Zirconium as a coagulant for enhanced natural organic matter removal

Abstract

Coagulation is the most common way to remove the bulk of natural organic matter (NOM) from moorland source waters during drinking water treatment. Deteriorating water quality and tightening regulations have created the need for more effective treatment options. A review of the literature identified a range of enhanced treatment options that are available for NOM removal. A novel highly charged Zr coagulant (referred to as Zr-OCl) has also been proposed to enhance NOM removal. The aim of this thesis was to evaluate the use of Zr-OCl as a coagulant in continuous operation at pilot scale by benchmarking the performance of Zr-OCl against a conventional ferric coagulant (referred to as Fe-Coag). The potential use of Zr-OCl in a blend with Fe-Coag was also investigated. The removal of NOM, turbidity and disinfection by-product (DBP) precursors as well as the zeta potential and the strength of flocs was measured. The characteristics of Zr were related to the mechanisms of the coagulation process. It was clear that Zr-OCl could remove more NOM than Fe-coag displaying 7-10 % increased removal of dissolved organic carbon (DOC), 6-10 % increased removal of absorption of ultraviolet light at 254 nm (UV254), 31- 62 % increased turbidity removal and 23-38 % lower THM-FP. Zr-OCl also added more charge to the system and operated over a wider zeta potential range which explained the increased removal. The higher charge and wider operational range was explained by the characteristics of Zr found in the literature review. The Zr-OCl flocs were stronger than the Fe-Coag flocs resulting in less breakage during solid-liquid removal. When different blends of Zr-OCl and Fe-Coag were used there was an incremental increase in performance as a result of an increased amount of Zr in the blend for a given dose. The possible use of Zr-OCl a coagulant was put into context by comparing it to the other treatment options investigated in the literature review.