Enhanced Coagulation of Low Turbid Water for Drinking Water Treatment: Dosing Approach on Floc Formation and Residuals Minimization

Abstract

Abstract

Polyaluminum chloride (PACl) has been commonly used in drinking water treatment. However, it is not easy for drinking water treatment plant to carry out an effective coagulant dosing with PACl for low turbidity water with high pH where the excessed Al would remain in the water, which has been a concern for its risk to human health. The goal of this study is to investigate the effects of dosing approaches with alternative coagulants on the minimization of residual particulate matter and Al after coagulation-sedimentation process, and their coagulation behaviors. Two coagulant dosing approaches, including single and dual dosing, were adapted to evaluate coagulation performance in terms of turbidity, remaining Al and filterability for natural low turbidity water. A real-time floc image analyzer (i.e., FlocCAM^™) was used to evaluate the floc size during coagulation with commercial PACl or ferric chloride (FeCl₃) and the combination. The results showed that FeCl₃ coagulation gives the fastest and biggest growth in floc size with active Fe(OH)₃, but single FeCl₃ dosing brings an adverse effect on filterability of low turbidity water treatment. At dual dosing (PACl/FeCl₃ = 1:1), the residual turbidity (RT) and Al minimization are not further improved even though floc formation is facilitated. However, the single dosing with alternative PACl coagulant (PACl-C) would result in equivalent RT and dissolved Al with change in dosage. This study showed that a tailored PACl-C with low monomeric Al and high colloidal Al content can be used for the substitution of dual dosing (PACl+FeCl₃) to achieve minimization of RT and Al in low turbid water treatment.