AssimilableOrganic Carbon as a Measure of Bacterial Growth Potential in Water Supplies

ReportNo WSAA 66

September 1993




Theaim of this study was to determine the assimilable organic carbon levels ofdifferent water supplies in Australia and to attempt to correlate these levelswith identified bacterial growth.


Theconcentration of assimilable organic carbon (AOC) in water can be determinedindirectly by the measurement of the growth of indigenous microorganismsinoculated into the water. This method directly determines the abilityof a water to support bacterial growth and is better described as a bacterialregrowth potential (BRP) method. Bacterial growth can be measured by theautomated monitoring of turbidity changes in the sample. The growth potentialof the water can then be estimated by the increase in turbidity (ratiofinal/initial) expressed as the growth factor (f) while the growth rate (m) can be obtained directlyfrom the increase in turbidity during the log growth phase. Changes in totalcell numbers and the concentration of dissolved organic carbon (DOC) can alsobe determined.


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


Theresults indicate that although there is some relationship between the growthparameters, plate counts and DOC, there is no direct correlation. Neither DOCnor plate counts alone were sufficiently reproducible or sensitive as a measureof BRP.


Conventional,dissolved air flotation and Sirofloc treatment generally resulted in anincrease in BRP. It is possible that the increased growth potential followingtreatment was a result of the removal of high molecular weight organics orother inhibiting agents from the raw water. In one of the treated supplieswhere granular activated carbon was used the BRP of the treated water was negligible.


TheBRP was found to decrease with time through water supply distribution systems,consistent with bacteriological data indicating increased bacteriologicalactivity at the ends of systems. From the limited data available, there was nosignificant difference in BRP resulting from the use of chlorine or chloramine.


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