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Composting is a complex biological process which, when engineered and controlled, can deliver a high quality product which returns nutrients to the soil and improves its structure. This chapter looks at the fundamental processes which drive the composting process and then the key features which the process needs to deliver a quality product.

The biological degradation process can be represented by a series of exponential decay terms, each one representing a different component of the feedstock. Researchers have found that, in practice, the decay of most material mixtures can be represented by using two components, one of which has a ‘fast’ rate, e.g. starch and sugars, and the other having a ‘slow’ rate, e.g. cellulose. The decay of typical wastes used in composting, such as domestic refuse and mixtures of sewage sludge and woodchips, can be represented by using two rate constants. The degradation releases heat, the use of which is the key to effective composting, producing pile core temperatures in the 55 to 70°C range. The elevated temperature serves two purposes, firstly to sanitise the material (kill off most of the pathogens) and secondly to maximise the rate of degradation. Each 10°C increase in temperature, up to around 55°C, effectively doubles the reaction rate, making the process far more rapid than if it occurred at ambient temperatures. The key requirements to produce these conditions in a practical situation, i.e. pile structure (to facilitate aeration), available nutrients (primarily the C : N ratio) and the appropriate moisture content, are considered.

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