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 be used as indicators of compost maturity. One treatment was a mixture of sewage sludge and sawdust (TS); another was a mixture of sewage sludge, pig manure and sawdust (TS-PM). 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 TS-PM, 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.