A new approach to the solubilization of waste activated Sludge (WAS) using an alkaline protease-producing bacterial isolate, Exiguobacterium sp. YS1, was investigated under controlled mild alkaline conditions at pH 10. Compared with the noninoculated experiment, the inoculated experiment in an anaerobic bioreactor increased soluble chemical oxygen demand concentration and alkaline protease activity by more than 40%, indicating a synergistic effect could be achieved when both bacterial inoculation and alkaline treatment were combined. Indeed, this combination led to 56.6% COD solubilization after 5 days of reaction time. However, the inoculant was not effective in the aerobic bioreactor. Denaturing gradient gel electrophoresis of PCR-amplified 16S rRNA fragments revealed that the inoculated Exiguobacterium sp. YS1 became the predominant population in the bacterial community during the anaerobic solubilization processes. These results Suggest that bioaugmentation of the organism might be useful for enhancing the solubilization of WAS at mild alkaline pH. (C) 2009 Elsevier Ltd. All rights reserved.