For decades, officials wrestled with how to treat the 40 million gal per day (mgd) of untreated sewage that flowed into Halifax Harbor in Nova Scotia, Canada. As one of the largest deep-water ports on Canada’s east coast, the harbor offers both economic and recreational benefits to the city of Halifax, but pollution problems were starting to overwhelm the ecosystem.
According to Halifax officials, the top problems included:
- Prohibition of shellfish harvesting in the harbor;
- Large areas of contaminated sediment around 40 separate outfalls;
- Poor water quality along the shorelines;
- Widespread bacterial contamination; and
- Poor aesthetics along the Halifax/Dartmouth waterfronts due to particulates, floating trash and odor.
The Halifax Regional Municipality initiated a massive clean-up project in 1997, which included the construction of three wastewater treatment plants, a collection network through three communities, 22 combined sewer overflow chambers and several pump stations.
Environmental & economic benefits
In a 2000 survey of city residents, 71% said they were willing to pay $100 to $150 more in order to improve water quality in the harbor, according to a government-funded study conducted by GPI Atlantic. The report also estimated the economic benefits of a clean-up at roughly $1.6 billion over a 60-year period—including allowing shellfish farmers to return to shorelines for harvesting and reselling. The city also receives economic benefits from increased real estate values, tourism and recreational sports, according to the report, which examined harbor clean-ups across North America, such as the Boston Harbor clean-up in the 1990s.
“In many ways, this is an economic development project as well as an environmental rehabilitation and enhancement project,” Halifax officials wrote in the closing of their planning report.
Collection network
For the massive sewage treatment project, which will serve the cities surrounding Halifax Harbor, contractor Black & McDonald placed an order for 22 Storm Monster overflow screens from JWC Environmental. During heavy rainstorms or overflow events, the 22 Storm Monsters, each weighing more than a ton and some as long as 35 ft, will play a key role in protecting the harbor by screening out municipal and industrial pollutants, discharging them into the downstream sewage flow and preventing them from escaping into the environment.
“The selection of the Storm Monster was a combined effort between ourselves, the general contractor and the design engineers, based on the required performance specification,” said Robert Burns, the project team leader for Black & McDonald.
The Storm Monster is a breakthrough technology providing capture efficiency and high flow rates in the extremely demanding sewage overflow application. Rotating stainless-steel panels with ¼-in. (6-mm) perforated openings capture pollutants and solids and move them to a solid-clad cleaning brush that returns them to the sewage flow where they continue on to the treatment plant for processing. Because the Storm Monster allows flow to enter through the top, bottom and front of the screen, it achieves nearly twice the flow rate of a single entry screen.
Burns, who heads Black & McDonald’s Harbor Solutions Team, pointed to three key Storm Monster features that influenced his decision:
- Perforated plate screens offer better capture efficiency than slotted bar screens;
- Solids are moved downstream without removing, conveying or handling screenings, as a raked bar screen would require; and
- JWC Environmental has a reputation for handling projects from start to finish.
Installation of the Storm Monsters began in 2005 and continues today.
“We are proud to provide the Storm Monsters for the Halifax Harbor Solutions Project,” said Art Melanson, JWC’s Nova Scotia representative and director of Engineered & Environmental Products Co. “The overflow screens are a crucial component of the project. Swimming will once again be possible and fun at Point Pleasant Beach, and it’s great to make a positive impact on the environment and help improve the lifestyle of the people living here.”
The Storm Monster uses a unique “dogleg” shape for the screening loop. The design places the cleaning mechanism above the highest emergency overflow level, allowing large flow fluctuations without risking brush submergence. This helps ensure screen function is maintained during periods of high blinding and large flows. In addition, two returning panels are partially submerged, allowing additional screening capacity to maximize the hydraulic capacity of the unit. The largest screen can process flow up to 365 mgd.
The panels have moving stainless steel plates that form a continuous seal against fixed ultra-high molecular weight plastics side strips located around submerged areas to prevent unscreened material from passing through the screen and into the overflow. This design helps ensure small trash items such as rags, cigarette butts and latex items are unable to escape into the harbor. Many bar screens with horizontal bars allow this material to slip through, lowering their screening effectiveness.
In a two-year study of wastewater screens conducted in the UK, researchers discovered fine screens capture roughly 75% of material, while bar screens capture only 15 and 40% of the material. The study, conducted by the UK Water Industry Research organization, found that band screens, where material touches only one side of the screening loop, did even better—capturing up to 97% of the material.
High-quality expectations
In addition to the sewage collection network, the project also involves the construction of three advanced primary treatment plants with UV disinfection systems installed prior to discharge. Each of the three cities surrounding Halifax Harbor—Dartmouth, Herring Cove and Halifax—will receive a sewage treatment plant, and all are scheduled to start operations in 2008.
Each facility has a unique architectural and landscaping design so it will blend in with the surrounding community. The municipality has set aside $1 million for each site in “community integration” funding to build green spaces and recreational pathways around the sites. Many of the pumping stations also were designed to blend in with the neighborhood—including shutters and clapboard siding consistent with the architectural style in the area.
“The city overall is quite excited about the project,” Burns said, who is also a 25-year resident of Halifax. “The expectations of the water quality have some in the city talking about applying for international sailing events and the return of recreational activities on the water.”
