Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
Composting is an aerobic bacterial decomposition process to stabilise organic
wastes and produce humus (compost). Compost contains nutrients and organic
carbon which are excellent soil conditioners. Composting takes place naturally
on a forest floor where organic materials (leaf litter, animal wastes) are
converted to more stable organic materials (humus) and the nutrients are
released and made available for plant uptake (Section 2 (2.2)). The process is
slow on a forest floor, but can be accelerated under optimum conditions.
The optimum conditions for composting are a moisture content of about 50 %,
a carbon to nitrogen ratio of about 25 to 30, and temperature of 55 °C.
Because wastewater sludge is rich in nutrients its carbon to nitrogen ratio
is low (5 to 10). It is also high in moisture. Addition of dry saw dust, which
is very high in carbon to nitrogen ratio (500) can adjust both the moisture
and carbon to nitrogen ratio. Other waste materials that can be used for this
purpose are mulched garden wastes, forest wastes and shredded newspaper.
Composting can be carried out in a specially built composter, such as an inclined
rotating cylinder, fed on one end with the raw materials, and the aerated product
collected at the other end. As the materials are slowly tumbled over a period
of about one week, they are mixed and aerated. Because bacterial decomposition
produces heat, temperatures in the insulated composter can easily reach 55 °C.
The immature compost is then windrowed for at least 12 weeks to allow the composting
process to complete, with occasional turning of the windrow.
Composting can be more simply carried out in windrows (Figure 2.40). Regular
turning of the windrows assists with mixing of the materials and more importantly
supply the oxygen to the bacteria. Temperatures can reach 55 °C, because
compost has a good heat insulating property. Turning of the compost also ensures
that all parts of the windrow reach the required 55 °C essential for pathogen
destruction. Turning is required every two to three days in the first two weeks
when temperature is 55 °C or above. After this period frequent turning of
the compost windrow is not required as less heat is generated and less oxygen
is required while the compost undergoes maturation.
Figure 2.40: Windrow composting of faecal sludge (FS) (from Heinss et al.,