in pressure and the level inside the digester may affect the cover. The biggest danger associated with the operation of fixed cover digesters occurs when the pressure relief device mounted on top of the digester fails or the sludge overflow line blocks and the liquid level in the digester continues to rise. In such a situation, the excess gas pressure inside the digester can exceed the maximum design pressure and damage the cover or its mountings. Fixed covers can also be damaged by excess negative pressure (vacuum) or if the rate of waste sludge withdrawal exceeds the feed СКАЧАТЬ