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Phagotrophic algae can consume bacteria that are the predominant microorganisms present in the waste activated sludge (WAS) generated from municipal wastewater treatment processes. In this study, we developed a combined ultrasonicationphagotrophic algal process for WAS conversion. The ultrasonic pretreatment released small volatile solids (VS) including bacteria from WAS flocs. A phagotrophic alga Ochromonas danica then grew by consuming more than 80% of the released VS, with approximately 30% (w/w) algal cell yield. The process reduced the overall WAS VS by 42.4% in 1 day, comparing very favorably with the 27% reduction in 10 days by aerobic digestion. For stabilizing the solids remaining from the ultrasonic step, the total oxygen uptake required was 65%-92% lower than that for the original WAS, indicating substantially reduced aeration cost. Overall, this novel process enhanced the WAS digestion at lower energy requirements and produced microalgae for other potential uses. © 2021 Water Environment Federation • Practitioner points• At least 80% of released VS from WAS can be processed by phagotrophic algae.• Significant amounts of algae can be produced from WAS.• Ultrasonication-phagotrophic algal process can make sludge management more sustainable.