Chemical evaluation of sewage sludge composting as a mature indicator for composting process

Several chemical parameters were studied during a 100 days period in order to identify parameters that can bo used as indicators of compost maturity. Ono treatment was a mixture of sewage sludge and sawdust (TS); another was a mixture of sewage sludge, pig manure and sawdust (TSPM). The pH of water extracts increased during the thermophilic phase and then decreased in two treatments. While the electrical conductivity (EC) decreased throughout the composting time. The 4.99% in the change of volatile matter of TS-PM was higher than 1.73% in that of TS, at the same time, the change of organic carbon were 5.04% in TS-PM more than 3.82% of TS. Total nitrogen content in TS and TS-PM had a markedly increase tendence during composting, and the change in TS-PM was very more obvious than in TS, so reduction of C/N ratio in TS-PM was higher than in TS, C/N ratio of TS-PM and TS was 14.39 and 18.57 in mature production, respectively. Carbon content in HA plus FA and HA in two composts decreased from 0 to 100 day, but carbon content of FA decreased, so HA-C/FA-C ratio, HR, HI has a markedly increase in TS and TS-PM, and the parameters in TS-PM were higher than in TS. The points expressing maturity degree in TS-PM was more in TS. The nitrogen content of HA plus and HA showed a increased tendence in TS and TSPM, the change of nitrogen of these fractions in TS-PM was higher than in TS, so C/N ratio of all fractions including HA plus FA, HA and FA, indicated a reduction tendence, and the refuction in FA was larger than in HA. E4/E6 ratio of HA plus FA and HA decreased during composting, and one FA was increase. E4/E6 ratio of HA plus FA and HA in TS-PM was very smaller than in TS, HA in TS-PM was a larger molecular size than HA in TS. The changes of organic matter compositions in composting: the crude fat and carbohydrates had a markedly reduction and cellulose plus hemicellulose had a slight decrease; crude protein and lignin content in TS and TS-PM had an increase tendence.