Application of a combination of oxidants improves treatment performance for NOM and manganese

Abstract

Increasing natural organic matter (NOM) measured as dissolved organic carbon (DOC) in surface water presents a challenge to drinking water treatment plants using direct filtration processes. This paper seeks to improve the performance of a direct filtration plant treating water from Lake Nepean in Sydney, Australia, with ferric chloride and PDADMAC (pH = 6), by understanding the impacts of oxidant doses (potassium permanganate (KMnO₄) and chlorine (Cl₂)) and dosing locations (oxidation prior to and post coagulation) on NOM character and NOM and soluble manganese removal. Higher oxidant doses led to increased degradation in high molecular weight fractions, such as biopolymers. When coupled with subsequent coagulation–flocculation (C–F), the NOM removal efficiency increased by 4.7–9.8% compared with the C–F process without oxidation, while applying oxidation post coagulation resulted in lower DOC removal. NOM flocs generated from oxidation prior to C–F were smaller in size and more resistant to breakage compared with flocs generated from C–F alone, which could be beneficial for reducing turbidity breakthrough during separation. The addition of Cl₂ prior to coagulation, compared with dosing KMnO₄ alone, significantly improved the removal of both biopolymer and humic substances but led to trihalomethane (THM) formation. Increasing KMnO₄ dose did not vary the NOM removal efficiency but increased the residual manganese concentration in the finished water irrespective of the oxidation location. Overall, optimal performance was achieved by oxidation using 0.5 mg L⁻¹ KMnO₄ in combination with 1.0 mg L⁻¹ Cl₂ prior to the C–F process.