Comparisons of coagulation efficiency of conventional aluminium sulfate and enhanced composite aluminium sulfate/polydimethyldiallylammonium chloride coagulants coupled with rapid sand filtration

Abstract

Conventional aluminium sulfate (AS) coagulant's inability to effectively meet stringent water quality requirements have created the need to explore enhanced composite coagulants. The coagulation efficiency of AS and composite AS and polydimethyldiallylammonium chloride (AS/PDMDAAC) coagulants were investigated using jar test and then, coupled with rapid sand filtration (RSF). The various composite AS/PDMDAAC at different intrinsic viscosity of PDMDAAC and mass ratio of PDMDAAC to AS in coagulant solutions were optimized and operated at same supernatant turbidity (SST) or same dosage mode (SD). The results showed about 7–10% reduction in dosage for all composite AS/PDMDAAC and also improved water qualities compared to AS only. The patterns of water quality improvements at lower (0.65 dL/g), medium (1.60 dL/g), and higher (2.60 dL/g) intrinsic viscosity of cationic polymer PDMDAAC among different mass ratio of PDMDAAC to AS in the composite mixtures were 5:1 > 20:1 > 10:1; 10:1 ≥ 5:1 > 20:1; 20:1 > 10:1 > 5:1 respectively. The percentage removal efficiencies for AS and optimal composite coagulant at intrinsic viscosity (1.60 dL/g) and mass ratio (10:1) operated at SST or SD after filtration for CODMn, NH3N and turbidity were (71.20, 72.72, 73.33%); (69.15, 69.23, 73.08%) and (93.89, 95.07, 97.26%) respectively. It was concluded that enhanced composite AS/PDMDAAC coagulants operated at SST mode showed significant water quality improvement than conventional AS having 7% and 31.8% for COD and NH3N removal while operational mode at SD produced more significant water quality improvements than conventional AS with 9.49%, 45.45%, and 39.55% for COD, NH3N, and turbidity removal before filtration respectively. More importantly, all composite AS/PDMDAAC met stringent water quality requirements before and after filtration. This research provided new insight about application of economical and effective alternative composite AS/PDMDAAC coagulant useful in the treatment of water for large population.