An evaluation of a modified membrane filter procedure for enumerating stressed fecal coliforms in chlorinated sewage effluents

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Virginia Polytechnic Institute and State University

Wastewater samples were collected from both the secondary settling and the chlorine contact tanks at a secondary sewage treatment plant (trickling filter) in Blacksburg, Virginia and analyzed for fecal coliforms using three procedures. Physical parameters including total suspended solids, DO, pH, turbidity, temperature and total chlorine residual were measured in effort to ascertain their effect on fecal coliform recoveries.

The three procedures employed included the multiple-tube fermentation technique that yields the most probable number (MPN), the standard MF technique (SF-MF), and a modified MF technique (IF-MF) which consisted of a lactose overlay and a 5-hour incubation period at 44.5°C. A statistical analysis of the data showed that the means of the recoveries by the IF-MF technique were significantly greater (0.01 level) than those by the SF-MF technique in both the secondary settling tank and the chlorine contact tank samples. Recoveries by the IF-MF technique were comparable to those by the MPN technique when samples from the secondary settling basin were analyzed, but not in samples from the chlorine contact tank. However, the means of the IF-MF recovery procedure were within the 95 percent confidence interval associated with the MPN. No relationships could be established between the observed variations in the physical and chemical characteristics of the treated sewage samples and the fecal coliform densities.