BACKGROUND The production of microalgal biomass as a potential biofuel source is limited by expensive harvesting costs. However, air flotation has been applied as an effective method for harvesting microalgal cells from diluted liquid suspensions. In this study, Chlorella vulgaris and Scenedesmus obliquus harvesting experiments were performed with thermal pretreatment prior to air flotation. RESULTS An orthogonal test showed that the harvesting efficiency was affected by the heating temperature and pH but not by the microalgal concentration or stirring rate. Under a heating temperature of 90 degrees C and pH 3, the optimal heat-aided air flotation efficiency achieved for harvesting C. vulgaris was 26.5%, with a concentration factor of 5.5. For harvesting S. obliquus, the optimal harvesting efficiency was 93.6% and the concentration factor was 25.0 under a heating temperature of 70 degrees C and pH 3. CONCLUSION High temperature and low pH increased the zeta potential value of the microalgal cell surface, and high temperature promoted the secretion of extracellular organic matter, causing microalgal cells to gather and form flocs. The smaller flocs and higher zeta potential value of S. obliquus led to its higher harvesting efficiency. These conditions provided the possibility of waste acidic gas and thermal energy from industries replacing flocculants for the harvesting process. (c) 2019 Society of Chemical Industry