Integrated disinfection strategies for mitigating Escherichia coli in distribution systems

The focus of this project was to assess the impact of UV disinfection in combination with chemical disinfectants on suspended and biofilm bacteria in annular reactors (ARs) that were operated under conditions relevant to drinking water distribution systems. In addition, this work compared the potential synergies of UV treatment and disinfection on E. coli in distribution systems. The experimental setup addressed the following factors in drinking water supply: (1) pipe material (cast iron and polycarbonate), (2) UV treatment (with or without) and (3) chemical disinfectant (no disinfectant, chlorine (Cl2), chlorine dioxide (ClO2) or monochloramine (NH2Cl).; Twelve ARs, six polycarbonate and six cast iron, were used to simulate conditions and model pathogens in a drinking water distribution system. Four of the ARs were chemically disinfected, while eight of the ARs were pre-treated with a low pressure UV lamp (Trojan UV-Max) followed by secondary disinfection. E. coli was spiked prior to the UV lamp for four of the ARs, while the other eight ARs were spiked after UV disinfection to simulate a breech in distribution system integrity. Tap water from the Halifax Regional Water Commission (HRWC) was the primary source water for the ARs. Prior to entering the ARs, the tap water passed through a granular activated carbon (GAC) filter to neutralize any chlorine present in the source water. Then the water entered a biologically active GAC filter which provided an inoculum for the ARs. The ARs were given three weeks to form an indigenous biofilm and were then inoculated with one of two strains of E. coli K12 (ATTC strain #MG1655 and mutS Mutant). After a four week inoculation period, low level disinfection (0.2mg/L residual [Cl2, ClO2]; 1.0 mg/L residual [NH 2Cl]) proceeded for four weeks followed by high level disinfection (1.0mg/L residual [Cl2, ClO2]; 2.0 mg/L residual [NH2C1]) for three weeks. Effluent and biofilm of the ARs were sampled and analyzed for heterotrophic bacteria (HPCs) and E. coli. Additional analysis of the AR effluent included pH, temperature, turbidity, Total Organic Carbon (TOC), Dissolved Organic Carbon (DOC), UV-254 and disinfectant residuals.; This bench scale study showed that UV disinfection had little effect on the initial concentration and growth of biofilm and effluent heterotrophic bacteria in both cast iron (CI) and polycarbonte (PC) ARs. However, a trend was observed where UV treatment in combination with a chemical disinfectant was in most cases more effective at reducing heterotrophic bacteria in PC ARs than the chemical disinfectant alone. In cast iron reactors this trend was also observed for effluent HPCs, however not for biofilm HPCs. The best disinfectant regimes for reducing biofilm HPCs were UV + ClO2 for PC ARs and UV + NH2Cl for CI ARs. The most effective disinfectant treatments for lowering effluent HPCs were UV + ClO2 for PC reactors and UV + Cl2 for CI reactors. UV disinfection was extremely effective at reducing effluent and biofilm E. coli. In addition, all chemical disinfectants performed similarly against effluent and biofilm E. coli on both pipe materials. Finally, it was concluded that pipe material, UV and chemical disinfection likely affected the nature of the biofilm and E. coli behaviour. Phenotypical changes were observed in E. coli isolated from all annular reactors. The E. coli may have exhibited these changes as a result of stress from lack of nutrients, acid conditions or organism competition within the biofilm.