Determination of a Preferred Disinfection Strategy for the City of Toronto's Ashbridges Bay Treatment Plant

The Ashbridges Bay Treatment Plant (TP) currently uses chlorine addition to disinfect treated effluent prior to discharge through an outfall pipe with diffusers that distribute effluent into Lake Ontario. The Ashbridges Bay TP is fed by a collection system that includes many portions with combined sewers, and therefore experiences high, dilute flow during periods of rainfall and snow melt. During high flow periods, when the flow to the plant exceeds the capacity of the secondary treatment process, primary effluent (secondary bypass), which is only partially treated, is blended with the secondary treated effluent and discharged through the outfall. The existing outfall does not provide adequate hydraulic capacity for extreme peak flows. In addition, adequate chlorine contact time for disinfection is not provided. In light of the hydraulic capacity limitation and the insufficient disinfectioncontact time, the City is planning to construct a new outfall, with a compatible disinfection process to provide adequate capacity to disinfect and convey all flow into Lake Ontario. An environmental assessment study was conducted to determine the preferred disinfection strategy for both secondary effluent and secondary bypass flow from the plant and the results of that study are presented in this paper.