Coagulation-ultrafiltration efficiency of polymeric Al-, Fe-, and Ti- coagulant with or without polyacrylamide composition

Abstract

Coagulant is the key variable determining the coagulation-ultrafiltration efficiency. As commonly used coagulants, polymeric aluminum chloride (PAC), polymeric ferric sulfate (PFS), and titanium xerogel coagulant (TXC) exhibit different coagulation behaviors and mechanisms. This study investigated their coagulation-ultrafiltration efficiencies in algae-laden water. Results showed that PAC with the strongest charge neutralization ability performed the best in accelerating the filtration flux under neutral and alkaline conditions. PAC with dose of 8 mg Al/L effectively removed algae cells (> 95%) and organic matters (about 50%) during coagulation process, and subsequently improved the normalized flux (J/J0) from 0.18 to 0.59–0.71. Under acidic condition, pre-coagulation with TXC (8 mg Ti/L) was slightly superior to the equal-dose of PAC and PFS in terms of filtration flux (J/J0 was 0.90) and recovery rate (91%), since its flocs caused the minimal irreversible fouling resistance. Overdosed coagulant (30 mg M/L, M represents metal ion) under acidic condition also facilitated the subsequent membrane filtration (J/J0 was 0.70–0.73), even though the pollutants were re-stabilized in solution. Co-coagulation with cationic polyacrylamide (CPAM) promoted the aggregation of algal cells, thereby reducing the reversible fouling resistance and then improving the filtration flux. Increasing CPAM dose did not further improve the ultrafiltration efficiency, due to its limited removal ability toward organic matter. For CPAM-enhanced coagulation-ultrafiltration treatment, the dose of CPAM must be strictly controlled at the least level corresponding to the effective removal toward turbidity.