Composting 97In order for the composting process to function effectively, a suitable number of
organisms must be present which are capable of attacking the types of wastes to be
stabilized. These organisms are naturally present in the wastes, such as nightsoil,
animal manure, and wastewater sludges; hence, compost seeding is usually not
necessary. Although packages of compost inoculum are commercially available,
controlled scientific tests showed no increased benefits over natural sources of
organisms (Dindal 1978). However, some types of agricultural wastes such as rice
straw, leaves, and aquatic weeds which do not readily have these organisms may
require the seeding of nightsoil or sludge at the starting period.
3.4 Environmental requirements
The effectiveness of a composting process is dependent upon the groups of
organisms that inhabit and stabilize the organic wastes. Any process failure may
be due to some unbalanced chemical and physical conditions in the compost
piles which are unfavorable for microbial growth. The major environmental
parameters needed to be properly controlled in the operation of composting
processes are as follows:
3.4.1 Nutrient balance
The most important nutrient parameter is the carbon/ nitrogen or C/N ratio.
Phosphorus (P) is next in importance, and sulfur (S), calcium (Ca) and trace
quantities of several other elements, all play a part in cell metabolism.
Alexander (1961) reported that between 20-40% of C substrate in the organic
wastes (compost feed) is eventually assimilated into new microbial cells in
composting, the remainder being converted to CO 2 in the energy-producing
processes. However, these cells contain approximately 50% C and 5% N on a
dry weight basis or the C/N ratio is about 10/1. If 30 % of C substrate is
converted into microbial cells, to make a balanced substrate proportion of cell
growth, the initial C/N ratio of the compost feed should be adjusted to 30/1.
Accordingly, the requirement of N in the composting feed is 2 - 4% of initial C,
i.e. a C/N ratio range of about 20/l - 40/1 should be considered optimum for
biological processes.
The C/N ratios of various wastes are shown in Table 3.1. Except for horse
manure and potato tops, the C/N ratios of other wastes should be adjusted to the
optimum value of 25/1 prior to being composted. In practice, accurate
calculation and adjustment of optimum C/N ratio are made difficult by the
following factors (Gray et al. 1971).