In the presence of surfactant sodium dodecylbenzene sulfonate (SDBS), the development of main hydrolysis products (soluble
carbohydrate and protein) and hydrolytic enzyme activities (amylase and protease) were monitored during microaerobic
thermophilic digestion (MATD) of waste activated sludge (WAS). Results showed that the presence of SDBS remarkably
increased the solubility of protein and carbohydrate into aqueous phase. The maximal concentration of carbohydrate and protein
was 990.39 and 2832.67 mg/L at SDBS dosage of 0.02 g/g dry sludge (DS), while it was only 513.92 and 1791.37 mg/L in the
blank test (without SDBS). But the activities of amylase and protease were improved only at lower SDBS dosage (0.02 g/g DS),
the maximum of whose showed a 1.23-fold and 1.20-fold as those of the blank test, respectively. The maximum activity of
enzymes decreased with the SDBS dosages increasing to 0.1-0.2 g/g DS. The effect of pH on the hydrolytic enzyme activities
was studied at 4.0, 6.0, 7.0, 8.0 and 10.0, respectively. Results revealed that the higher enzyme activities were produced at the
neutral condition (pH 7.0), which suggested a predominance of neutrophilic populations during MATD.