Thing: The water factory

>> >> How the slimy green waters of the St. Lawrence are made fit for human consumption

by PHILIP PREVILLE

People want their tap water to be as clear and pristine as a fresh mountain stream. But mountain streams aren't as pristine as people think--they're filled with animal dung and all sorts of bottom-feeding microorganisms. And if that's the case even in the deep wilderness then you can imagine what it's like in the St. Lawrence, where Montreal gets its tap water.

Down at the Charles-J. Des Baillets water filtration plant in LaSalle, engineers siphon over 1.3-million cubic metres of watery ecosystem out of the St. Lawrence every day. Then they embark on a four-step process of microbiological genocide to make it suitable for human consumption.

Step 1: Intake. Water travels to the plant through a pipe 1.6 km long. Everything travels up that pipe, including some pretty big catfish. ("Some employees bring their rods to work and fish them out during their lunch break," says plant manager Michel Gagné.) From there it flows through a grill and is pumped upwards to the concrete reservoir shown here. The grill keeps the catfish out of the water supply, but everything else gets through this first step--from suspended solids to insect larvae to coliform bacteria to baby fish. "Every December the system fills up with minnows," says Gagné.

Step 2: Sand filtration. From there, the water flows through a bed of sand 1.2 metres deep. The sand filter removes most of the suspended particles and toxic chemicals--and some of the living organisms (most notably the minnows). But many bacteria still get through.

Step 3: Ozone injection. These pods are the gas chambers of water purification. Inside, pure oxygen atoms (O2) pass through an electrical current and recombine to produce ozone (O3), which is injected into the water. There, oxygen-starved bacteria gravitate to the rich but highly unstable ozone and are asphyxiated. The same thing would happen to a human being if they drank ozone-laced water, but the compound is so unstable it evaporates from the water within 10 minutes.

Step 4: Chlorination. Once ozonated, the water is as pure as human engineering can make it. However, just in case any contamination enters the water between the time it leaves the plant and the time it reaches your house, they add chlorine to it as well.

Final factoid: Despite all this, bacteria sometimes make it through to your tap anyway. Once a week, city officials take water samples from 70 different points in the network and test for contamination. Last year they issued five advisories, compared to only two in 1998. :