Disinfection of physico-chemically treated wastewater using medium pressure ultraviolet lamps

An ultraviolet pilot plant was installed at the Montreal Urban Community Wastewater Treatment Plant (MUCWTP) to assess the feasibility of employing a full scale UV disinfection system. Wastewater characteristics such as suspended solids (SS), turbidity, iron content, transmittance at 254 nm, and dissolved organic carbon were monitored to assess their effect on UV disinfection. Fecal coliforms were used as indicators to measure disinfection performance. UV disinfection was capable of reducing the fecal coliform counts from typical values of 0.2-3 million CFU/100ml (depending on the process and season) to thousands, hundreds or tens per 100 ml, depending on the dose applied, the process used for phosphorus removal, and wastewater quality. High turbidity values yielded higher surviving microbial counts; disinfection effectiveness decreased by 10% for each NTU of turbidity increase. SS in the wastewater scattered the UV light, leading to dose underestimations and the illusion that higher SS would yield better disinfection effectiveness. A photoreactivation increase of 8.1 and 11.7 fold was observed for iron-treated and alum-treated effluents respectively. Dark repair increases were very different for iron-treated and alum-treated effluents; however, in terms of dark repaired fraction, both effluents had values within the same range. UV disinfection was not found to cause any additional toxicity in the wastewater