Heat treatment has been used for the reduction of excess sludge. To clarify the characteristics of lysis process during the heat treatment, bacterial population and hhydrolytic enzyme (protease) activity succession of a municipal activated sludge have been studied. Culture-dependent method and culture-independent method by denaturing gradient gel electrophoresis (DGGE) were applied to analyze the change in bacterial population. The cultivation method showed the thermoduric protease-secreting bacterium of Bacillius subtilis and thermophilic protease-secreting bacterium of Geobacillus stearothermophilus were the dominant bacteria during the heat treatment. While the PCR-DGGE fingerprint showed that some bands disappeared and some bands increased in density during the heat treatment, which could be pertaining to the lysis-cryptic growth of microbial community in the sludge. After the heat treatment, the Bacilli became dominant class in the community. The protease activity in the supernatant of the sludge before the heat treatment was nearly zero, however after 1 h heat treatment, the activity increased apparently, and the alternation in protease activity indicated that the lysis process was induced by the heat treatment.
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