Water quality changes caused by microbial activity in the distribution network can cause serious problems. Reducing the amount of microbial available substrate may be an effective way to control bacterial aftergrowth. The purpose of the present study was to study the kinetics of substrate utilisation and bacterial growth at low nutrient conditions in a model distribution system. The model system consisted of two loops in series, where flow rate and retention time were controlled independently. Spiking the drinking water of the model system with two different environmentally realistic concentrations of carbon allowed for a close monitoring of the kinetics of substrate turnover (less than 10 μmg C/L 14C-benzoic acid was added). The mineralisation of benzoic acid was rapid and could be modelled by a no-growth Monod expression using a maximum degradation rate of 0.59 μmg C/L/h and a half-saturation constant of 2.6 μmg C/L. Only 2-4% of the carbon being degraded was incorporated into the biofilm. The results from our study suggest that the cell-specific respiration of biofilm was much higher than for suspended bacteria, and that the growth rate of the bulk phase bacteria was approximately 10 times higher than the biofilm bacteria.

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