The application of phagotrophic algae in waste activate sludge (WAS) treatment can largely accelerate WAS organics reclamation rate, because the phagotrophic capability of phagotrophic algae on intact microbial cells provides an efficient and fast way to digest microbial cells, which eliminate the need of the time-consuming hydrolysis of WAS solid organics. In this research work, different pretreatment methods of anoxia, alkali, and ultrasound were compared in terms of efficiency in releasing microbial cells from WAS flocs and subsequent growth of phagotrophic algae. Results show that ultrasonic pretreatment was the most efficient method since it yielded the highest microbial cell release and subsequent algae growth, while requiring the shortest pretreatment time. Mathematical modelling shows that the release of microbial cells in anoxic and ultrasonic pretreatment was achieved mainly through breaking physical forces of attraction, while the release of microbial cells in alkali pretreatment was achieved through breaking ionic bonds. Re-floc of released microbial cells was observed during algae growth, and the microbial cells released through breaking ionic bonds were less liable to reform floc. The electricity consumption of ultrasonic pretreatment was the highest, which was 6.9 times and 17.9 times of that of alkali and anoxic pretreatment. The electricity consumption of ultrasonic pretreatment can be reduced by performing alkali pretreatment prior to ultrasonic pretreatment. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.