Table 6-3 of the Disinfection Profiling and Benchmarking guidance document is giving alternative disinfection benchmark for water systems with heavily contaminated source water, based on the 90th percentile of indicator (Fecal Coliforms/E.coli) concentrations obtained through source water characterization monitoring. The table lists the indicator concentrations as being reported as CFU (Colony Forming Units)/100 mL. However, the concentrations obtained through the source water characterization – if using the methods listed on page 6-13- would have been reported as MPN (Most Probable Number)/100 mL results (there are no methods listed on p. 6-13 that give CFU results – all would give MPN results).
The Indicator Concentration presented in Table 6-3 (p. 6-8) are calculated data – extrapolations of data analyzed by EPA to generate the Alternative Disinfection Benchmarks presented in the table, and likely were just labeled CFU/100 mL for ease of use. For determination of 90th Percentile Indicator Concentration, one could use approved fecal coliform or E.coli enumerative methods, such as those listed on p. 6-13, and whether CFU/100 mL data (for instance from a membrane filtration method), or a Most Probable Number (MPN) number (for instance from a multiple tube method) were generated – the same table (6-3) would be used.
It is important to note that since the publication of this guidance document (August 1999), the numbers of approved methods available for use has increased significantly. For enumeration of fecal coliform bacteria in a source water sample, any of the methods approved in CFR §141.74 would be acceptable – and there are membrane filtration methods that would provide CFU/100 mL data approved. There are also methods approved for E.coli enumeration under LT2 – the list of these methods can be found at §136.3, Table 1H.
At the time of the preparation of this guidance document, methods approved under the Total Coliform Rule for E.coli were included (those given on page 6-13), these methods, and others, are still approved for use under the TCR (141.21/141.852). However not all of the methods currently approved under the TCR rule have an enumerative component (since that rule only requires presence/absence testing). Also methods approved under the TCR are intended for finished drinking water, and as such, some of the methods may not be effective if used for heavily contaminated source water.