Abstract

The City of Toronto’s sewer system, parts of which date back to the mid 1800's was originally developed as a combined system, carrying both sanitary and storm flows. As the City rapidly expanded in the early half of the century, the capacity of the combined sewer system was quickly exceeded and basement flooding became a recurring problem. To solve this problem, the City constructed approximately 680 km of storm sewers in the mid 1960's which separated approximately 70% of the road drainage from the combined sewer system. The combined sewer system which the City today, however, continues to pick up sanitary flow and storm runoff from homes and buildings constructed prior to 1965, as well as storm drainage from the remaining 30% of the roadways which have not been separated.

Naturally, during periods of heavy rainfall, the capacity of the combined sewers and interceptor sewers are exceeded, resulting in the excess combined flows discharging directly to the receiving rivers and Lake Ontario. The Ontario Ministry of Environment investigated the impact of combined sewer overflows (CSO) and storm outfalls in a 1990 Wet Weather Outfall Study for the Toronto waterfront and determined that the 24 combined sewer overflows and the 29 stormwater outfalls (Figure 14.1) discharge approximately five million cubic metres of flow annually into the City's waterfront from May to October and contribute approximately 1,700 metric tonnes of total suspended solids and 6,650 metric tonnes of chemical oxygen demand to the near shore waters, along with concentrations of phosphorous, lead, zinc, aluminum and iron (Metropolitan Toronto Waterfront Wet Weather Outfall Study Phase II, August 1995).

In total, these outfalls severely impact the recreational use of the Toronto Waterfront and result in frequent unsuitable for swimming beach posting by the Medical Officer of Health. To solve this problem, the City undertook a Sewer System Master Plan (SSMP) in 1990 with the objective of virtually eliminating the pollutant loadings associated with combined sewer overflows and the control and treatment of stormwater runoff where required. Virtual elimination of CSO in the context of the SSMP was defined as reducing CSO to an average of one overflow or less per year for the Western Beaches or, alternatively, reducing the average total annual volume of CSO discharging to the Western Beaches by more than 90%. The City chose this criteria which exceeds the Provincial Guidelines for CSO discharges due to the extensive parkland system and swimming beaches along this section of the shoreline.

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