Effects of drinking water biodegradability and disinfectant residual on bacterial regrowth

In this research an empirical biofilm accumulation model was developed which relates steady-state heterotrophic biofilm bacterial (HPC) numbers to chemical and physical system conditions in a drinking water distribution system. Biodegradable organic matter (BOM), which is a surrogate for the available organic carbon, disinfectant type and the concentration of the disinfectant are the chemical conditions. The investigated physical parameters are the shear stress or flow velocity, temperature, and the material of a pipe surface (substratum). Although biofilm detechment and the relationship between biofilm accumulation and suspended cell numbers are complicated phenomena, in general, reducing biofilm accumulation would be expected to reduce suspended cell numbers.; The model was developed utilizing data from experiments with both synthetic and real waters. These waters were fed to bench-scale annular reactors (AR), which represent a section of a distribution system. The feed water as well as a BOM cocktail, and either chlorine or chloramine disinfectants were dosed directly into the ARs. Shear conditions could be adjusted by the rotational speed of an inner drum in the AR. Liquid phase temperature was controlled using a recirculating temperature control unit. Biofilm was mechanically removed from flush-mounted polycarbonate or ductile iron coupons and quantified by HPCs. The quantitative variables were investigated at design levels which are typical in actual distribution systems.; Both experimental and modeling results clearly show the importance of the disinfectant. The increase of free chlorine residual from zero to 0.5 mg/L reduced HPC numbers by 3 to 4 orders of magnitude. When applied to an established biofilm, the efficacy of the disinfectant was somewhat lower. In a system with little or no disinfectants, the pipe material appears to affect the accumulation of biofilm, such that the corrosive ductile iron surface supports significantly higher net accumulation. Model output indicates a positive correlation between BOM levels and the steady-state HPC numbers. The rate of increase in BPC numbers, however, declines as BOM level increases.; Evidence suggests that the effect of shear on bacterial growth is a function of the BOM and disinfectant residual in a system. In the presence of a BOM supplement but in the absence of a disinfectant, HPCs were little affected by shear conditions. Shear appeared to be a significant factor for net accumulation only in the absence of both BOM and a disinfectant. This suggests a bioreaction limitation at higher BOM levels and mass transfer or diffusion limitation at lower nutritional conditions. It was speculated that in the presence of a disinfectant residual, higher flow velocities may lead to lower biofilm accumulation. The practical implication of this is that in the design of distribution system flow velocities, due consideration should be given to BOM and disinfectant conditions in the system. HPC numbers were less affected by temperature than by other factors such as disinfection residual and BOM level.; A user-friendly interface of the model was written in Visual Basic (c) programming language. The executable file of the interface is appended on two distribution (3 1/2&inches;) disks.