Composting 101bulking materials are needed to be added to these wastes to raise the C/N ratio,
provide structural support for the compost pile, and create void spaces in case of
aerobic composting. Organic amendments are materials added to the composting
feed to increase the quantity of degradable organic C, reduce bulk weight, and
increase air voids of the compost mixture; examples of these materials are
sawdust, rice straw, peat, rice hulls, and domestic refuse. Bulking materials can
be either organic or inorganic of sufficient size which, when added to sludge,
will provide structural support and maintain air space in the compost mixture.
Dried water hyacinth (Eicchornia crassipes) and rice straw, shredded into small
pieces (2-3 cm long), and were found to be suitable as both organic amendments
and bulking materials for the composting of nightsoil (Polprasert et al. 1980).
3.4.3 Moisture control
An optimum moisture content of the compost mixture is important for microbial
decomposition of the organic waste. Since water is essential for nutrient
solubilization and cell protoplasm, moisture content below 20% can severely
inhibit the biological process. Too-high moisture content will cause leaching of
nutrients and pathogens from the compost pile. In aerobic composting, too much
water will block air passage, causing the compost pile to become anaerobic. A
moisture content of between 50-70% (average 60%) is most suitable for
composting and should be maintained during the periods of active bacterial
reactions, i.e. mesophilic and thermophilic growth (Figure 3.2).
Since nightsoil, sludge, and animal manure usually have moisture contents
higher than the optimum value of 60% (see Chapter 2), the addition of organic
amendments and bulking materials will help reduce the moisture content to a
certain degree. On the other hand, most agricultural residues have moisture
contents lower than 60% and some water have to be added during the
composting period of these wastes. For batch operation of composting, the
moisture content of the compost mixture can be controlled by adding water to
the compost piles one or two times daily. The moisture content should be
controlled at the optimum range until the thermophilic period is completed
which is evidenced from the decline of temperature in the compost pile and the
occurrence of the second- and third-level consumers (Figure 3.3).
For a composting system which is continuously operated, the control of
moisture content of compost mixture can be achieved through the recycle of
composted product, as schematically shown in Figure 3.4 (Haug 1979).