A novel Tubificidae-reactor with special porous carrier and combined aeration system was designed to investigate the sludge reduction. The influences of change in frequency of high-intensity aeration (FHIA), dissolved oxygen (DO) content, initial sludge concentration (ISC) and sludge retention time (SRT) on the immobilization of Tubificidae and sludge reduction were evaluated. Furthermore, response surface method was applied to optimize the process conditions using the sludge reduction rate and stable immobilization of worms as the target parameters to optimize, and DO as the control factors to be optimized with the fluctuation of influent ISC, while keeping FHIA and SRT at optimum level. Attractive sludge reduction rate (470 mg L(-1) d(-1)) can be obtained using the strategies indicated by the highly correlated model. The optimum conditions were found to be DO of 1.0-1.6 mg/L and ISC of 3000-4000 mg/L respectively, while keeping FHIA at 12 times/d and SRT at 2 d. (C) 2010 Elsevier Ltd. All rights reserved.