COVER

SCIENTIFIC SAFETY NETS

HOW TREATMENT CAN REDUCE RISKS

NORA UNDERWOOD January 15 1990
COVER

SCIENTIFIC SAFETY NETS

HOW TREATMENT CAN REDUCE RISKS

NORA UNDERWOOD January 15 1990

SCIENTIFIC SAFETY NETS

COVER

HOW TREATMENT CAN REDUCE RISKS

During the 1800s, tens of thousands of Canadians died from cholera, an acute bacterial infection of the intestines. Cholera disappeared almost

entirely in the West during the 20th century after scientists discovered that the bacteria was in many cases being transmitted in untreated drinking water. As a result, governments began treatment programs. Initially, methods were simple, involving merely filtering to rid the water of relatively large impurities. Now, most large cities in the Western world not only filter their water, but also

disinfect it with chlorine and other chemicals to kill disease-causing bacteria.

Source: The procedures for treating water before it reaches consumers vary depending on the source. River water, such as the sewagecontaminated water from the North Saskatchewan River that is used by Edmonton residents, usually requires extra treatment because it is “hard”—meaning that it contains high levels of calcium and magnesium, making soap difficult to lather. Water for Halifax residents comes from Pockwock Lake, 24 km northwest of the city limits.

The provincial department of lands and forests owns all the land surrounding the lake and does not permit any houses to be built near it or sewage to be dumped into it. As a result, according to Joseph McCarthy, a water treatment technician at the facility there, the

process needs less chemical treatment. Said McCarthy: “When the raw water is of good quality, it is a little easier to treat.”

For its part, Toronto has one of the most sophisticated treatment facilities in North America to serve its 2.5 million customers. Four filtration plants draw in raw water from Lake Ontario and convert it into 300 million gallons of drinkable water every day. In the plant’s intake well, the water is pretreated with a small amount of bacteria-killing chlorine and the coagulant aluminum sulphate, known as alum. In the next stage, mechanical mixers whip up the mixture, causing the alum to turn into a so-called floe, a fluffy, gelatinous mass that carries an electrical charge and magneti-

cally attracts such small particles as fine clay and algae in the water. The floe, with the impurities clinging to it, settles into a basin. Pumps then push the water through to a filtration process that is supposed to catch whatever the floe has missed.

Toronto uses a system called “dual media” filtration. The water seeps down through two layers of absorbant material. The top layer is about 12 inches of anthracite, a coarse, granular form of carbon. Below that is a much finer layer of 18 to 24 inches of sand. During that stage, according to John Camduff, director of

the Toronto water supply, bacteria levels in the water can also be reduced. Said Camduff: “It is virtually impossible to say that any processing will remove all bacterial pollutants. It will probably only reduce them.”

Disinfection: In the final stages of the process, the water flows into large underground wells, where technicians subject it to what is called super-chlorination. That involves adding large quantities of chlorine, ranging from two to 10 parts per million, to eliminate as much of the remaining bacteria as possible. If that requires so much chlorine that it gives the water an unpleasant taste or odor, sulphur dioxide is added later to neutralize at least some of the chlorine after it has done its work.

Then, 1.2 parts per million of fluoride—roughly 1.7 tons per day—is mixed into the water, a step that most municipal governments gradually introduced in the 1950s to help prevent tooth decay. Finally, following disinfection, ammonia is added to the water to combine with the residual chlorine. That combination, according to Henry Jakubiec, process engineer at Beak Engineering Ltd. in Toronto, provides longer-lasting disinfection.

Pollutants: Water treatment systems like Toronto’s are in use in Calgary and Winnipeg, and most across Canada are designed to kill

potentially dangerous bacteria. But they are not capable of removing many of the mineral and chemical pollutants now found in some Canadian waters. Currently, one of Toronto’s four filtration plants is undergoing expansion, and officials say that, during the next five years, Toronto’s entire system will be reassessed and upgraded if necessary. The improvements would help the system tackle some of the chemicals and minerals found in Lake Ontario. Although Camduff maintains that Toronto’s water is among the best in Canada, he added, “There’s always room for refinement.”

NORA UNDERWOOD

with

BARBARA WICKENS

in Toronto