1995 Volume 4 Pages 60-74
To utilize sewage sludge in agriculture, the decomposing process of dry sewage sludge and sludge compost after incorporation into soil and their effects on plant growth were investigated by the following methods : (1) Mineralization of nitrogen of sewage sludge was investigated by a kinetic study at three different temperatures (10, 20, 30℃). Dry sludge showed an nonlinear parallel equation having two fractions of rapidly decomposable and slowly decomposable nitrogen, but sludge compost showed a simple nonlinear equation having only slowly decomposable nitrogen fraction. The period in which 99% of rapidly decomposable nitrogen was decomposed was calculated by computer simulation from the obtained equations for nitrogen mineralization, and it was named a "dangerous period to cultivation". These periods for dry sludge were 33, 11 and 4 days at 10, 20 and 30℃, respectively. The dangerous periods did not exist in sludge compost. (2) Cultivation experiments using komatsuna indicated that safe amounts of an application of dry sludge and sludge compost were about 0.5 and 2% on a dry weight base, respectively. (3) From the analysis of thermograms measured with a microcalorimeter, it was found that heat generation of dry sewage sludge in soil was 81.0 kjoule g^<-1>, while that of sludge compost was only 7.7 kjoule g^<-1>. Thus, it was confirmed that the most part of organic matter in sewage sludge had been decomposed by composting and that harmful effects of decomposition in soil on plant growth could be avoided.
