Assimilable Organic Carbon as a Measure of Bacterial Growth Potential in Water Supplies

This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.

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Assimilable Organic Carbon as a Measure of Bacterial Growth Potential in Water Supplies

Report No UWRAA 66

September 1993

 SYNOPSIS

The aim of this study was to determine the assimilable organic carbon levels of different water supplies in Australia and to attempt to correlate these levels with identified bacterial growth.

The concentration of assimilable organic carbon (AOC) in water can be determined indirectly by the measurement of the growth of indigenous microorganisms inoculated into the water. This method directly determines the ability of a water to support bacterial growth and is better described as a bacterial regrowth potential (BRP) method. Bacterial growth can be measured by the automated monitoring of turbidity changes in the sample. The growth potential of the water can then be estimated by the increase in turbidity (ratio final/initial) expressed as the growth factor (f) while the growth rate (m) can be obtained directly from the increase in turbidity during the log growth phase. Changes in total cell numbers and the concentration of dissolved organic carbon (DOC) can also be determined.

A total of 18 Australian supplies were examined in this study. These were chosen to give as wide a range of treatment processes and disinfection regimes as possible, although the majority of the samples were obtained from South Australia because of the proximity and ease in the collection of samples. A number of the supplies are treated using conventional treatment, consisting of flocculation ,sedimentation and filtration. In some supplies, different treatment processes are incorporated such as granular activated carbon filters or dissolved air flotation while in one plant an alternative treatment process is used (Sirofloc). The number of supplies studied was limited due to the fact that only two samples were able to be analysed concurrently and to the time required per analysis (3 days).

The results indicate that although there is some relationship between the growth parameters, plate counts and DOC, there is no direct correlation. Neither DOC nor plate counts alone were sufficiently reproducible or sensitive as a measure of BRP.

Conventional, dissolved air flotation and Sirofloc treatment generally resulted in an increase in BRP. It is possible that the increased growth potential following treatment was a result of the removal of high molecular weight organics or other inhibiting agents from the raw water. In one of the treated supplies where granular activated carbon was used the BRP of the treated water was negligible.

The BRP was found to decrease with time through water supply distribution systems, consistent with bacteriological data indicating increased bacteriological activity at the ends of systems. From the limited data available, there was no significant difference in BRP resulting from the use of chlorine or chloramine.

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