Pilot Studies and Assessment of Downstream Effects of UV and Ozone Disinfection of a Physicochemical Wastewater

Abstract

UV radiation and ozonation were investigated as disinfection alternatives for the Montreal Urban Community physicochemical wastewater treatment plant. The final effluent flows through a 5-km-long outfall tunnel before discharge into the St Lawrence River; the critical point for microbial quality is 4 km downstream. The study, therefore, also examined possible biofilm growth in the tunnel and dilution behaviour in the river. For UV disinfection, a newly developed, high-intensity medium-pressure lamp unit was tested because of its space-saving features. Typical two-stage dose-response curves were obtained with the transition between disinfection of free-swimming and particulate-associated organisms occurring at approximately 25 mW s/cm2. Disinfection behaviour improved when alum was substituted for FeCl3 as the coagulant. Photoreactivation decreased with increasing UV dose. Ozonation was adversely effected by changes in COD and H2S in the wastewater. A dose of 17 mg/L was needed to reach target disinfection levels of <5,000 CFU/100 mL in 90% of the cases; the CT value was approximately 0.4 mg min/L. Replacement of FeCl3 by alum reduced the required dose by 35%. Effects of biofilm growth in the tunnel were inconsistent: effluent SS and microbial concentrations were sometimes higher at the exit than at the entrance and vice versa. Effective dilution of microbes in the river decreased as water temperature increased, reaching a minimum of approximately 20.