Restoring Halifax Harbour
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THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS |
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HALIFAX, NOVA SCOTIA is an important commercial container port and military naval base. |
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THE CITY WAS FOUNDED as a military outpost in 1749. |
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THE HARBOUR stretches from the Atlantic (background) far inland to a sheltered basin. |
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THE HALIFAX HARBOUR Solutions Project, is a $333-million plan to dramatically improve water quality. |
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HALIFAX IS HOME to about 350,000 people, and each day about 200 million litres of raw sewage are
released from 40 points into the surrounding waters. |
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By 2010, three sewage treatment plants, made almost entirely of stainless steel, will help to
restore this spectaular natural harbour to its original purity by Dean Jobb
Nickel Magazine, December 2006 -- It’s one of the world’s largest and deepest natural harbours, and ice-free to boot. Halifax Harbour, on the rocky east coast of the Canadian province of Nova Scotia, stretches from its broad approaches on the Atlantic Ocean far inland to a sheltered basin, with depths of up to 18 metres at low tide.
Convoys assembled here to shuttle troops and supplies to Europe during two world wars, and today the largest container ships and cruise liners afloat are frequent visitors. Not long after the city was founded as a military outpost in 1749, British officers declared it the finest harbour they had seen, capable of accommodating the entire Royal Navy of the day.
Unfortunately, the harbour has also served as a dumping ground for the city’s sewage. The Halifax Regional Municipality is now home to about 350,000 people, and each day about 200 million litres of raw sewage are released from 40 points into the surrounding ocean.
But that’s about to change. The $333-million Halifax Harbour Solutions Project will improve water quality dramatically while transforming a murky, foul-smelling waterway into what James Campbell, spokesperson for the project, calls “the jewel of the city.”
At the heart of the project are three treatment plants being built by Degrémont Halifax, a division of Paris-based global water treatment giant Degrémont Suez. The material of choice for a great many of the pipes, pumps, screens, conveyors and other components of the treatment plants is nickel-containing stainless steel.
Metal components must be able to withstand more than an onslaught of up to 650,000 cubic metres of foul water each day. Degrémont’s project manager, Philippe Cantareil, says untreated sewage gives off hydrogen sulphide and other noxious, highly corrosive gases. In addition, the plants must be able to withstand the dumping, whether accidental or illegal, of oil and industrial chemicals into the sewer system.
Galvanized steel would last just a few weeks under such conditions, he notes. Stainless steel is preferable by far, in combination with concrete for holding tanks and plastics for some of the pipes. Practically all the stainless used in the plants will be S30400,, except for the last stages of the treatment process, when more corrosion-resistant S31600 is needed.
The three plants vary in size. The largest, on the Halifax side of the harbour, can handle up to four cubic metres of sewage per second. The plant is gravity-fed, and the flow enters at 19 metres below sea level through a 3-metre-diameter concrete channel. A stainless steel sluice gate, 9 metres square, controls the flow as needed. Once inside, sewer water passes through a 25-mm stainless steel screen so that solids such as bottles, rocks, bits of wood, and other debris can be removed. The debris is then hoisted to the surface on a stainless steel track.
The water enters one of five 5.5–tonne pumps and is pumped 25 metres upwards through 800-mm stainless pipes and into the plant. It flows through a 10-mm screen to remove smaller debris, then passes through an aeration system that causes fine grit to settle. The heart of the operation is Degrémont’s Densadeg system, which removes suspended particles quickly using settling tanks a fraction of the size of traditional ones (vital for the Halifax plant, which is within the downtown core). Chemicals cause particles to clump and settle at the bottom of two concrete tanks, 10 metres deep and 11 metres square, for removal. The scrapers that rotate through the sludge as it settles to the bottom are made of S31600 stainless. Water is conveyed through various stages of treatment through stainless pipes “in all diameters you can imagine,” Campbell adds, down to as little as 25 mm.
The Halifax plants provide what is known as advanced primary treatment. Once suspended solids have been reduced to an acceptable 40 parts per million, the water passes through an array of ultraviolet lights that kill fecal coliform bacteria before it is discharged into the harbour. Bacterial counts now in the millions will be reduced to below 5,000 per 100 millilitres of water. The screened-out debris will be trucked to a landfill for disposal while the sludge is processed off-site to make nitrogen-rich fertilizer. Even the air inside the plant will be processed before release into the atmosphere, through a system of scrubbers and stainless steel ductwork that can process 90,000 cubic metres of air per hour.
Degrémont is building the treatment plants in partnership with Dexter Construction Company Ltd., the largest civil contracting firm in Atlantic Canada. Dexter also has the $112-million contract to lay 20,000 metres of new sewer pipe to feed the plants.
The Halifax plant will be in operation by spring 2007. The Dartmouth plant, on the opposite side of the harbour, will come on-stream in August of that year and be joined by the smaller Herring Cove plant, near the harbour’s mouth, in mid-2008. Tidal action will ensure a dramatic and almost immediate improvement in water quality.
“The harbour has quite a high potential to clean itself,” says Dr. Tony Blouin, Manager of Environmental Policy, Environmental Management Services, Halifax Regional Municipality. “Once you stop putting in the dirty water, the harbour ‘flushes’ itself, so to speak, quite often, so it’s a matter of literally days to weeks before the water is replenished.” The water willclear odour and “floatables” will disappear, and most areas will be safe for swimming and, eventually, shellfish harvesting.
The cleanup will restore pride to a city that has suffered from the stigma of using its harbour as a sewer, says Campbell, who adds that the number of residents near the waterfront has increased in recent years as boardwalks and waterside condominium developments take over empty lots, and vacant warehouses are converted into museums and cruise ship terminals.
PHOTOS: Nickel Institute
Degrémont Dexter Construction Company Ltd. Halifax Harbour Solutions Project Phone: 902-490-4604 |
