Ordering Information:
ORDER NUMBER:  91221
DATE AVAILABLE: Summer 2008

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PRINCIPAL INVESTIGATORS:
Gregory W. Harrington, Prapakorn Assavasilavasukul, Boris L. T. Lau, Rebecca M. Hoffman, and Mark A. Borchardt

OBJECTIVES:
The overall goal of this study was to evaluate pathogen removals in conventional water treatment at pathogen concentrations nearer to those found in the aquatic environment. Specific objectives included (1) measurement of pathogen removal at concentrations of 10¹ pathogens/L rather than 10⁶ pathogens/L, (2) comparison of grab and composite sampling methods, and (3) comparison of alternative enumeration procedures.

BACKGROUND:
In the Long-Term 2 Enhanced Surface Water Treatment Rule, conventional treatment plants received a 3-log credit for oocyst removal if they met the filtration requirements of the rule. This credit was based largely on experiments with pathogen concentrations 6 to 8 orders of magnitude higher than those observed in practice. Thus, there is considerable uncertainty about pathogen removals that would be attained under more realistic situations.

HIGHLIGHTS:

• The removal of Cryptosporidium oocysts and Giardia cysts through conventional treatment was observed to depend on initial pathogen spike dose. Removal increased with increasing spike dose and statistical analyses suggested that log removal differences between low and high concentration trains were statistically significant in most cases. This was consistent with fundamental expectations.
• Similar results were observed for the unit processes of sedimentation and granular media filtration.
• Composite sampling with continuous flow separation channel centrifugation allowed for better detection of pathogens in filter effluent due to increased sample volume.

APPROACH:
Experiments were conducted with different initial pathogen concentrations and sampling methods at the University of Wisconsin pilot plant. To minimize the hindrance of raw water quality as a confounding variable, treatment trains were run in parallel to facilitate better comparison of treatment performance between trains due to the effect of pathogen spike dose. Continuous separation channel centrifugation was used to concentrate pathogens while immunofluorescence microscopy was used for pathogen enumeration. Particle sorting with flow cytometry prior to microscopic enumeration (FC sorting-microscopy) was compared with the more conventional method of microscopic enumeration.

RESULTS/FINDINGS:

1. The removal of Cryptosporidium oocysts and Giardia cysts through conventional treatment was observed to depend on initial pathogen spike dose. Removal increased with increasing spike dose and statistical analyses suggested that log removal differences between low and high concentration trains were statistically significant in most cases. This is consistent with theoretical expectation.
2. As with conventional treatment, removal of Cryptosporidium oocysts and Giardia cysts by the unit processes of sedimentation and granular media filtration correlated with pathogen concentration at the filter influent. Lower pathogen removals were observed at lower pathogen concentrations.
3. When comparing previously published data with the results of this study, it is apparent that conventional treatment can achieve removals of 1 to 5 log for either Cryptosporidium oocysts or Giardia cysts. As expected, differences in raw water quality, treatment operation, and influent microorganism concentrations can all contribute to the reported differences in the pathogen removal capabilities of conventional treatment.
4. Continuous sampling allowed for the calculation of log removals in most cases when grab sampling produced non-detectable concentrations of Cryptosporidium oocysts or Giardia cysts. This approach provided an opportunity to evaluate pathogen spike doses of approximately 10¹ pathogens/L.

IMPACT:
For the water evaluated in this study, 3-log Cryptosporidium removal was only observed at initial oocyst concentrations that were orders of magnitude higher than observed in drinking water supplies. Thus, some surface water systems may not achieve the level of Cryptosporidium oocyst removal they are given credit for. Similar experiments conducted on other water supplies may produce different conclusions and water utilities are encouraged to participate in studies that elucidate the impacts of particle content on pathogen removal. Utilities are also encouraged to maintain optimal operational conditions and to be vigilant in monitoring filter effluent water quality.

ISBN 978-1-60573-023-3