A dynamic model describing early stages (up to the point when bacterial detachment becomes significant) of bacterial colonization of surfaces was proposed. Exponential, synchronous growth was postulated, and experimentally confirmed. Using this model, a new, simple experimental method for assessment of biological activity in drinking water treatment processes and distribution systems was developed. The method was based on direct determination of growth rates of bacteria attached to the surface of a rotating PVC disk. Growth rates of an attached mixed population were estimated for different concentrations of acetate in controlled laboratory conditions. Higher growth rates were measured with increasing acetate concentrations up to 1 mg acetate/L. A strong dependence of growth rate on testing time was found. Daughter cells detached from micro-colonies probably due to local substrate depletion. Detaching daughter cells may re-attach in the vicinity of the micro-colony causing an increase of single cells and invalidating the hypothesis on which the proposed model was based. Growth rates were estimated for effluents of dual media, dual media plus GAC, and GAC/sand filters, with and without ozonation, in a pilot plant installation. Ozone application to settled water resulted in higher attached growth rates. GAC/sand filtration brought biological activity of ozonated water to levels typical of settled water. Good correlations were found with AOC measured with Pseudomonas fluorescens strain P17 for non-ozonated effluents, and AOC measured with Spirillum strain NOX for ozonated effluents. Ozonated effluents supported higher growth rates at the same AOC concentrations. Biologically stable ozonated water occurred at AOC concentrations of 10-15 {dollar}mu{dollar}g C/L, whereas an AOC concentration on the order of 1 {dollar}mu{dollar}g C/L was necessary to ensure biologically stable non-ozonated water.
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