Investigating background aluminum and aluminum-based coagulants at a treatment plant and its role in downstream metal accumulation in a distribution system

Coagulants are essential to the water treatment process to remove large particles from source water. The coagulation process works in a way such that the positive charge of the coagulant neutralizes the negative charge of dissolved and suspended particles in water. This improves particle settleability by creating bigger floc which settle more easily. Coagulants are often used with flocculants to aggregate small particulates from the source water, adsorbing dissolved organic matter, settling out and reducing turbidity. Coagulants are chosen based on effectiveness, cost, and availability, with aluminum-based being the most common. Aluminum sulphate (alum) is widely used in treating drinking water. Al is naturally occurring and can exist in raw water at low concentrations but increases when Al-based coagulants are used to reduce pathogens. Coagulation processes, though efficient in improving turbidity, are known to have “carryover” or residuals. Potential methods to minimize coagulant residuals include dose optimization through jar test studies, pH control, alternative coagulant selection, or additional chemical additives. Residual Al carry-over can lead to the formation of micro-flocs and act as soft deposits in the DS, compromising the efficacy and effectiveness of pathogen removal. Although not yet thoroughly investigated, Al deposits are suspected to act as a sink for metal accumulation in a distribution system. Metals are known to accumulate downstream from water treatment plants, and they are suspected to increase with higher Al concentrations, contributing to water quality degradation in a distribution system. The aim of this paper is to characterize the association between background Al, and Al-based coagulant dosage at the treatment plant on metal accumulation downstream of the WTP in a Canadian treatment plant using historical data from 2012 to 2019. The objectives include calculating Pearson correlation to determine the relationship between the Al at the WTP and mean metal concentration over multiple hydraulic distances.