The evaluation of various schemes to eliminate Combined Sewer Overflows (CSO) from the City of Toronto to the lower Don River and the shores of Lake Ontario resulted in the selection of a storage tunnel system, from which flows are pumped to a treatment facility before discharge to the receiving waters. This paper discusses the procedure used and the results obtained to optimize the combination of the storage volume and treatment rate to meet the required objectives. To analyze the effectiveness of this scheme in reducing the number of overflow events and the total volume of the CSO, long term simulations were carried out. The City's Quantity-Quality Simulation model (QQS model, developed by Dorsch Consult) was calibrated and run for four years of rainfall record. These years were selected as two average precipitation years, a dry year and a wet year representing the last twenty years of precipitation record. The model was used to predict the frequency of overflows resulting from different storage volumes (represented by tunnel sizes) and different treatment rates (represented by pumping rates from the tunnel). A family of curves was plotted using the QQS results, to present the relationship between the storage volume and the treatment rate vs. the effectiveness of the scheme presented by the resulting number of CSO events per year, and the percentage reduction in the overall annual CSO volume. To optimize the scheme the total cost of the storage and treatment was estimated, and a family of curves was produced among alternative schemes consisting of combinations of storage sizes and treatment rates, their effectiveness and their overall cost. The curves were plotted to present the effectiveness in the entire waterfront area, as well as the effectiveness in separate areas, since different water quality objectives were set up according to the water use activities in those areas. The procedure then identified the most cost effective combination of tunnel sizes and treatment capacity to meet the objectives in the different areas.
Control and treatment of stormwater discharges and combined sewer overflows (CSO) from w•ban 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 stonnwater runoff prior to discharge to receiving waters. The CSO outfalls contribute approximately 71% of average annual volume and the stonnwater outfalls 29%. It has been detennined 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
The International Joint Commission has identified the St. Clair River in Sarnia as an area of concern requiring a remedial action plan in order to restore and protect the water quality. To attain this objective, the City of Sarnia has completed a Pollution Control Plan in March 1993 to develop the approach in resolving the pollution problems along the City of Sarnia waterfront and the St. Clair River. The Pollution Control Plan identified the following three areas for improvement: (1) upgrade the water pollution control plant to provide secondary treatment for three times the average dry weather flow; (2) install four tanks to intercept combined sewer overflows; and (3) intercept storm runoff discharges along the waterfront and provide their treatment through sedimentation and/or flow through a wetland. The City of Sarnia is presently installing the first of the CSO control tanks at Devine Street. During the operation of combined sewage or stormwater detention facilities, settling of solids will occur along the invert of the facility. These sediments have to be removed after every storage event to minimize future problems such as caking and/or odour. This paper presents the evaluation and selection process carried out in developing a cost effective cleaning system for the Devine Street detention tank.
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