BACKGROUND: Reduction of excess sludge production has become an urgent issue. An investigation into the influence of thermochemical sludge pretreatment on sludge reduction in a bench-scale anoxic-aerobic membrane bioreactor was performed. Two systems were operated. In one system, part of the mixed liquid (1.5% of the influent flow rate) was pretreated thermochemically (at 80 degrees C, pH 11 and 3 h) and returned to the bioreactor. This study examined and evaluated the effect of thermochemical sludge pretreatment on the reduction of excess sludge and on the performance of the system. RESULTS: The average solubilization efficiency of the pretreated sludge was found to be about 0.2. The sludge production rate of the experimental system (E-MBR) was less than that of the control (C-MBR) by about 33%. The total phosphorus was removed mainly by normal cell synthesis, with removal efficiencies of 38-40% and 40-42% for the E-MBR and C-MBR, respectively. The total nitrogen removal in the E-MBR was slightly higher than in the C-MBR due to supply of soluble chemical oxygen demand (SCOD) from the digested sludge solution as an external carbon source. The mixed liquor volatile suspended solids (MLVSS) and mixed liquor suspended solids (MLSS) ratios for the two systems were almost identical, in the range 74-77%, indicating that the inorganics from the disintegrated cells do not accumulate as particulates in the reactor. The TMP was maintained at less than 6 cmHg for 180 days without membrane cleaning. CONCLUSION: Thermochemical sludge pretreatment can play an important role in reducing sludge production. The qualities of the effluent water were not significantly affected during 6 months of operation. (c) 2009 Society of Chemical Industry