Biological Conversion – 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. A digester system (the anaerobic digester) is a device that promotes the decomposition of manure or digestion of the organics in manure to simple organics and gaseous biogas products.

      During anaerobic digestion of an organic material such as biomass, a varied mixture of complex compounds is converted to a very narrow range of simple compounds, mainly methane and carbon dioxide. The anaerobic bacteria are responsible for the biochemical transformation of the biodegradable organic fraction (BOF). The AD of organic material basically consists of hydrolysis, acidogenesis, acetogenesis, and methanogenesis. These transformations are involved in the breakdown of complex polymers, such as cellulose, fats, and proteins to long and short chain fatty acids, and finally to methane, carbon dioxide, and water.

      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 anaerobic digestion process focuses on hastening the natural process of biomass conversion to a gaseous fuel (biogas). Research has been conducted to ascertain optimal conditions for anaerobic digestion. These include (i) the feedstock, (ii) the nutrients, (iii) the temperature, (iv) the moisture content of the feedstock, (v) the pH of the system, and (vi) the atmospheric conditions. Most of the biomass waste feedstocks (municipal solid waste, agricultural waste, farm waste, crop waste, and forestry waste) studied have produced a biogas rich in methane. This medium to high Btu gas can, in some instances, be upgraded to a substitute natural gas (SNG). Depending on the feedstock, sulfur may also be produced.

      Anaerobic digestion is a multi-stage biological waste treatment process whereby bacteria, in the absence of oxygen, decompose organic matter to carbon dioxide, methane, and water. 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. 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% methane and the remainder largely being carbon dioxide.

The anaerobic digestion process is a multi-stage biological waste treatment process whereby bacteria, in the absence of oxygen, decompose organic matter to carbon dioxide, methane and water. 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. 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. The end result of the process is a well-established sludge in which 40 to 60% v/v of the solids are consumed by the process. Finally, a combustible gas consisting of approximately 60 to 75% v/v methane is produced with the remainder being predominantly carbon dioxide.

      Anaerobic digestion is a complex process which requires strict anaerobic conditions [oxidation reduction potential (ORP) < -200 mV] to proceed and depends on the coordinated activity of a complex microbial association to transform organic material into mostly carbon dioxide and methane. Despite the successive steps, hydrolysis is generally considered as rate limiting. The hydrolysis step degrades both insoluble organic material and high molecular weight compounds (lipids, polysaccharides, proteins, and nucleic acids) into soluble organic substances (amino acids and fatty acids). The components formed during hydrolysis are further split during acidogenesis, the second step.

      Volatile fatty acids (VFAs) are produced by acidogenic bacteria along with ammonia (NH₃), carbon dioxide (CO₂), hydrogen sulfide (H₂S), and other by-products. The 3^rd stage in anaerobic digestion is acetogenesis, where the higher organic acids and alcohols produced by acidogenesis are further digested by acetogens to produce mainly acetic acid and as well as carbon dioxide and hydrogen. The final stage of methanogenesis produces methane by two groups of methanogenic bacteria: (i) the first group splits acetate into methane and carbon dioxide and (ii) the second group uses hydrogen as an electron donor and carbon dioxide as the acceptor to produce methane. Thus: