Rethinking filtration performance assessment for public health protection

Abstract

Minimizing acute health risks from waterborne pathogens such as Cryptosporidium is the paramount goal of drinking water treatment. Because oocysts of the protozoan Cryptosporidium resist chlorine-based disinfectants and require complex and expensive detection methods, removal by physico-chemical filtration (CAF) is critical. Regulatory frameworks such as the US EPA suite of Surface Water Treatment Rules (SWTRs) and their Canadian analogs prescribe treatment credits based on filtered water turbidity, assuming the achievement of turbidity targets must reflect well-operated treatment and ≥3-log oocyst removal. However, in systems reliant on low turbidity (<2 NTU), low dissolved organic carbon (DOC; <2 mg/L) source water, raw water turbidity often meets or exceeds filtered water targets, obscuring whether coagulant dosing is resulting in sufficient particle destabilisation. As a result, treatment plants may unknowingly operate at coagulant doses that fail to achieve the particle destabilization required for pathogen removal. This challenge is further compounded because low turbidity, low DOC waters have fewer interactions between particles (as particle concentrations are low) resulting in the need for higher doses of coagulant to increase contact opportunities by precipitating additional particles. Here, pilot-scale CAF investigations confirmed that oocyst removal was dependent on sufficient particle destabilization. Filter challenge studies using Cryptosporidium oocysts were run using low turbidity, low DOC source water and various coagulant doses. The experiments were replicated at similar operational conditions over several years and consistently demonstrated an increased risk of oocyst passage when insufficient coagulant was added and inadequate particle destabilization occurred. The results incontrovertibly demonstrated that turbidity is inadequate as a sole indicator of particle destabilization necessary for ensuring sufficient oocyst removal by CAF, particularly for low turbidity, low DOC source waters. Zeta potential analysis proved to be a useful tool for indicating sufficiency of particle destabilization. Zeta potential values within ±5 mV of zero were required to consistently achieve ≥3-log removal of Cryptosporidium oocysts by filtration of low turbidity, low DOC Lake Ontario source water; in these cases, ≥4-log removal of oocysts was often achieved. As expected, higher coagulant doses than those typically practiced for low turbidity, low DOC source waters—specifically, coagulant doses that led to aluminum hydroxide solid precipitation—were required to achieve these levels of Cryptosporidium removal by CAF. These findings highlight the dangers of sole reliance on turbidity as an indicator of post-filtration water quality and treatment performance and underscore the need for complimentary monitoring tools to ensure protection of public health in systems reliant on higher quality source waters. Integrating zeta potential monitoring into routine coagulation control could provide operators—especially those dealing with the challenge of determining coagulant dose in the absence of substantial turbidity—with an indication of sub-optimal particle destabilization and associated poor filtration performance. These insights also point to broader implications for regulatory policy, as current turbidity-based treatment credits may not always adequately reflect true pathogen removal performance. Further operational guidance for zeta potential operationalization alongside turbidity analysis is needed to help ensure sufficient Cryptosporidium removal by CAF.