Water supply - Zebra mussel control in Toronto's water supply system
What are zebra mussels?
Zebra mussels are tiny animals, members of the mollusc family which includes clams and oysters. With dark black shells and white stripes, their appearance leads to the obvious zebra comparison. They are native to European fresh waters and were first detected in North America in Lake St. Clair (the small lake between Lake Huron and Lake Erie). It is thought that the zebra mussels first migrated to North America by smuggling themselves in the ballast of a European tanker that travelled through the Great Lakes in 1987.
Are they a problem for Toronto's water treatment plants?
The zebra mussels will naturally attach themselves to hard surfaces such as concrete intake pipes. Zebra mussels have been found on the exterior of the raw water intakes of the Toronto's four water treatment plants. However growth of the mussel colonies has been sporadic, to date has not affected water flows through the intakes.
What is Toronto doing to control zebra mussels?
Various methods were considered by Toronto to control the zebra mussels. However, since the only short-term control method approved by the Ontario Ministry of Environment (MOE) is chlorine application, this is the only method being used by Toronto. Chlorine is already used in the treatment of Toronto's water to control algae and bacteria at the beginning (pre-chlorination), and at the end (post-chlorination) of the water treatment process.
Will this increase the level of chlorine in Lake Ontario and, ultimately, in our drinking water?
No. The use of chlorine to ward off zebra mussels from the water intake pipes only requires the extension of the pre-chlorination solution lines to the mouth of the intake pipes in the lake. All of the chlorine solution would then be drawn back into the treatment plant so that there would be no addition of chlorine to the lake environment. Levels of chlorine in Toronto's drinking water are continually monitored and regulated at the end of the water treatment process so this control method would not increase chlorine levels in our drinking water.
What construction is involved?
In three of Toronto's water treatment plants the new chlorine solution lines and the new raw water sampling lines were installed directly inside the intake pipes, minimizing the need for extensive new construction. Unfortunately, the intake pipes for the R.C. Harris water treatment plant are so deep and difficult to work inside that provincial safety regulations did not allow divers to install the new lines. Instead, new chlorine solution lines and new raw water sampling lines were laid inside an excavated trench at the bottom of the lake. The pipes are protected against accidental damage with a bed of armour stone