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ARS Home » Pacific West Area » Maricopa, Arizona » U.S. Arid Land Agricultural Research Center » Water Management and Conservation Research » Research » Publications at this Location » Publication #256316

Title: False positive identification of E. coli in treated municipal wastewater and wastewater-irrigated soils

item McLain, Jean
item ROCK, CHANNAH - Maricopa Agricultural Center
item LOHSE, KATHLEEN - University Of Arizona
item WALWORTH, JAMES - University Of Arizona

Submitted to: Canadian Journal of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/6/2011
Publication Date: N/A
Citation: N/A

Interpretive Summary: In the United States, an estimated 1.7 billion gallons per day of treated wastewater is used for municipal and agricultural irrigation, and this number grows yearly as water reclamation technology improves and public acceptance increases. With the growth in use of treated wastewater, accurate monitoring of water quality increases in importance, but it is unknown whether current methods for water quality monitoring are accurate for use on treated wastewater samples. Over two years, we tested an E. coli assay widely used by wastewater professionals, and used molecular methods developed in our laboratory, to calculate rates of false positive E. coli identification (bacteria that are counted as E. coli in the assay but are actually not E. coli). We found that false positive rates in treated wastewater samples averaged 40%, suggesting that water managers throughout the U.S. could be relying on data which may not accurately represent the actual quality of the water produced. Inaccurate data could result in unnecessary waste of disinfectant and an increase in the release of disinfection byproducts into the environment. This is the first study to utilize cultivation and molecular methods to establish that E. coli false positive rates may be strongly affected by the unique chemistry and biology of reclaimed water. This work also highlights the extreme importance of validation of microbiological results from widely used commercial media for accurate analysis of wastewater quality.

Technical Abstract: With the growth in use of treated wastewater for municipal and agricultural irrigation, accurate monitoring of water quality parameters, including Escherichia coli (E. coli), increases in importance. Chromogenic media, because they are easy to use and provide rapid sample analysis, are often used for detection of E. coli in environmental samples. The presence of unique levels of organic and inorganic compounds, however, alters the chemistry of treated wastewater, potentially hindering accurate detection of E. coli using enzyme-based chromogenic technology. We used MI agar and molecular confirmatory methods to assess false positive identification of E. coli in treated wastewater samples collected from municipal utilities, a wastewater irrigation holding pond, soils irrigated with treated wastewater, and in stormwater flows being directed to groundwater recharge. Over several months, false positive rates in stormwater flows (4.0%) agreed closely with those reported by USEPA, but were far higher in samples collected from the holding pond, soils, and treatment facilities (33.3, 38.0, and 48.8%, respectively). Sequencing of false positive isolates confirmed that the majority were, like E. coli, of the family Enterobacteriaceae, and most have been reported to produce the ß-D glucuronidase enzyme targeted by MI agar. False positive identification rates were higher in winter months, and were significantly related to collection date and temperature, suggesting that seasonal variations in water quality may be influencing false positive identification. Studies examining the interactions of water quality and the performance of chromogenic media will ultimately increase the accuracy of future water quality monitoring programs.