90 Organic waste recycling: technology and management
fertilizers/soil-conditioners. Figure 3.1 shows the influence of time and
temperature on die-off of selected pathogens in nightsoil and sludge. The
higher the temperature the shorter the time required for pathogen die-off.- Nutrient and land reclamation. The nutrients (N, P, K) present in the
wastes are usually in complex organic forms, which are difficult for the
crops to uptake. After composting, these nutrients would be in inorganic
forms such as NO 3 - and PO 4 -3 suitable for crop uptake. The application of
composed products as fertilizer to land reduces loss of nutrients through
leaching because the inorganic nutrients are mainly in the insoluble forms
which are less likely to leach than the soluble forms of the uncomposted
wastes. In addition, the soil tilth is improved, thereby permitting better
root growth and consequent accessibility to the nutrients (Golueke 1982).
The application of compost to unproductive soils would eventually
improve the soil quality and the otherwise useless lands can be reclaimed. - Sludge drying. Human excreta, animal manure and sludge contain about
80-95% water which makes the costs of sludge collection, transportation
and disposal expensive. Sludge drying through composting is an
alternative in which the waste heat biologically produced will evaporate
the water contained in the sludge.
A major drawback of composting concerns the unreliability of the process in
providing the expected nutrient concentrations and pathogen die-offs. Because
the characteristics of organic wastes can vary greatly from batch to batch, with
time, climates, and modes of operation, the properties of the composted products
would also vary accordingly. The heterogeneous nature of materials in the
compost piles usually causes uneven temperature distribution (except in well-
operated compost reactors - see section 3.6), resulting in incomplete inactivation
of pathogens present in the composted materials. Other limitations of
composting relate to socio-economic factors. For example, the handling of
nightsoil during composting can be unappealing, unaesthetic and obnoxious in
odor. Most farmers still prefer to use chemical fertilizers because they are
relatively not too expensive and, in short term, produce reliable results on crop
yields.