The purpose of this study was to evaluate the impact of blending filtered water with treated waste residuals streams from a water treatment plant on finished water quality and biofilm formation in a modelled distribution system. A mixed waste residual stream composed of an equalized blend of filter backwash water (FBWW) and clarifier sludge blowdown was treated with a pilot-scale ultrafiltration (UF) membrane. The UF treated residuals stream was blended with plant finished water to model a recycle return location to the clearwell of a drinking water treatment train. Field evaluations involved the use of bench-scale annular reactors (ARs) to investigate two parallel model distribution systems: (i) a control system representing 100% plant filtered water (AR-filtered) and (ii) a blended system representing 10% by volume of the UF permeate blended with plant filtered water (AR-blended). To evaluate the impact of chemical disinfection on bacterial regrowth and the fate of inorganic constituents, free chlorine, monochloramine and chlorine dioxide were fed to the ARs during three distinct disinfection trials. An initial disinfection trial consisted of a control run during which no disinfectant was added. The results of the field experiments showed that blending 10% by volume of the UF permeate with plant finished water did not significantly impact regrowth of heterotrophic bacteria in the bulk or biofilm phases. Throughout each disinfection trial, the largest difference in HPCs noted between the two systems was 1 log in the bulk and biofilm samples. Free chlorine and monochloramine disinfection resulted in the greatest mitigation of microbial biofilm in both AR-filter and AR-blended systems. Aluminum, manganese, and iron in the bulk phase were found to be below Canadian Drinking Water Guideline levels. However, the detection of manganese-oxidizing bacteria (MOB) and elevated manganese in biofilm samples from AR-blended could hold potential implications for blending UF treated residual streams on biofilm development and inorganic post-precipitation occurrences in systems treating source waters containing elevated concentrations of manganese.

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