Abstract

Control and treatment of stormwater discharges and combined sewer overflows (CSO) from urban areas are issues of increasing importance in the field of water quality management. The City of Toronto has identified water quality problems resulting from these discharges and has undertaken to develop a Sewer System Master Plan for the virtual elimination of CSO and the control/ treatment of stormwater runoff prior to discharge to receiving waters. The CSO outfalls contribute approximately 71% of average annual volume and the stormwater outfalls 29%. It has been determined that for the City of Toronto the most effective means of controlling the CSO problem is to intercept CSO in a storage facility and then direct the intercepted CSO to a treatment facility at an acceptable rate of treatment. This paper presents a comparison of two methods of end-of-pipe control for CSO and stormwater to improve water quality. The two methods considered include a storage tunnel with one storage element and storage in scattered subsurface storage tanks. The tunnel system would consist of a 13,000 m long storage element along the lakefront from Parkside Drive to east of the Main Treatment Plant and a 3,400 m long tunnel along the Don Valley connecting to the lakefront tunnel. The subsurface storage tank system would consist of fifteen separate CSO and eleven stormwater tanks located at various outfall locations within the area identified above. Comparison of the two methods was undertaken in terms of effectiveness, environmental impacts, capital cost, treatment flexibility and operation and maintenance requirements.

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