The effects of ozone and alkaline pretreatment of sewage sludge on the performances of membrane-coupled aerobic sludge digestion were investigated. In particular, the effects of sludge pretreatment on the solubilization, sludge biodegradation and the stability of membrane filtration were evaluated. Three sets of reactors were operated under different conditions of sludge treatment; 1) ozone treatment at 0.1 g O3 g(-1) suspended solids (SS) of the influent sludge, 2) alkaline treatment at pH 11.4-12.0, 3) no treatment. Without sludge pretreatment, 27% of suspended solids (SS) reduction was obtained at the hydraulic retention time(HRT) of 5 days. With ozone and alkaline treatment, the average SS reduction increased to 83 and 76%, respectively, at the same HRT. Membrane fouling occurred earliest with the non-treated sludge, followed by the alkali-treated sludge. With ozonated sludge, stable membrane filtration for more than 150 days was possible without chemical cleaning of the membrane. The dynamic viscosity of the mixed liquor in the reactor fed with ozonated sludge was relatively low, indicating the role of ozone treatment in controlling membrane fouling. In conclusion, sludge pretreatment followed by aerobic sludge digestion in a membrane-coupled bioreactor can achieve a very high rate of sludge reduction at a relatively short HRT.