Biochemical Aspects of Composting

The composting mass is at ambient temperature, but a rapid rise occurs as the microorganism multiply. When the temperature moves above 40°C, the mesophilic stage is replaced by the thermophilic stage. The time required to reach the thermophilic stage varies, but it is frequently achieved in 2 or 3 days (Olds 1968). The temperature stabilizes around 70°C, followed by a gradual cooling to ambient temperature. This temperature pattern has been observed by many investigators for typical garden compost as well as for municipal compost (Webley 1947; Kortleven 1951; Eastwood 1952; Chang and Hudson 1967; Kochtitzky et al. 1969). The temperature gradient from the center outward lessens as the pile size increases. Since heat loss is proportional to surface area and heat generation is proportional to volume, the larger pile, having a smaller surface area to volume ratio loses relatively less heat. In large compost piles, the temperature increases steadily 70°C with time, in smaller piles there is usually a pause or leveling at 40°C while the transition from mesophilic to thermophilic microorganisms occurs (in: Anonymus 1953). Decomposition of organic matter is fastest in the thermophilic stage. The optimal temperature, based on oxidation of organic matter into CO2 and H2O has to be 60°C (Waksman et al. 1939; Wiley 1957; Schulze 1961, in: Poincelot 1972). Eklind et al. (2007) reported that biowaste composting can be optimized to obtain both a high decomposition rate and low ammonia emissions by controlling the process at about 55°C in the initial, high-rate stage. To reduce ammonia emissions it seems worthwhile to reduce the temperature after an initial high-temperature stage. Grinding the materials to be composted speeds their decomposition by increasing their surface area and hence their susceptibility to microbial invasion (in: Anonymus 1953). Better initial aeration is also achieved due to increased availability of oxygen at particle surfaces. In addition, the material is more easily handled and moistened. Gray and Sherman (1969) observed that grinding might double the amount of evolved CO2 as compared to unground material. The greater part of difference due to grinding was observed in the thermophilic temperature range (40-60°C), while little difference was seen in the mesophilic stage (Poincelot 1972). Oxygen is required by aerobic microorganisms during the decomposition process. For a mixed garbage-sewage sludge compost Schulze (1962) found that 5-9 ft3 of air per pound of volatile matter per day was required. Windrows may be aerated by turning (outer edges mixed in with center of pile) or by thoroughly mixing by mechanical means (Poincelot 1972).

Organic Gardeners Composting

Organic Gardeners Composting

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