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The Miami Dade High-Rate Disinfection Pilot Study at the South District Wastewater Treatment Facility compared high-level disinfection with chlorine to high-rate disinfection using chlorine dioxide and to disinfection with ultraviolet light. This presentation discusses some of the analytical challenges presented by the Florida Department of Environmental Protection (FDEP) pathogen guidelines, how those challenges were met and suggests innovative method changes to meet the guidelines in the future.

FDEP' rigorous pathogen guidelines define protozoan and virus limits for reuse water and set the standards for the pilot plant testing. For the protozoans, Cryptosporidium and Giardia, limits are defined in terms of “viable” oocysts and cysts. The standard EPA test, Method 1623 for Cryptosporidium and Giardia, gives total counts and information about protozoan morphology but does not measure viability. The results of Method 1623 counts were used to calculate physical removal and destruction with pilot scale tests, but those counts could not be used to calculate reduction in viability. Viability measurements were made by the focal detection cell culture method for Cryptosporidium and by live animal studies for Giardia with bench scale tests.

For viruses the FDEP guidelines recommend acceptable average limits as 0.044 plaque forming units (PFU)/100 liters or less than one 1PFU/ 2273 liters. The most probable number (MPN) assay used for the pilot study is considerably more sensitive than the plaque assay implied by the guidelines. The large volume and high solids used to simulate “worst case” conditions increased the analytical challenges. Method modifications were developed and validated at Environmental Associates Ltd. to accommodate each of the analytical challenges.

Surrogate assays were included to attempt to validate process control tools that would be cheaper and faster to run than pathogen assays. These assays included spores, coliphages and enterococci. Surrogate assays yielded mixed results with only some surrogates providing good predictive value. Pathogen levels varied by season, rainfall, time of day, day of week and year-to-year. The variability indicates that risk evaluation and treatment decisions need to be based upon large enough data sets to encompass these variations.

For new disinfection studies, method modifications are presented that will reduce cost and improve the accuracy and usefulness of the data.
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Document Type: Research Article

Publication date: 2007-01-01

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