WATER: Neptune’s Trident

The Halifax Regional Municipality in Nova Scotia uses a three-pronged approach to renewing, rehabilitating and protecting a growing water distribution system

Maintaining the distribution infrastructure is a critical challenge for the Halifax Regional Water Commission (HRWC), known locally as Halifax Water.

The HRWC, centered in Halifax Regional Municipality (HRM), the capital of Nova Scotia, supplies water for the area’s 350,000 residents and businesses. Created under a 1996 amalgamation, HRWC is an autonomous, self-financed utility.

The amalgamation brought together several water systems, including the former city of Dart-mouth across Halifax Harbour. Faced with providing the same standard of service to all customers and providing a basis for future system growth, Halifax Water embarked on an ambitious three-pronged program of water main replacement, main rehabilitation, and main protection.

Badly deteriorating water mains are replaced mainly by conventional methods. More sound mains receive a spray-on epoxy lining. Mains in good condition are preserved through cathodic protection, which combats corrosion.

Jamie Hannam, manager of engineering and information services with HRWC, observed that the program has been effective in preserving high-quality water service and in the long run will provide part of the foundation for distribution system growth.

Wide area

The water service area covers more than 200 square miles of some 2,000 square miles included in the amalgamation. Although the water systems of the former cities of Halifax and Dartmouth are nominally connected by a single 32-inch emergency main that crosses Halifax Harbour beneath the Angus L. Macdonald Bridge, the two systems are distinct entities. In an emergency, millions of gallons of water could be sent from either system to the other through the connecting main.

All told, the system encompasses almost 800 miles of water mains, two major water treatment plants (one 24 mgd, the other 60 mgd), seven smaller treatment plants, 16 storage reservoirs, more than 7,000 fire h­ydrants and 77,000 customer connections. A series of glacial lakes provide abundant fresh water to the ocean-bound region. One large lake supplies Halifax and another supplies Dartmouth. The watersheds are protected by the province to guarantee the quality of supply.

“Although the HRM is a fairly new entity, this is a historical fort city at its heart, with a lot of very old infrastructure,” says Hannam.

“The oldest main in service dates back to the 1850s, although there’s not a lot of it left in the system. About 60 percent of the system is cast iron pipe that spans two generations. The first generation, which was being installed up to 1940, is thicker and more robust than the thinner-walled pipe installed during World War II and the years that followed. In the 1960s, the system switched to ductile iron.”

Halifax Water is largely committed to replacing metal pipe with more metal. That’s because the city’s aggressive leak detection campaign relies on the delicate acoustics of metal-to-metal connection.

“We need to maintain the integrity of the system,” says Hannam. “If you replace a length of iron pipe with plastic, you throw off the acoustics, so we virtually always replace the pipe with new ductile, with only a nod to PVC pipe in isolated usage.”

Looking for leaks

The system is broken down into more than 90 district meter areas (DMAs) that can be temporarily isolated by a series of valves to run remote pressure checks. Rather than monitoring pressure during the day, when flows are variable, the utility uses its Supervisory Control and Data Acquisition (SCADA) system to monitor low flow rates, which are steadier.

The system can monitor water entering or leaving the system. “We record the night-time flows into each of the isolated areas, and if we notice flows greater than the historic night-time lows, we send a field crew to that zone within 24 hours,” says Hannam. “We pinpoint the leak hydrant-to-hydrant and valve-to-valve using acoustic gear from Sewerin. We use the ground mike for its range and sensitivity for both surveying and pinpointing, and we use correlators from Plamer Environmental to assist in pinpointing. Once the leak is identified, we dig and repair within three days.”

Leaks are generally repaired with stainless steel clamps tightened around a neoprene gasket, a low-cost fix that has served the system well.

The leak detection program is already mature: Most historic leaks have been taken care of. Hannam notes that the system experiences 200 to 250 leaks annually, a number that has remained constant over the past few years.

“We follow up with an annual leak detection survey and noise mapping to pick up any leaks that aren’t reported by customers or escape our SCADA system,” he says. “Some systems comparable to ours run checks annually or semi-annually, but that isn’t good enough for us. It’s not only a drain on revenue, but it robs the system of pressure and causes considerable long-term damage by undermining the support around water mains.” All mains in the system are graded according to three categories:

• Structurally deteriorated and ready for replacement.

• Structurally sound, but short on performance and ready for cleaning or lining.

• Structurally sound with acceptable or good performance and candidates for cathodic protection.

Making the old new

“Once the pipe reaches the end of its service life, as measured by its structural integrity, it’s ready for replacement,” says Hannam. “Obviously, lengths of main with multiple leaks are prime suspects. We were traditionally inspecting pipe and replacing it at about 0.4 percent of the system per year. We’re now ramping that up to 0.8 percent for 2010, with an eventual goal of replacing one percent annually.”

The utility uses Class 52 ductile iron pipe supplied by the Canada Pipe Co. for replacement. Much of the work is open-trench replacement, although the utility has begun to examine the potential of trenchless technologies. In 2007, more than 6,000 feet of main was rehabilitated with Aqua-Pipe structural liner, a thermoset epoxy system from Sanexen Environmental Services Inc.

Trenchless pipe repair is not a high priority at this point because the utility is coordinating much of its replacement work with a large-scale street improvement program. Most of the pipe is buried a little more than 4 feet down, just below the frost line.

“We’re able to complete 90 percent of the water main renewal program with streets already under construction, which reduces costs for both our program and for street construction,” says Hannam. “We go in just after the asphalt has been stripped from the street. At that point, they’ve already established a construction zone, so interruption of traffic flow is limited as well.”

The utility is also coordinating some of its renewal with construction of a large wastewater treatment system now under way, although there is less synergy between water main and wastewater pipe locations. Once complete, the wastewater system will be handed over to the HRWC to operate on the same independent model as the water utility.

Cleaning and lining

For structurally sound mains with performance problems, the utility engages in cleaning and lining. The biggest problem affecting pipe performance is tuberculation in pre-1950 cast iron pipe with a relatively rough interior surface that encourages microbiological growth. As the tuberculation increases, it gradually chokes off water pressure, reduces fire flows and vexes customers by tinging the water red.

“We use lining technology extensively here,” says Hannam. “We dig holes at 200-foot intervals and use a rack-and-bore system that drags a line full of prongs through the system that knocks nodules off the inside of the pipe and scrapes it clean. That’s followed by a high-pressure jet-spray cleaning.”

Once the pipe is clean, the utility resurfaces the interiors with air-cured, spray-on epoxy. Halifax Water lined about 15,000 feet of water main in 2004 using COPON Hycote 162 PWX epoxy liner from COPON Pipelinings, E. Wood Ltd. In 2006, about 9,500 feet of main were lined using Huntington Waterline Epoxy Product #8001.

“Using these systems, we can line about 500 feet of main at a time,” says Hannam. “We’re also using CARUS 3250 water treatment chemical, a phosphate dry blend, on the water to discourage microbiological tuberculation and eliminate the redness from the water. It’s not as much about water quality as it is an aesthetic treatment for customers who don’t like to see red water when they’re doing the laundry.”

Protecting assets

For metal pipe that is in good condition from both a structural and performance perspective, the utility uses cathodic protection to extend service life. More popular in Canada than in the United States at present, cathodic protection spares water mains from exterior corrosion.

The soils in the area are moderately corrosive, and some lines have shown signs of exterior damage. The program involves welding a sacrificial anode, usually a 30-pound magnesium cylinder, to contacts on the main.

“We auger down and dig a hole about 6 inches in diameter to about a foot away from the main, and use a vacuum truck to remove the remaining soil,” says Hannam. “The leads of the anode are arc-welded into place, and then the anode is lowered onto a bed of sand or limestone and buried.”

Each length of main receives two anodes, but since cathodic protection relies on a continuous electrical connection between sections, the utility needs to take note of previous repairs that use neoprene gaskets and break the connection.

“We began our three-pronged approach of replacement, rehabilitation and protection about 10 years ago, as a response to the needs of the system and of our customers,” says Hannam.

“That became more formal about five years ago as we realized that these three activities were effectively addressing the major needs of the system. Halifax Regional Municipality continues to grow, and our assertive program is ensuring that the water system is in good shape to grow with it.”



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