Removal of Cryptosporidium parvum by granular media filtration

Abstract

Increasingly stringent regulations for drinking water quality have placed increased emphasis on a multi-barrier approach for providing protection from waterborne pathogens. Widely found in surface waters, the pathogen Cryptosporidium parvum is particularly resistant to chemical disinfectants commonly used in drinking water treatment, underscoring the need for multiple treatment strategies for inactivation or removel of C.parvum from drinking water. When operation is optimized, granular media filtration systems are particularly effective barriers against C.parvum passage into potable water; however, less is known about the pathogen removal capacity of these systems at the beginning and end of the filter cycle (i.e., filter ripening and breakthrough) or when particle removal processes are challenged (e.g., coagulation upsets, hydraulic changes, etc.)

The research presented in this thesis examined the passage of C.[arvum and potential surrogates for C.parvum through granular media filters during periods of optimal and non-optimal filter operation. A thorough review of the relevant filtration and C.parvum literature emphasized the difficulty in accurately enumerating C.parvum from water samples. A relatively simple analytical method for concentrating and enumerating C.parvum during filtration studies was implemented and optimized. Then, to address the uncertainty or reliability of C.parvum concentration and removal data, a new quantitative tool that incorporated several sources of error (representative sampling, random analytical error, and non-constant analytical recovery) was developed. The statistical model assumed a Poisson distribution for the true sample counts, a binomial distribution for modeling the recovered fraction of oocysts, and a Beta distribution for describing the uncertainty of oocyst recovery. A numerical technique (Gibbs sampler) was then applied to the statistical model to determine confidence intervals for C.parvum concentration and removal data. This moethod of describing the uncertainty associated with C.parvum data (confidence intervals calculated via the Gibbs sampler) was used throughout this thesis research because it allowed for comparison between different data sets with, in some cases, different analytical recoveries.

Bench-scale experiments were performed to determine if viable and chemically-inactivated C.parvum oocysts were similarly removed by granular media filters at a variety of operating conditions. These experiments were critical because of the potential health risks associated with the experimental use and release of viable oocysts. Since uncoagulated, chemically-inactivated oocysts have demonstrated slightly different surface charge properties (described by zeta potential) than viable oocysts, it had been speculated that the different oocysts might also be removed differently by granular media filters. Dual- (anthracite/sand) and tri-media (anthracity/sand/garnet) investigations demonstrated similar removals of viable and chemically-inactivated C.parvum oocysts during optimized operation, filter ripening, and coagulation failure when it decreased by >3-log relative to stable operation. C.parvum removals were not statistically different in and tri-media filters, though increased replication may yield statistically significant differences in the marginally higher removals achieved by tri-media filtration.

Pilot-scale experiments represented the majority of the experimental efforts and focused on investigating design and operational strategies for maximizing C.parvum removal by filtration. Multiple research platforms permitted investigation of different types of raw waters, water temperatures, coagulation regimes, and filter designs. Formalin-inactivated C.parvum oocysts were seeded at all of the experimental locations. In addition to turbidity and particle concentration evaluations, polystyrene microspheres were evaluated as potential surrogates for C.parvum because they were similar to oocysts in size and easy to identify and enumerate.

The pilot-scale experiments demonstrated that excellent removals (>5-log) of C.parvum could be achieved during optimal operating conditions, even at temperatures as low as 1oC and during spring runoff conditions. These removals deteriorated substantially (by 3- to 4-log) during end-of-run and early breakthrough filtration, even at filter effluent turbidities below 0.1 NTU. This result suggested that filter operation during breakthrough, as measured by turbidity or perhaps even particles, should be avoided. Coagulation failure and sub-optimal coagulation conditions (reductions in coagulant dose) also resulted in deteriorated C.parvum removals. Relatively rapid changes in hydraulic loading demonstrated varied effects on C.parvum removal by filtration, though in most cases only little to no deterioration in filter effluent C.parvum concentrations occurred. Turbidity monitoring proved more useful than particle counting in gauging the effects of hydraulic steps on C.parvum passage through filters. These events should be investigated further to better define how and when they impact pathogen passage. C.parvum removals by filtration were moderately (~o.5-1 log) lower during ripening than during stable operation, these less substantial differences occurred over a relatively short duration of the ripening period. During most of these operating conditions, oocyst-sized polystyrene microspheres appeared to be reasonable surrogates for C.parvum removal by filtration; however, they should continue to be evaluated relative to oocysts to better define the limits of their applicability as surrogates. The pilot-scale investigations resulted in several operational and design implications and strategies for maximizing C.parvum removal by granular media filtration, the most notable of which were the importance of optimized chemical pretreatment (coagulation prior to filtration) and the potential for increased pathogen passage during end-of-run operation.