Evaluation of the impact of coagulant choice on phosphorus removal from municipal wastewater

Abstract

Jefferson, Bruce - Associate SupervisorPhosphorus removal is a critical objective in municipal wastewater treatment due to its role in eutrophication and the tightening of regulatory discharge limits. Chemical coagulation remains the most widely adopted method for phosphorus control; however, its effectiveness is influenced by coagulant type, pH conditions, dosing location, and wastewater matrix composition. This thesis aimed to advance the understanding of how these operational and chemical variables govern the mechanisms of phosphorus removal, with the goal of optimising coagulant selection and application strategies under real-world conditions. A comprehensive screening of 17 coagulants, including ferric, aluminium, rare earth, zinc, and calcium-based formulations, was conducted under both uncontrolled and pH-adjusted conditions. Ferric sulphate, polyaluminium chloride (PACL), and aluminium sulphate emerged as the most effective agents, achieving residual total phosphorus concentrations as low as 0.35 mg/L, 0.15 mg/L and 0.6 mg/L, respectively, under controlled pH conditions, particularly under neutral pH, where stable hydroxide flocs are favoured. Rare earth coagulants demonstrated high phosphate affinity but formed fragile flocs, limiting their practical application. Floc characterisation revealed that compact, shear-resistant aggregates correlated strongly with higher removal efficiency. To investigate the role of pH, a detailed comparative analysis of ferric sulphate (FS), aluminium sulphate (ALS), and PACL was performed across a pH range of 4-8. The results confirmed that coagulant solubility, hydrolysis potential, and metal speciation significantly impact phosphorus removal efficiency. FS and ALS were better than PACL under acidic conditions due to more complete hydrolysis and formation of stable flocs. Phosphorus fractionation and turbidity data supported these trends, identifying pH 6-7 as the optimal window for coagulant performance and floc settleability. The final phase of the study examined how dosing location in the crude influent, after primary settling tanks (PST), and in the final effluent (FE) influences coagulant performance. FS showed enhanced phosphorus removal even in high-strength crude wastewater, though required careful pH control to avoid over-acidification. ALS and PACL were more effective at PST and FE, where organic loading and particulate interference were lower. A two-point dosing strategy applied to crude wastewater was found to enhance phosphorus removal while reducing total coagulant demand, offering a practical route for chemical cost optimisation. Collectively, this thesis delivers critical insights into the physicochemical and operational factors driving chemical phosphorus removal. The findings inform coagulant selection and deployment in diverse wastewater environments, support compliance with future phosphorus discharge standards, and contribute to the development of cost-effective and environmentally sustainable treatment strategies.PhD in Wate