Alkaline pretreatment is an effective technology to disintegrate sewage sludge, where alkali dosage and sludge concentration are two important factors. pH value or alkali concentration is usually adjusted in order to determine a proper dosage of alkali. Our work has found that this is not a good strategy. A new parameter, the ratio of alkali to sludge (R-a/s), is more sensitive in controlling the alkali dosage. The sludge concentration C-s and retention time t are two other important factors to consider. The validity of these arguments is confirmed with modeling and experiments. The individual effect of R-a/s, C-s and t was studied separately. Then the combined effect of these three factors was evaluated. The sludge disintegration degree of 44.7% was achieved with the optimized factors. Furthermore, an alkaline-microwave combined pretreatment process was carried out under these optimized conditions. A high disintegration degree of 62.3% was achieved while the energy consumption of microwave was much lower than previously reported. (C) 2015 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.