Nutrients for bacterial growth in drinking water: bioassay evaluation

350 210 Stroud Water Research Center

Kaplan, L.A., and T.L. Bott. 1990. EPA Report No. EPA/600/S2-89/030. U.S. Environmental Protection Agency, Cincinnati, Ohio.

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The regrowth of bacteria in drinking water distribution systems can lead to the deterioration of water quality. Pathogenic bacteria are heterotrophs, and heterotrophs are probably the dominant bacteria associated with the regrowth phenomenon. Only a portion of the total organic carbon in drinking water is biologically labile to heterotrophic bacteria, and a bioassay developed to quantify this assimilable organic carbon (AOC) has been proposed as an index of the regrowth potential of drinking water (van der Koolj et al. 1982b). We have evaluated both biological and chemical assays for determining AOC as related to regrowth of bacteria in drinking waters from surface water and groundwater sources. Pseudomonas fluorescens strain P-17 was used in bioassays for AOC. Dissolved organic carbon (DOC), uv-labile DOC, DOC < 10,000 daltons, monosaccharides, and primary amines were the chemical assays used to predict concentrations of AOC. Growth of P-17 was enumerated as viable and total cells with spread plates and direct epifluorescence microscopy, respectively. OC concentrations in surface waters ranged from 48 to 607 μg liter-1 and in a groundwater supply from 40 to 146 μg liter-1. AOC remained relatively constant or declined in distribution systems with distance from the treatment plant. Incubation vessel surface to volume ratio influenced the AOC value by enhancing wall growth of reversibly attached cells. The bioassay assumes that 1) organic carbon limits growth of the bioassay organism, 2) the yield of the bioassay organism on naturally occurring AOC is constant and equal to yield on model organic compounds, and 3) the bioassay organism is an appropriate surrogate for the native microflora of distribution systems in utilizing AOC. We have found that phosphorus additions to some test waters were required to generate carbon limitation and that yield of P-17 on naturally occurring AOC approximates the yield on acetate. Correlations of the bioassay AOC with chemical determinations were poor, but with improvements we have made in the handling of the test water, glassware, and P-17, we suggest that the bioassay holds promise for a simple, routine measure of drinking water regrowth potential.