Going Off Chlorine

In 2005, the City of Montreal launched a major project to upgrade its eight drinking water treatment plants. The project includes studies, engineering, procurement, project management and construction management for the upgrading and rehabilitation of eight plants serving a population of two million people.

One of the mandates involves replacing the existing chlorine gas disinfection systems at two of the largest plants: the Atwater and Charles-J. Des Baillets plants, located close to downtown Montreal. The Atwater plant is 100 years old and has a capacity of 680,000 m3/d, while the Charles-J. Des Baillets plant, with a capacity of 1,159,000 m3/d, was built in 1978. The treatment process at both plants primarily includes screening, sand filtration and chlorine gas disinfection.

In the wake of incidents at drinking water treatment plants in the U.S. several years ago when chlorine gas was accidentally released into the atmosphere, the city of Montreal became concerned about its own exposure to this risk, both when transporting the gas to its treatment plants through densely populated residential areas, and in handling the gas in routine plant operations. The city asked SNCLavalin, as the lead member of the SNC-Lavalin/DESSAU consortium doing the plant upgrades, to examine these risks, evaluate the potential impacts, and seek alternative methods of disinfection.

The studies revealed that the two plants use between two and four tons of chlorine gas per day, which is transported in one-ton cylinders over a distance of about 100 kilometres. The plants have approximately 80 cylinders in-waiting, plus 36 cylinders connected to the chlorination system. The cylinders are unloaded outdoors at the Atwater plant and indoors at the Charles-J. Des Baillets facility. One of the risks of handling the cylinders is valve failure, which might cause a cylinder to release about one ton of chlorine gas.

To bring the existing disinfection systems to current standards and reduce risks at the plants, SNC-Lavalin recommended replacing the existing chlorine gas disinfection systems with an on-site sodium hypochlorite disinfection system.

An on-site sodium hypochlorite system uses three common consumables: salt, water and electricity. The salt is used to prepare a brine solution, which is then reduced in concentration with demineralized water. The diluted brine solution is pumped through an electronically-charged electrolytic cell where a chemical reaction results in a 0.8% sodium hypochlorite solution.

The salt can be delivered by truck and easily stored in tanks. Handling a much weaker 0.8% sodium hypochlorite solution greatly reduces the risk of accidental exposure to hazardous materials.

As a result, in contrast to chlorine gas and bulk hypochlorite, an on-site generation system eliminates the bulk storage of chemicals, lowers risk to plant personnel because of drastically reduced hazardous material storage and handling requirements, eliminates transportation liabilities, and reduces the threat to public safety.

According to SNC-Lavalin’s study, six hypochlorite generation systems, each with a capacity of 1,500 lbs/day of chlorine equivalent, are required at Atwater, and five at Charles-J. Des Baillets. The new disinfection system is under construction at Charles-J. Des Baillets, while installation at Atwater is set to begin in mid-2011 with completion in 2012.

Jacques Trottier, M.A. Sc. is Director of Processes and Urban Infrastructure; Jing An is an engineer. Both are with SNC-Lavalin in Montreal.