Development of a Revised Method for Increased Sensitivity for Recoveryof Total Coliform and E. coli in Drinking Water

摘要

Although it is not a conclusive indicator, total coliform (TC) bacteria have been used fordecades as indicators of fecal contamination and/or distribution system breaches.Despite new data indicating TC are not useful as indicators of fecal contamination, TCsampling is still seen by water quality managers as a useful indicator of the integrity oftheir distribution system. A number of researchers have identified the difficulty inestimating the underlying spatial and temporal distributions of TC in distribution systemsbased on presence/absence sampling. For TC to be a useful indicator there must be anunderlying assumption that monitoring will result in observing TC when present.However, the sampling plans currently employed by water utilities are not based onoptimizing sampling to identify TC when present but rather they are optimized byaccessibility and logistics. TC sampling of even large numbers of samples (≥480/month)tests an infinitesimal portion of the water flowing through the system in a month; sinceTC are highly heterogeneous, mostly zeros and perhaps a few positives, result. In orderto optimize a sampling plan to detect TC when present, the plan, including locations,periodicity and sample volumes, should be based on the underlying distribution. Thisdistribution cannot be measured directly, but the distributions of the sampling results canbe evaluated. A recent study on sample volume determined that when taking 100mLand 2L samples side by side and comparing the results, the 100mL samples significantlyunderestimated the occurrence of TC. While these results suggest that the currentsampling and analytical strategies may not be optimal, they do not resolve the issue ofhow many samples of what size are needed to meet the objectives listed above. It maybe that different objectives require different sampling and analytical plans. To explorehigh volume sampling, we have developed a method using capsule filtration and enzymesubstrate technology that can accommodate samples up to 20L. We evaluated theability of the process to filter, recover and identify total coliform and E. coli at differentconcentrations and sample volumes ranging from 100 mL to 20L.