Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 2/9/2011
Publication Date: N/A
Citation: Interpretive Summary:
Technical Abstract: With the growth in use of treated wastewater for municipal and agricultural irrigation, accurate monitoring of microbial water quality indicators, including Escherichia coli (E. coli) and Enterococcus, increases in importance. Enzyme-based chromogenic media, because they are easy to use and provide rapid sample analysis, are often used for detection of these two microbial groups in environmental samples. The presence of unique levels of organic and inorganic compounds, however, alters the chemistry of treated wastewater, potentially hindering accurate detection using chromogenic technology. We have been monitoring water quality within two Arizona municipal reclaimed water distribution systems using EPA-approved most-probable-number (MPN) methods for E. coli and Enterococcus (Colilert® and Enterolert®, respectively) and comparing these results to those obtained from EPA-approved membrane filtration (MF) and selective agar techniques (DifcoTM MI and M-EI agar). Results from both MPN methods were substantially higher than the MF over a one year period. The MPN methods averaged 27.6 and 237.9 MPN 100 mL-1 for E. coli and total enterococci, respectively, compared to results from MF methods which averaged 6.4 and 10.3 CFU 100 mL-1 for E. coli and enterococci, respectively. Identity confirmation by polymerase chain reaction (PCR) was performed on colonies isolated from selective agar plates. A total of 58 E. coli isolates assayed to date show a false positive rate (isolates counted as E. coli but actually were not) of 36.2%, much higher than the accepted rate of 4.3% referenced in USEPA method 1604 for MI agar. In contrast, no false positive enterococci have been identified to date by PCR. Statistical comparison of false positive rates and chemical and biological water quality parameters may aid in identification of factors contributing to variations in microbial results. Such validation/optimization and side-by-side comparative testing of microbiological methods is critical to ensure accurate reporting and to provide further information/confirmation on the ability of microbial indicators to accurately predict water quality. Such testing is of heightened importance for treated wastewater, where quality is assessed under a permitted system in which a high false-positive rate could result in unnecessary waste of disinfectant, an increase in the release of disinfection byproducts into the environment, or an erroneous violation of a discharge permit. This work, examining the interactions of water quality and the performance of chromogenic media, will ultimately aid in increasing the accuracy of treated wastewater quality monitoring programs.