A comparison of two membrane filter techniques for the enumeration of total coliforms in potable water

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Virginia Polytechnic Institute and State University

A laboratory study was conducted to compare the coliform recovery capabilities of two membrane filter techniques on coliforms exposed to varying chlorine residuals and contact times.

An enrichment-step membrane-filter method (E-MF) involved the incubation of a filtered water sample on lauryl tryptose plus 1.5 per cent agar for two hours at 35°C prior to transfer of the filter to LES-Endo agar and a final incubation for 22 hours at 35°C. A single incubation of the membrane filter for 24 hours at 35°C was employed in a non-enrichment membrane filter method (NE-MF). A free chlorine residual was established in a synthetic test water at a neutral pH and constant temperature prior to inoculation of the water from a suspension of bacteria comprised of four coliform genera commonly found in natural waters. Timed samples from 5 seconds to 15 minutes, analyzed for coliform recovery by the E-MF and NE-MF methods revealed an average increase in recovery of the E-MF to NE-MF method of 1.6:1. When the initial inoculum was kept below 1 x 10⁴ cells/100 ml, and the test water was chlorine demand- free no coliforms were recovered by either method after two minutes of contact time with free chlorine residuals greater than 0.12 mg/l. Many of the recovered colonies failed to produce a characteristic sheen but were verified as coliforms.

It was concluded that all coliforms were effectively killed at chlorine contact times greater than two minutes and free chlorine residuals greater than 0.13 mg/l, in chlorine demand-free water. Free chlorine residuals of 1.0 mg/l effectively killed all coliforms within five seconds.