Factors affecting Composting Process - 2
14.4.4 Temperature
The aerobic decomposition of a gram mole of glucose releases 484 to 674 kilo calories (kcal) energy under controlled conditions, while only 26 kcal are released when it is decomposed anaerobically. Municipal solid waste is known to have good insulation properties and hence the released heat results in increase in temperature of the decomposing mass. 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 windrows, the resultant heat loss results in drop in temperatures. Under properly controlled conditions temperatures are known to rise beyond 70oC in aerobic composting. Under properly controlled conditions temperatures are known to rise beyond 70oC in aerobic composting. During anaerobic composting as the released heat is quite small and as part of it is lost from the surface only a marginal rise in temperature occurs.
This increased temperature results in increased rate of biological activity and hence results in faster stabilisation of the material. However, if the temperature rise is very high, due to inactivation of the organisms & enzymes the rate of activity may decrease. The studies carried out have shown that the activity of cellulose enzyme reduces above 70oC and the optimum temperature range for nitrification is 30o to 50oC beyond which nitrogen loss is known to occur. The temperature range of 50o to 60oC is thus optimum for nitrification and cellulose degradation.
The high temperature also helps in destruction of some common pathogens and parasites (Table 14.1). According to Scott, during aerobic composting when the material is turned twice in 12 days Entamoeba histolytica is killed and the eggs of Ascaris lumbricoides are killed in 36 days when turned thrice. The studies carried out at NEERI have shown that the destruction of these organisms is not ensured under anaerobic conditions.
Knoll has proved that the high temperature and long retention during aerobic composting along with the antibiotic effect results in destruction of parasites and pathogens. Thus, if the process is so controlled that the temperature is kept between 50o to 60 o C for 5 to 7 days, destruction of pathogens and parasites can be ensured.
14.4.5 Carbon/Nitrogen (C/N) Ratio
The organisms involved in stabilisation of organic matter utilise about 30 parts of carbon for each part of nitrogen and hence an initial C/N ratio of 30 is most favourable for composting. Research workers have reported the optimum value to range between 26-31 depending upon other environmental conditions.
The C/N ratio considers the available carbon as well as the available nitrogen while the available carbon and nitrogen in the MSW may vary from sample to sample. Whenever the C/N ratio 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.
