Performance improvement and mechanism of composite PAC/PDMDAAC coagulant via enhanced coagulation coupled with rapid sand filtration in the treatment of micro-polluted surface water

Abstract

The coagulation and filtration are widely used for surface water due to cost effectiveness therefore the need to study its’ improvements for safe drinking water that meet national standards. The performance of enhanced coagulation of composite coagulants (ECC) using poly aluminium chloride/poly di-methyl di-allyl ammonium chloride (PAC/PDMDAAC) coupled with rapid sand filtration (RSF) was studied. This was performed using jar testing, optimized, and operated at same dosage and supernatant turbidity (SDST) mode; also, the interception points for PAC and composites at baseline turbidity between 1.0 and 1.50 NTU with fixed RSF operating conditions while water parameters like turbidity, CODMn, NH3N, and zeta potentials were used as indicators for improved removal rates. Furthermore, the floc size and morphology analysis at optimum dosage combined with zeta potentials were used to explained the mechanism of ECC-RSF. The results showed that most composites out-performed PAC only, before and at the interception points; however, beyond this point, the superior performance disappears with the increasing PAC dosage above 3.0 mg/L. At the optimum dosage, composite PAC/PDMDAAC at 1.60 dL/g, 10:1 showed the best overall removal rate at SDST mode of 99.36%, 64.50%, and 66.18%, respectively for turbidity, NH3N, and CODMn. The dominant mechanism of composite coagulation is charge neutralization within the dosage range used and strengthened by adsorption-bridging due to the presence of PDMDAAC while the major mechanism during filtration is adsorption. It was concluded that depending on the raw water quality, the coagulation mode of SDST at baseline turbidity near 1.0 NTU can maximized the water quality removal for composite PAC/PDMDAAC. New operational insights about maximizing the performance of composite PAC/PDMDAAC coagulants have been put forward.