136 Organic waste recycling: technology and management
of these elements, the greater the fertilizer value of the compost). Most of the N
in the compost in an organic form and must be mineralized (such as through
curing) to inorganic ammonium or nitrate before it is available to plants.
Composted organic wastes function as slow-release N fertilizers when applied to
soil. The K content of sludge compost is usually low (0.5%) because it is water
soluble and remains in the effluent after wastewater treatment. However, in
refuse compost the level of K may exceed 1 percent. The N, P and K values of
compost do not only depend on the type of materials composted but also the
initial C/N ratio of those materials.
Table 3.8 shows the composition of compost produced from municipal refuse
of Bangkok city, Thailand. The percentages of N, P, K contents were 2.58, 1.67,
and 0.58, respectively, which are several times lower than those present in
chemical fertilizers. This compost thus needs to be mixed with chemical
fertilizers prior to being used as fertilizer.
Table 3.8 Composition of compost produced from municipal refuse
Items % of composition
(based on dry solids)
Total-N 2.58
Total-P (P 2 O 5 ) 1.67
Total-K (K 2 O) 0.58
Magnesium (Mg) 0.49
Calcium (Ca) 6.20
Sulfur (S) 0.37
Iron (Fe) 4.39
Manganese (Mn) 0.16
Copper (Cu) 0.09
Zinc (Zn) 0.30
Baron (B) 0.051
Molybdinum (Mo) 0.001
Chlorine (Cl) 0.66
Moisture Content 23.86
pH 7.2 (dimensionless)Source: The office of Bangkok municipal fertilizer (personal communication)
An experiment was carried out at AIT by composting night soil, water
hyacinth and leaves (compositions are shown in Table 3.9) with different initial
C/N ratios (Table 3.10). The method of composting employed was similar to
that shown in Figures 3.11 and 3.12. The percent N, P and K of the composted