Composting without oxygen results in fermentation. This causes organic compounds to break down by the action of living anaerobic organisms. As in the aerobic process, these organisms use nitrogen, phosphorus, and other nutrients in developing cell protoplasm. However, unlike aerobic decomposition, this reduces organic nitrogen to organic acids and ammonia. Carbon from organic compounds, is released mainly as methane gas (CH4). A small portion of carbon may be respired as CO2.
This anaerobic process takes place in nature. Examples include decomposing organic mud at the bottom of marshes and buried organic materials with no access to oxygen. Marsh gas is largely methane. Intensive reduction of organic matter by putrefaction is usually accompanied by unpleasant odors of hydrogen sulfide and of reduced organic compounds that contain sulfur, such as mercaptans (any sulfur-containing organic compound).
Since anaerobic destruction of organic matter is a reduction process, the final product, humus, is subject to some aerobic oxidation. This oxidation is minor, takes place rapidly, and is of no consequence in the utilization of the material.
There is enough heat energy liberated in the process to raise the temperature of the putrefying material. In the anaerobic dissolution of the glucose molecule, only about 26 kcal of potential energy per gram of glucose molecules is released compared to 484 to 674 kcal for aerobic decomposition. The energy of the carbon is in the released methane (CH4). The conversion of CH4 to CO2 produces large amounts of heat. This energy from anaerobic decomposition of organic matter can be used in engines for power and burned for heat.
Pathogens could cause problems in anaerobic composting because there is not enough heat to destroy them. However, aerobic composting does create high enough temperatures. Although heat does not play a part in the destruction of pathogenic organisms in anaerobic composting, they do disappear in the organic mass because of the unfavorable environment and biological antagonisms. They disappear slowly. The composted material must be held for periods of six months to a year to ensure relatively complete destruction of Ascaris eggs, for example. Ascaris are nematode worms that can infest the intestines. They are the most resistant of the fecal-borne disease parasites in wastes.
Anaerobic composting may be accomplished in large, well packed stacks or other composting systems. These should contain 40% to 75% moisture, into which little oxygen can penetrate, or 80% to 99% moisture so that the organic material is a suspension in the liquid. When materials are composted anaerobically, the odor nuisance may be quite severe. However, if the material is kept submerged in water, gases dissolve in the water and are usually released slowly into the atmosphere. If the water is replaced from time to time when removing some of the material, odor does not become a serious nuisance.
Both aerobic and anaerobic composting require bacteria. Some bacteria work better in one or the other environment. Compost piles under aerobic conditions may attain a temperature of 140° to 160° F in one to five days depending upon the material and the condition of the composting operation. This temperature can also be maintained for several days before further aeration is needed. The heat necessary to produce and maintain this temperature must come from aerobic decomposition, which requires oxygen. After a period of time, the material will become anaerobic unless it is aerated. There is probably a period between the times when the oxygen is depleted and anaerobic conditions become evident, during which the process is aerobic.
“Aerobic composting” requires a considerable amount of oxygen and produces none of the characteristic features of anaerobic putrefaction. Aerobic composting can be defined as a process in which, under suitable environmental conditions, aerobic organisms utilize considerable amounts of oxygen in decomposing organic matter to fairly stable humus.
“Anaerobic composting” describes the process of putrefactive breakdown of organic matter by reduction in the absence of oxygen where end products such as CH4 and hydrogen sulfide (H2S) are released.