The objectives of this study were to investigate the behavior of Phormidium sp. during flocculation and negatively or positively charged bubble flotation in order to optimize algal removal processes and identify mechanisms underlying the efficiency of flotation with positively charged bubbles. The nuisance of Phormidium sp. significantly decreases water quality in natural watershed and clogs filter bed in water treatment plant. Although dissolved air flotation has been recently adopted for algae removal, the best method has not been fully investigated. According to theories on dissolved air flotation, the operational conditions affect removal of the process and in this study, the optimum bubble generations was also investigated for better algal removal. Bubbles were generated at two levels of saturated pressure and measured at different bubble concentrations (10%, 20% and 30%), in the absence and presence of coagulants. Bubbles forming at 6 bars and 3 bars were observed at zeta potentials of -30 mV to +27 mV. The chain-like algae were cultured in the laboratory for 20 days. At the stationary phase, Phormidium sp. sizes ranged from 2 mu m to 10 gm in diameter and about 100-200 mu m in length. Over a pH range of 4.0-7.0 (increments of 0.5), the negative zeta potentials were -4 mV to -12 mV. Algal removal by flocculation was determined by jar tests and by the batch dissolved air flotation (BDAF) method with bubble generation and flotation. We obtained optimal Phormidium sp. removal with positively charged bubble flotation at a 30% bubble rate at >16 mV and a bubble formed at 6 bars, with removal of up to 85% and 93% of cells and chlorophyll a, respectively. We also demonstrated the efficacy of using positively charged bubbles to remove Phormidium sp. cells and the importance of positively charged bubbles in the rarely reported interaction between bubbles and chain-like algae. (C) 2014 Elsevier Ltd. All rights reserved.