Composting

Composting is the controlled decay of organic matter in a warm, moist environment by the action of bacteria, fungi, and other organisms.

The organic matter may be in municipal solid waste, sewage sludge, septage, agricultural waste, manure, leaves and other yard waste, or combinations of these materials and other organic wastes.

Compost is particularly useful as organic manure which contains plant nutrients as well as micronutrients which can be utilized for the growth of plants. For obtaining optimum results it should be used in conjunction with chemical fertilizers.

Principles of the Composting Process

The organic material present in the municipal wastes can be converted to a stable form either aerobically or anaerobically. During aerobic decomposition, aerobic microorganisms oxidise organic compounds to $CO_2$, $NO_2$, and $NO_3$.

Carbon from organic compounds is used as a source of energy while nitrogen is recycled. Due to the exothermic reaction, the temperature of the mass rises. Anaerobic microorganisms while metabolising nutrients, breakdown the organic compounds by a process of reduction. A very small amount of energy is released during the process and temperature of the composting mass does not rise much. The gases evolved are mainly $CH_4$ and $CO_2$.

As anaerobic decomposition of organic matter is a reduction process, the final product is subject to some minor oxidation when applied to land.

The factors affecting the composting process are as follows:

• Organisms:

1. Aerobic composting is a dynamic system where in bacteria actinomycetes, fungi and other biological forms are actively involved.
2. Facultative and obligate forms of bacteria, actinomycetes and fungi are most active in this process. In the initial stages mesiphilic forms predominate and thermophilic bacteria and fungi soon take over except in the final stage of composting
3. Thermophilic bacteria are mainly responsible for the breakdown of proteins and other readily biodegradable organic matter.

• Use of cultures:

1. Under proper environmental conditions the indigenous bacteria adapted to municipal solid waste rapidly multiply, as compared to the added cultures which are more attuned to controlled laboratory conditions and carry out decomposition.
2. Hence addition of similar and extraneous organisms in the form of inoculums is unnecessary. However, such inoculums will be required during composting of industrial and agricultural solid waste which do not have the large mix of indigenous bacterial population.

• Moisture:

1. The moisture tends to occupy the free air space between the particles. Hence, when the moisture content is very high anaerobic conditions set in.
2. However, the composting mass should have certain minimum moisture content in it for the organisms to survive.
3. The optimum moisture content is known to be between 50 to 60%. Higher moisture content may be required while composting straw and fibrous material which soften the fiber and fills the large pore spaces.

• Temperature:

1. Municipal solid waste is known to have good insulation properties and hence the released heat leads to increase in temperature of the decomposing mass.
2. As some of the heat loss occurs from the exposed surface, the actual rise in temperature will be slightly less. When the decomposing mass is disturbed, as during turning of windows, the resultant heat loss results in drop in temperature.
3. The increased temperature results in increased rate of biological activity and hence results in faster stabilization of the material.

• C/N ratio:

1. The organisms involved in stabilization of organic matter utilize about 30 parts of carbon for each part of nitrogen and hence an initial C/N ratio of 30 is most favorable for composting.
2. Whenever the C/N ratio of the waste is less than the optimum carbon source such as straw, sawdust, paper are added while if the ratio is too high, the sewage sludge, slaughter house waste, blood etc. are added as a source of nitrogen to the waste during its composting.

• Aeration:

1. It is necessary to ensure that oxygen is supplied throughout the mass and aerobic activity is maintained. During the decomposition, the oxygen gets depleted and has to be continuously replenished.
2. This can be achieved either by turning of windows or by supplying compressed air.
3. During turning it is necessary to bring inner mass to the outer surface and to transfer the outer waste to the inner portion. In the case of artificial air supply the quantity of air supply is normally maintained at 1-2 cu.m/day/kg of volatile solids.