Microbiology of Composting

Microbiology of Composting

	Microbiology of Composting - Composting involves different groups of microorganisms which act on the substrates in succession. Although the decomposition is mainly carried out by the thermophilic microorganisms, the initial process is carried out by mesophilic microorganisms. In the initial stages of composting the heap of materials to be composted will be at ambient temperature and the pH will be more or less neutral. At first the biodegradable materials, such as proteins and carbohydrates, are decomposed naturally by the mesophilic microorganisms. The degradation products are usually simple organic acids which causes a drop in pH. As the initial reactions proceed heat is produced which rises the temperature within the heap.
The insulating effect of heap further causes an increase in temperature. However, the maximum temperature achieved and the time taken to achieve it depend on various factors like composition of organic wastes, moisture content, size of heap and degree of aeration and agitation. As the temperature increases, the activity of mesophiles are gradually reduced and finally at about 60°C the mesophiles are killed by the high temperature. They may become the source of nutrients for higher forms like protozoa, rotifers and nematodes. They may even add to the total organic content. At this temperature, thermophilic microorganisms take over the degradation process. Hemicellulose, cellulose and lignin are degraded rather slowly. Thermophilic bacteria like Thermonospora and Thermoactinomyces and thermophilic fungi such as Thermoascus carry out these reactions.
Once the readily degradable materials are degraded, the reaction rate slows down. Eventually the temperature decreases once again to the mesophilic range through heat loss from the surface of the heap. Once again the mesophiles, either through re-invasion from outside or through germination of heat resistant spores, dominate. The thermophilic process has certain advantages as follows: - acceleration of the process - killing of pathogenic microorganisms - destruction of ungerminated seeds of weeds.
Although the pathogenic microorganisms are destroyed, sometimes spores of the fungi Aspergillus may be found in the vicinity of composting areas. However, any adverse effects caused by these microorganisms are highly localized and hence there will not be any threat to residential areas. In certain cases, starter culture from old compost fermentations may be used as an inoculum for new ones.

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