Raw Water Intake -– Island Treatment Plant
The Island Treatment Plant takes raw water from Lake Ontario and converts it into safe potable/drinking water for final pumping into the City’s distribution system. Here’s how the treatment works:
Water from Lake Ontario enters the two intake mouths located approximately 740 and 824 metres from shore, one at a depth of 17.0 metres and one at 11 metres. It proceeds through the two 1800 millimetre (mm) diameter riveted steel intake pipes to a common well in the raw water pumping station. During construction of the plant, a 2250 mm diameter pipe was laid from the new suction well to the junction point of the two intake pipes and a branch was left for future connection of a new 2250 mm diameter intake.
Screening and Pre-chlorination
Raw water travels from the entrance chamber to the suction of the raw water pumps, through three 1.5 m wide by 11.3 m high travelling screens, which remove the larger particles of suspended debris. The water is pre-chlorinated at the inlet side of the screens. During zebra mussel season, pre-chlorination is applied at the intake mouths as a control measure.
Raw Water Pumps
Four 114 ML/d pumps lift the water from the lake to the elevated from which it flows by gravity through the rest of the plant and to the John Street Pumping Station on the mainland.
Raw Water Treatment
As the water is discharged from the pumping units into the flume, alum is added, mixed by in-line blenders. The addition forms a jelly-like substance which joins together with impurities forming larger particles called floc.
Mixing and Flocculation
In the flume, the water passes through a hydraulic jump, the turbulence of which assists in thorough mixing of the alum and formation of floc. Polyelectrolyte filter aids can be added at this point.
Filtration
From the flume, the water passes to the six dual media filters. The filter consists of 0.56 metres of anthracite and 0.2 metres of sand, resting on a bed of graded gravel. Suspended impurities, including the floc, as well as some of the bacteria and other microorganisms, are removed in the filtration process.
Disinfection and Fluoridation
Following filtration, the water is post-chlorinated to completely destroy harmful bacteria and is then fluoridated to about 1.2 milligrams per litre. When high levels of bacteria are present, the chlorine dosage is increased through a process called "super-chlorination" and the resulting chlorine content is reduced by adding sulphur dioxide at the exit from the reservoir.
Ammoniation
Following disinfection, ammonia is added to combine with the residual chlorine. As the combined product persists much longer, it will ensure some protection at the remote ends of the distribution system.
Distribution
On completion of the process, a small quantity of the water is pumped into the Island Supply System, the balance flowing by gravity to the John Street Pumping Station which contains one 45.5 ML/d and eight 118 ML/d pumps to pump the water from the Island or Harris plants into the distribution system, to storage and to the consumer.
Metering
As the water leaves the John Street Pumping Station, the flow is accurately measured to indicate station output and for billing purposes.