The Canadian city of Moncton, population 70,000, is located in the geographic center of the Maritime Provinces. For this reason, and because it’s historically been a center of shipping and rail, Hub City has a notably strong economy. It has grown fast at times, and its infrastructure has to grow fast too.
“We’re part of a tri-city region, and our wastewater collection systems are struggling to keep up,” says Sewer Systems Department Project Engineer Darryl Bonhower, PE. “Anything we can do to ease that pressure translates into significant current and future savings.”
Sewer Systems is especially concerned about the potential effects of stormwater inflow and infiltration (I&I) on treatment plant capacity, electricity usage, and sewer overflows. When Bonhower and his team analyzed the problem, they found that off-road and rural manholes, in particular, were a problem.
Most are below grade, so inflow is dramatic during storms. Almost 10 feet of snow falls on the city every winter and melts every spring.
There were problems inside the manhole as well.
Some sewers date to 1895, with some systems made of vitrified clay or brick; and roughly half the manholes fall into the “old” category. Newer concrete manholes didn’t completely eliminate the problem. They have numerous joints and concrete-to-iron sealants have limited life expectancy. Water was getting into the collection system between the concrete cover and the iron frame and in other ways.
It all added up to a lot of unwanted wastewater.
“The extraneous flows were really robbing our system of capacity,” he says.
The department produced an internal paper on the subject: Surface Water Inflow through Manhole Lift/Vent Holes into Sanitary Sewer Systems. They found:
- When a cover is submerged, the lift/vent holes first perform as a weir and then as an orifice, allowing extraneous flows to enter into the manhole. As the depth of submergence increases, the inflow into the manhole increases.
- When submerged by 2 inches, a cover with four .98-inch-diameter lift/vent holes may allow 15.8 gallons per minute (gpm) to enter. At 5.9 inches, 26.4 gpm may enter.
Note that this is the inflow only through the lift/holes. Additional inflow occurs between the steel cover and the steel grate, and between the steel grate and the concrete manhole.
Extraneous inflows like these may not seem significant, but they have a grave impact on system performance. A typical 7.9-inch-diameter PVC sanitary sewer running at a minimum grade of 0.5% has a full flow capacity of 475.5 gpm. If 10 covers are leaking, bout one-third to two-thirds of capacity is lost.
Losing that much capacity was a problem, of course, but determining the source of so much I&I was also a tremendous opportunity. If Moncton could find a way to dramatically reduce water leaking in through manholes, construction or expansion of wastewater collection systems could be deferred for years, maybe decades.
Fortunately, surface inflow is easier, and cheaper, to detect and mitigate than subsurface infiltration through pipe joint and crack leaks. Some studies conclude that eliminating I&I at manholes can, over the life of the project, reduce inflow for as little as 1 cent per gallon compared to hundreds of dollars per gallon for sewer replacement or rehabilitation.
With this determination made, operators formulated an asset-management plan based on two technologies:
- Conventionaln epoxy coatings to repair subsurface leaks
- Watertight frames and covers on manholes in areas prone to excessive surface inflow.
Epoxy coatings work particularly well when the manhole is in good shape, structurally, with leaks confined mainly to joints.
“Some of the newer, spray-on epoxy systems have worked really well for us,” Bonhower says. But because epoxy doesn’t address the biggest source of unwanted inflow—surface flow through manhole covers—it couldn’t by itself achieve the significant reductions his team was hoping for.
As a result, the department is also installing more frame-and-cover systems made by Toronto-based Hamilton Kent, a turnkey manufacturer and distributor of resilient-rubber sealing systems and components. After learning of the Lifespan System in 2011, they installed a couple in 2012 to test them out. That was followed by about 15 more in 2013. Going forward, the product will be installed annually as manholes need rehabilitation.
“We’ve put these out to tender this year, and have been receiving bids for $1,500 – $2,000 plus labor,” says Bonhower. Although that’s double what the department paid for self-leveling steel covers, he’s convinced the covers pay for themselves.
“Look what we’re avoiding or deferring—sewer renewal at approximately $400/meter—and all the other costs of extraneous flow,” he says. “When you look at the big picture, this system is extremely cost-effective.”