Effect of Dissolved Air Flotation on Algae Removal in Seawater Desalination Pretreatment During Harmful Algal Blooms

To secure the stable operation of seawater reverse osmosis during harmful algal blooms (HABs), this pilot-scale study evaluated an emergency pretreatment strategy combining sodium hypochlorite (NaClO) pre-oxidation, dissolved air flotation (DAF) and postsand filtration (SF) to optimize algae and turbidity removal. Results demonstrated that moderate NaClO pre-oxidation effectively avoided the risk of flocculant oxidation while enhancing algal and turbidity removal by approximately 6% and 18% after DAF, respectively. About 12 mg/L polyaluminum chloride (PAC) and 15 mg/L FeCl₃ showed optimal algae removal performance, but PAC had a better coagulation effect on DAF than FeCl₃, which was 26.8% higher than FeCl₃. Low-dose cationic polyacrylamide (PAM) could further enhance flocculation. The SF unit after DAF served as a key physical barrier, elevating overall algae removal to > 95% and achieving 100% turbidity removal under the optimal dosing process (0.5 mg/L NaClO + 12 mg/L PAC + 0.2 mg/L PAM). This study provided a technically feasible emergency strategy for seawater desalination pretreatment against HABs shocks, ensuring water supply security.