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This study evaluates the synergistic effects of ultraviolet (UV) and secondary disinfectants on water quality in a model distribution system. Chemical disinfectants evaluated for residual disinfection include chlorine dioxide, monochloramine, and free chlorine. Results suggest that there may be synergistic effects between UV and the chemical disinfectants for controlling microbiological re-growth in drinking water distribution systems. UV disinfection appeared to increase the vulnerability of bacteria located in suspension or within biofilms to chemical disinfection, although only at low disinfectant residual concentrations of 0.25, 0.50, and 1.0 mg/L for chlorine dioxide, free chlorine, and monochloramines, respectively. At high disinfectant residual concentrations of 0.50, 1.0, and 2.0 mg/L for chlorine dioxide, free chlorine, and monochloramines, respectively, results indicated that chemical disinfection alone would be adequate for microbial control. In the reactors receiving UV pre-treatment, both chlorine dioxide and free chlorine disinfection were far more effective in reducing the growth of suspended heterotrophic bacteria than without UV pre-treatment. Similar biofilm and suspended bacterial levels were observed without subsequent chemical disinfection regardless of the presence or absence of UV treatment. Overall, these findings imply that UV treatment prior to chemical disinfection enhances microbial control in distribution systems. These results indicate the use of UV pre-treatment can be effective at reducing the disinfectant concentration necessary for microbial control, therefore reducing the potential for the formation of disinfectant by-product in the system.Key words: drinking water, UV pre-treatment, chlorine dioxide, chlorine, monochloramines, disinfection.

Membrane bioreactors (MBRs) in combination with appropriate disinfection may provide sufficient wastewater treatment to produce effluent suitable for non-potable reuse. However, the ability of this technology combination to remove endocrine disrupting compounds (EDCs) has not been well studied. This study found that prior to disinfection a MBR removed greater than 96% of the influent cholesterol, coprostanol, and stigmastanol from municipal wastewaters compared to greater than 85% removal in a conventional treatment plant receiving the same influent. MBR effluent EDC concentrations were also lower for estrone, 17α-ethynylestradiol, and bisphenol A. Disinfection of MBR effluent with chlorine, chloramines, and chlorine dioxide provided no significant additional removal of estrone, 17β-estradiol, or 17α-ethynylestradiol but could remove 95% of added bisphenol A. Ultraviolet light at a dose appropriate for non-potable reuse did not affect removal of the target EDCs.Key words: endocrine disrupting compounds, membrane bioreactor, conventional treatment, disinfection.

This bench-scale study investigated the hypothesis that humic acids may thinly coat the surfaces of viruses (bacteriophages MS2 and T4) and thereby protect them from inactivation by UV light (254 nm). The model viruses were mixed in buffered water (pH 5.5) that contained Aldrich® humic acid (AHA) concentrations of 0, 10, 50, and 150 mg/L and were exposed to UV light at 40 and 80 mJ/cm2 for bacteriophage MS2 and 2 and 7 mJ/cm2 for bacteriophage T4 using a low pressure UV collimated beam. No steps were taken to form particles (i.e., no coagulant was added). The UV inactivation of both bacteriophages decreased to a statistically significant degree (based on 95% confidence level, Student's t test) at a humic acid concentration of 150 mg/L at the higher UV doses for each phage. This was despite the increased UV absorbance at 254 nm (UV254) caused by the humic acid being taken into account in the UV dose calculation. It was therefore hypothesized that the humic acid was coating the phage surfaces with a thin UV-shielding layer. As a further investigation of this hypothesis, samples containing humic acid, both with and without phage, were pelletized by ultracentrifugation and re-suspended in pure Milli-Q® water and the resulting UV254 values were compared. It was anticipated that the re-suspended pellets from the solutions containing humic acid and phage would exhibit higher UV254 than those of the pellets from solutions containing either phage or humic acid, due to the coating interaction. While there were statistically significant differences in UV254 among some of the re-suspended pellets, the results were inconsistent due to limitations of the experiment. As such, while some supporting evidence for the humic coating hypothesis was provided by this initial proof of concept study, further research is needed to more fully understand this phenomenon. Key words: ultraviolet light, disinfection, humic acid, bacteriophage, MS2, T4.

Computational fluid dynamics (CFD) modeling and full-scale tracer tests (using barium or fluoride) were used to determine the baffle factors of clearwells at three Canadian water treatment facilities (two in Ottawa, Ontario, and one in Peterborough, Ontario). A variety of clearwell baffling configurations and a range of flow rates (35 to 257 MLD) were considered. Two-dimensional CFD modeling (no depth dimension) was conducted using commercially available software (Fluent 6.0®). Virtual particle tracking allowed simulation of the residence time distribution for each clearwell configuration and flow rate condition. The baffle factors (t10/θ) derived from the CFD modeling closely matched the values obtained from full-scale tracer testing (

The impact of supplementing a raw drinking water source with reuse water was examined. Reuse water produced from municipal wastewater by a membrane bioreactor and reverse osmosis (MBR-RO) system was blended with untreated Lake Ontario water, then subjected to conventional water treatment processes, and evaluated in terms of disinfection by-products (DBPs), nitrate, and coliforms. The addition of reuse water to the lake water improved water quality in terms of total organic carbon (TOC) and bromide, which resulted in reduced trihalomethane (THM) formation. This appeared to be entirely due to dilution with no specific impact from the conventional treatment process. Nitrate levels in the reuse water (1.4 mg/L) were higher than the lake water (0.4 mg/L) and, therefore, an increase in the reuse water : lake water blend ratio resulted in a linear increase in nitrate levels. The conventionally treated blend water was shown to meet typical drinking water regulations for THMs, haloacetic acids (HAAs), total coliform, and nitrate. Key words: water reuse, drinking water supplement, reverse osmosis, membrane bioreactor, conventional water treatment, disinfection by-products, nitrate.

A simplified computational model of the ultraviolet (UV) disinfection dose delivered to wastewater was developed and the model outputs were compared with pilot-scale biodosimetry data. The model assumed plug flow, an assumption that may apply for some open channel UV reactors designed for wastewater disinfection and in which case may eliminate the need for involved and expensive computational fluid dynamics analyses of flow patterns. The reactor residence times derived from this assumption were combined with the output from a two-dimensional, point-source summation UV irradiance model to calculate UV dose. The output from the irradiance model was adapted to account for the cross-sectional distribution of UV irradiance within the reactor. Two UV reactor configurations were modeled and pilot-tested at two UV transmittance (UVT) values that are typical for wastewater. The simplified model predicted doses that fell within the range of the observed UV doses under some flow and UVT conditions, however it did so inconsistently and over-predicted the doses at the highest tested flow rates. This was concluded to be due to a breakdown of the simplified plug flow assumption due to deviations in the velocity field distribution at higher flows. Therefore, while this modeling approach may provide "back of the envelope" initial estimates of the UV doses supplied to wastewater and may allow a qualitative evaluation of the effect of adjusting UV reactor design parameters (e.g., channel width, lamp spacing) on the resulting UV dose, the precise quantitative prediction of UV dose must continue to rely on more sophisticated models. Key words: ultraviolet disinfection, computational modeling, wastewater treatment.

A bench-scale study was conducted using water from a drinking water treatment facility on Lake Ontario to investigate water quality and disinfection impacts of chlorine contamination in a chlorine dioxide solution applied for primary disinfection. Levels of chlorine contamination were varied between 0% (pure chlorine dioxide) and 20%, with a chlorine dioxide dose of 1 mg/L. The chlorine did not significantly affect the rate of chlorine dioxide decay in the water. The chlorine reduced chlorite formation, but led to a corresponding increase in chlorate formation. Trihalomethane and haloacetic acid concentrations varied with the amount of free chlorine present in the chlorine dioxide solution. Levels of Bacillus subtilis endospore inactivation were similar for solutions containing pure chlorine dioxide and a 20%:80% chlorine:chlorine dioxide mass ratio (for the same total chlorine concentration). Since chlorine alone has been reported to be very ineffective against these endospores, the results suggest that the disinfection effectiveness of the mixture was greater than the effectiveness predicted from each of the two disinfectants acting alone. Key words: chlorine dioxide, chlorine, disinfection, by-products, inactivation, drinking water.

Drinking water utilities may have to consider changing disinfectant to improve water quality and meet more stringent disinfection regulations. The objective of this study was to evaluate the response of a full-scale drinking water distribution system to a change in disinfectant from chlorine to chlorine dioxide, in terms of its impact on microbiological stability and disinfection by-product formation. Chlorine dioxide residuals were consistently present above detection limits throughout the distribution system (>0.20 mg/L). Over a study period of 3.5 months, no degradation of bacterial water quality occurred after implementing the new disinfectant. Chlorine dioxide maintained total bacteria (microscopic counts) and heterotrophic plate count levels below 2 x 105 cells/mL and 1000 CFU/mL, respectively. The change in disinfectant from chlorine to chlorine dioxide led to an 85% reduction in trihalomethanes (i.e., from 30 to 5 μμg/L) and a 60% reduction in haloacetic acids (i.e., from 20 to 8 μμg/L). Chlorine dioxide represents a valuable tool to produce high quality water and is a strong alternative to chlorine for certain distribution systems. Key words: distribution system, chlorine dioxide, drinking water quality, disinfection by-products.