A comparison of methods used to enumerate Escherichia coli in conventionally treated sewage sludge

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Abstract
Aims: This study examined the suitability of three analytical methods for isolating and enumerating Escherichia coli from conventionally treated sewage sludge. Methods and Results: Crude sewage, mesophilic anaerobic digested (MAD) sludge, and final product sludge samples were taken from six sewage treatment works for analysis. Two of the three methods tested were membrane filtration techniques, utilizing chromogenic E. coli/coliform (CEC) media and membrane‐lactose glucuronide agar (MLGA); the third method was a most probable number (MPN) technique utilizing Colilert in Quantitray 2000 (Idexx). The methods were evaluated for variation, consistency, false‐positive and false‐negative results, as well as method correlation. The methods gave good and consistent recovery of E. coli for a range of conventionally treated sewage matrices. All of the methods had a false‐positive rate of E. coli removed through the sludge treatment process and the numbers remaining in the final product. It is recommended that the Colilert 2000 (Idexx, Westbrook, Maine) and CEC methods would be equally suitable to assess the reduction of indigenous E. coli in conventionally treated sludges, and that MLGA be used with follow‐up confirmatory testing.

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