Stabilization Process of Sewage Sludge Compost in Soil

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Abstract
The stabilization processes of sewage sludge composts with different degrees of maturity, which were incorporated into a coarse sandy loam soil under a rather intensified loading rate of 5% in dry weight, were investigated during their incubation at 25°C. In the immature compost-soil mixtures, ammonification, decrease of nitrate concentration, and/or accumulation of nitrite were observed in the early stages of incubation and followed by nitrification. In the mature compost-soil mixtures, only nitrification was observed during the incubation. These nitrogen transformations were wen correlated with the transitions of characteristic patterns; I, II, and III of gel chromatograms of the water extracts. The redox potential decreased appreciably in the immature compost-soil mixtures in the early stages of incubation and increased in the later stages, when pattern III of the chromatogram was observed. When sludge composts became stable in soil, as shown by the chromatographic pattern III, the values of pH and NH4 +/NO8 − ratios in the mixtures were about the same independently of the degree of compost maturity. The time required for the stabilization in soil became shorter with the increase in the degree of maturity. It was suggested that the sludge compost in which ammonification had ended during composting is stable enough for practical land application.