The most frequently used DST method, Colilert®, detects coliforms and E. coli through the activity of the enzymes beta-D-galactosidase and beta-D-glucuronidase respectively.
Click here to enlarge imageNo false positives were found when DST tests were used on any of the 54 cultures isolated from the Seattle distribution system, even when spiked with concentrations as high as 500,000 per 100ml sample. This is well over the USEPA Surface Water Treatment Rule MCL of 500 cfu/ml of heterotrophs. Seattle concluded that new testing method reduced the possibility of false-positives due to this gram-negative bacterium.
These two studies brought about changes that have dramatically reduced coliform positives. Consequently, Seattle has switched from membrane filtration to Defined Substrate Technology methods for all of its finished water testing.
3. Improving Source Water Quality - After converting to DST for finished water, the Water Quality Lab evaluated its suitability for source water testing. Using samples from the Youngs Lake Reservoir, the lab compared Colilert®-18 reagent used with Quanti-Tray®/2000 to the multiple tube fermentation method. Each method can be used to enumerate coliforms and E. coli. The Quanti-Tray/2000 MPN method proved more accurate, and the data led to improved operations at Seattle's new treatment facility.
For example, the study showed that coliforms tend to stratify at changing depths in Seattle's reservoir. Seasonal changes in temperature shifted these strata, affecting the quality of the water drawn at the intake. Using accurate enumeration data, Seattle engineers now select the depth at which they draw the city's source water, optimizing for the highest water quality.
Conclusion
Defined Substrate Technology tests have helped the Water Quality Lab assess Seattle's entire water system. As a result, operational improvements have improved water quality for ratepayers. The testing method's lower false negative rate provides Seattle with a tool for ongoing process improvement, while its reduced false positives improve operational efficiency and compliance. As an added bonus, the water lab found the tests easy to use. Compared to the membrane filtration method, they require less hands-on time and their ability to provide a completed result eliminated many hours of media preparation, disinfection and confirmation.
About the Author: Ed Oliver is a Water Quality Analyst at the Seattle Public Utilities. He has worked as a microbiologist in the water industry for 25 year and has written eight papers on characterizing distribution system coliform bacteria. The topics on these papers have ranged from reducing coliform-positive distribution samples by flushing and increased chlorine residuals to comparing new methods for detecting coliforms in sediments and biofilms.