Algae-removing and algicidal efficiencies of polydiallyldimethylammonium chloride composite coagulants in enhanced coagulation treatment of algae-containing raw water

Enhanced coagulation treatment of algae-containing raw water was studied by using three series of composite coagulants, that were prepared by combining polydiallyldimethylammonium chloride (PDADMAC) with different intrinsic viscosity values (0.55–2.47 dL/g) and mass percentages (5–20%) with polyaluminum chloride (PAC), aluminum sulphate (AS) and the composite of aluminum sulphate and ferric chloride (A-F), respectively. The coagulants were tested by jar tests for the efficiencies to remove algae in raw water and to kill algae in settled sludge. It was found that when the residual turbidity of supernatant after sedimentation reached the control standard of 2 NTU, the algae-removing rates of PAC, AS and A-F in raw water were 89.5%, 83.2% and 86.0%, respectively, and the algicidal rates in sludge were 0.03%, 0 and 0.20%, respectively. The algae-removing rates of PAC/PDADMAC (0.55/5%–2.47/20%), AS/PDADMAC (0.55/5%–2.47/20%) and A-F/PDADMAC (0.55/5%–2.47/20%) with significantly reduced dosages compared with PAC, AS and A-F in raw water were 89.3–90.6%, 84.7–85.5% and 84.3–73.5%, respectively, and the algicidal rates in sludge were 5.32–10.4%, 6.09–19.1% and 5.43–12.3%, respectively. The algicidal efficiencies of composite coagulants stemmed from the algicidal action of PDADMAC, and the inorganic coagulants had little algicidal function under the experimental conditions.

► Treatment of algae-containing raw water is studied by using composite coagulant.
► Composite coagulant is prepared by combining PDADMAC with inorganic coagulant.
► Using composite coagulant can significantly improve algae-removing efficiency.
► Algicidal efficiency of composite coagulant stems from algicidal action of PDADMAC.
► Inorganic coagulant has little algicidal function under experimental condition.