In most parts of the city of Toronto (and in other cities) there are two main pipe systems; the first is the storm sewers system, which is for storm water coming from the street surface. The second is the sanitary sewers system, which is for wastewater that leaves your house. The sanitary sewer carries water from sinks, toilets, and household appliances such as your washing machine. In the oldest part of Toronto, the stormwater and sanitary sewers flow into the same combined underground piping system, and then are carried into wastewater treatment centres (as of 2010). Below is a map of the old Toronto sewershed boundaries.
This system has some advantages. For example, the pollutants trapped in the rainwater can be treated before being released into the environment (see urban runoff for more info). In times of heavy rainfall, the combined volume of water from the storm and sanitary sewers is too much to be treated at the treatment plant. In this situation, the water from the combined sewer will bypass the water treatment plant and go directly into the receiving water body, which in Toronto is Lake Ontario. Having raw (untreated) human sewage and other pollutants in the water is extremely dangerous to both aquatic ecosystems and humans. In fact, consequent high levels of Fecal Coliform Bacteria (from the sewage) can cause beaches to be closed during summer months. Fecal coliforms often include harmful strains of E. Coli which can cause human and animal sickness, and even death. This is why the use of combined sewer systems is a large issue. Even occasional occurrences of combined sewer overflow can cause significant damage to local ecosystems and our health, and can also impact our recreational activities.
There are many different solutions to this issue. They all have their advantages and disadvantages. Which would you pick?