THE MICROBIOLOGY OF ANAEROBIC DEGRADATION AND METHANOGENESIS WITH SPECIAL REFERENCE TO SEWAGE

Abstract The anaerobic degradation of organic matter to CO2 and CH4 as it occurs in sewage involves a consortium of interacting anaerobic microbes. The process includes three general groups of bacteria. The first group is a complex of fermentative species which hydrolyze the primary substrates such as cellulosic materials, lipids and proteins and ferment the products to acetic and other organic acids, alcohol, H2, CO2, NH3 and sulfide. The second group of bacteria, as yet mainly hypothetical, oxidizes the acids with longer carbon chains than acetate and alcohol with production of more acetate and the electrons generated are probably excreted as H2. The third group involves diverse species of methanogenic bacteria which utilize the H2 and CO2 and acetate for growth and production of the final products, CH4 and CO2. The fermentative group excretes products which are needed as energy and nutrient sources by the other two groups. In the effective removal of H2, the methane bacteria allow the nonmethanogens to produce a much greater amount of H2 which replaces other reduced electron sink products such as lactate, ethanol and propionate and results in the excretion of a greater proportion of the more oxidized end products, acetate and CO2. The removal of H2 allows more efficient growth of the fermentative bacteria and is essential for the nonmethanogenic acid and alcohol catabolizing bacteria. The methane fermentation is very important in the carbon and other cycles in nature because it degrades complex organic matter to the gaseous products, CO2 and CH4, with arelatively small growth yield of bacteria. Thus, a large amount of organic matter is destroyed but most of the substrate energy is retained in the CH4. The biological agents of methanogenesis are bacteria and the complete process involves a complex consortium of interacting bacterial species, most of which do not produce methane as such. The process occurs in environments in which oxygen and other electron acceptors such as sulfate or nitrate are present in minimal concent rations. Temperature wise, two different groups of bacteria are involved; thermop hilic species are active from about 45 to 70°C and mesophiles, at lower temperatures. The pH optimum is about seven and the rate drops rapidly as acidity increases. The fermentation occurs in many natural environments such as sediments of aquatic systems, tundra, peat bogs, and swamps. Fossil fuels represent indigestible organic residues of the fermentation such as lignin which were modified further by geophysical conditions. Man has long used the fermentation to stabilize sewage solids. Partial methane fermentation occur in the gastro - intestinal tract of many animals. The present discussion will refer mainly to the mesophilic fermentation involving sewage solids. In this process, the settled solids containing about 4% of organic matter are passed through a digestor with a hydraulic retention time of about 15 to 30 days. The material is often mixed and heated to about 35°C using a pump and heater powered by the CH4 produced. About 0.5 to 0.7 liters of methane are produced per gram of organic matter digested depending on the composition of the organic matter.