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If treated water from a water treatment plant contain excessive amount of biologically assimilable organic carbon, then the water quality may be deteriorated as caused by the multiplication of heterotrophic bacteria inside the distribution system. This phenomenon is called "regrowth" or "aftergrowth". Although strategies to control aftergrowth include maintain disinfectant-residual, flushing and mechanical cleaning, and nutrient control, there are disadvantages for some of these methods. For example, free chlorine residual is not very effective for controlling biofilm bacteria, and there is also disinfection by-products problem. Flushing and mechanical cleaning, to be effective, must be performed regularly. For highly branched pipe networks, it is difficult to implement, and for transmission mains and trunk lines, it usually involves the disruption of service to the customs, in addition to extreme effort and high costs.
As a result, the most effective method to control aftergrowth, considered nowadays, is by removal of growth-promoting compounds, specifically the dissolved organic carbon available as growth substrate for the microorganisms. This type of drinking water with limited bacterial growth potential has been characterized as "biologically-stable" drinking water.
Assimilable Organic Carbon (AOC) is that portion of the biodegradable organic carbon that can be converted to cell mass and expressed as a carbon concentration by means of a conversion factor. In this study, two organisms, Pseudomonas fluorescens strain P17 and Spirillum species strain NOX were selected for the AOC determination. First, their growth yields for acetate were determined. Then the sample to be measured were sterilized by pasteurization at 60℃ for 30 minutes, and the two pure cultures mentioned above were inoculated, and incubated at 15℃ in the dark. The growth of the bacteria was determined by periodic colony counts with spread plate technique on Lla (Lab-Lemco nutrient agar) cultivation medium until the growht reached maximum (maximum colony count, Nmax). Subsequently, the AOC concentration can be calculated using the Nmax value and the yield values as calibrated by acetate. Therefore, the AOC value is expressed as μg acetate-C equivalent/L.
Next, three water treatment plants and their distribution system in southern Taiwan were selected for AOC and other relevant water quality analysis. The results show that, for the part of treatment plant, AOCp17 contributed a major portion of the total AOC of the raw waters. However, the portion contributed by AOCNOX usually increased significantly after prechlorination. This probably is caused by the oxidation reaction between chlorine and organics, which increased the portion of total organics contributed by carboxylic acids. For the three distribution systems studied, it can be noticed that when the AOC values were between 30 and 70μg acetate-C/L, and free chlorine residual maintained at about 1 mg/L, then the variation of the AOC value during distribution was minor, and the HPC values were below 20 CFU/mL. However, for the system with AOC value higher than 150 μg acetate-C/L, then the HPC values were much higher than those systems with lower AOC values, even when free chlorine residual was maintained at higher than 1.0 mg/L. This probably indicates that aftergrowth can not be controlled by chlorination only.
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