There are several factors, some of them interdependent, which are fundamental in planning a composting project or in analyzing composting operations. Some of the methods of composting may be used most economically under different conditions. Analyzing methods in the light of fundamental factors permits: (a) selecting established procedure best for the particular circumstances, (b) selecting different techniques from different established procedures, or (c) developing other methods to economically meet requirements of the individual situation.
Important factors in composting operations are:
Grinding or shredding
Carbon vs nitrogen
Use of inocula
Placement of materials
Compost & chemicals
Quality of compost
Use of compost
When organic material decomposes with oxygen, the process is called "aerobic." When living organisms, which use oxygen, feed upon the organic matter, they develop cell protoplasm from the nitrogen, phosphorus, some of the carbon, and other required nutrients. Carbon serves as a source of energy for organisms and is burned up and respired as carbon dioxide (CO2). Since carbon serves both as a source of energy and as an element in the cell protoplasm, much more carbon than nitrogen is needed. Generally about two-thirds carbon is respired as CO2, while the other third is combined with nitrogen in the living cells. If the excess of carbon over nitrogen in organic materials being decomposed is too great, biological activity diminishes. Several cycles of organisms are required to burn most of the carbon. When organisms die, their stored nitrogen and carbon become available to other organisms. Nitrogen use from dead cells by other organisms forms new cell material and again requires burning excess carbon to CO2. Thus, the amount of carbon is reduced and the limited amount of nitrogen is recycled. Finally, when the ratio of available carbon to available nitrogen is low enough, nitrogen is released as ammonia. Under favorable conditions, some ammonia may oxidize to nitrates. Phosphorus, potash, and various micro-nutrients are also essential for biological growth. These are normally present in more than adequate amounts in compostable materials.
The aerobic process is most common in nature, such as the forest floor, where droppings from trees and animals are converted into a relatively stable humus or soil manure. There is no accompanying smelling nuisance when adequate oxygen is present. A great deal of energy is released as heat in the oxidation of carbon to CO2. For example, if a gram molecule of glucose is dissimulated under aerobic conditions, 484 to 674 kilogram calories (kcal) of heat may be released. If organic material is in a pile or is otherwise arranged to provide some insulation, temperatures during decomposition will rise to over 170º Fahrenheit. If the temperature exceeds 162º to 172º Fahrenheit, however, the bacterial activity is decreased and stabilization slows. When temperatures exceed about 120º Fahrenheit, thermophilic organisms, which grow and thrive in the temperature range 115º to 160º Fahrenheit, develop and replace the mesophilic bacteria in the decomposition material. Mesophilic organisms live in temperatures of 50º to 115º F. Only a few groups of thermophiles are active above 160º. Oxidation at thermophilic temperatures takes place more rapidly than at mesophilic temperature and, hence, a shorter time is required for stabilization.
High temperatures destroys pathogenic bacteria and protozoa (microscopic one celled animals), and weed seeds, which are detrimental to health and agriculture when the final compost is used on the land.
Aerobic oxidation does not stink. If odors are present, either the process is not entirely aerobic or there are materials present, arising from other sources than the oxidation, which have an odor. Aerobic decomposition or composting can be accomplished in pits, bins, stacks, or piles, if adequate oxygen is provided. Turning materials or other techniques for adding oxygen are necessary to maintain aerobic conditions.
Putrefactive breakdown of organic material takes place anaerobically. Organic compounds break down by the action of living anaerobic organisms. As in the aerobic process, the organisms use nitrogen, phosphorus, and other nutrients in developing cell protoplasm but reduce organic nitrogen to organic acids and ammonia. Carbon from organic compounds, which is not used in the cell protein, is liberated mainly in the reduced form of methane CH4. A small portion of carbon may be respired as CO2.
This process takes place in nature, such as decomposing organic mud at the bottom of marshes and in buried organic materials with no access to oxygen. Marsh gas, which rises, is largely CH4. Intensive reduction of organic matter by putrefaction is usually accompanied by odors of hydrogen sulfide and of reduced organic compounds which 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 when put on the soil. This oxidation is minor, takes place rapidly, and is of no consequence in the utilization of the material on the soil.
There is enough heat energy liberated in the process to raise the temperature of the putrefying material. In the anaerobic dissimulation of the glucose molecule, only about 26 kcal of potential energy per gram-molecule of glucose are released compared to 484 to 674 kcal for aerobic decomposition. The energy of the carbon is in the CH4 released. If the CH4 is burned to CO2, large amounts of heat are involved. In many instances, the energy of the CH4 from an aerobic destruction of organic matter is utilized in engines for power and burned for heat.
Since there is no significant release of heat to the mass in anaerobic composting, this could pose a problem for treatment of contaminated materials. High temperatures are needed to destroy pathogens and parasites. High temperatures do not play a part in the destruction of pathogenic organisms in anaerobic composting. The pathogenic organisms do disappear in the organic mass, because of the unfavorable environment and to biological antagonisms. The disappearance is slow and the material must be held for periods of six months to a year to ensure relatively complete destruction of Ascaris eggs. Ascaris are nematode worms that can infest the intestines. These 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 containing 40% to 75% moisture, into which little oxygen can penetrate, or in composting systems containing 80% to 99% moisture so that the organic material is a suspension in the liquid. When materials are composted anaerobically in this way, not covered with water, the odor nuisance may be quite severe. However, if material is kept submerged, 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, no serious nuisance is created.
While composting can be either aerobic or anaerobic, some bacteria can grow under either condition but may grow better under one condition. Compost piles under aerobic conditions attain a temperature of 140o to 160o 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. 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. In its modern sense, aerobic composting can be defined as a process in which, under suitable environmental conditions, aerobic organisms, principally thermophilic, utilize considerable amounts of oxygen in decomposing organic matter to a fairly stable humus
The term "anaerobic composting" is used to describe putrefactive breakdown of the organic matter by reduction in the absence of oxygen where end products such as CH4 and H2S are released.