TRAVERSE CITY — With more than 3 miles of city sewer line below Lake Michigan and groundwater levels, leaky sewer mains could be letting millions of gallons of groundwater gush into Traverse City’s sanitary sewers.
But thousands of service lines could be to blame, too, and city Municipal Utilities Director Art Krueger recently told city commissioners it’s probably a mixture of both.
City Commissioner Brian McGillivary said he saw firsthand how the tiny lines could be letting in groundwater, and described an inspection of an old cast iron pipe connecting a house and sewer main. That material, he said, “rots from the top,” and the level of rot was clearly letting in groundwater.
“Shortly after the camera leaves the building, it gets a little bit lower out into, and suddenly it’s in standing clear water,” he said. “It’s like the camera’s going through a series of little waterfalls, some are sprinkles and some are torrents.”
Failing service lines are another factor to consider as the city looks for solutions to keep groundwater and rain runoff out of city sewer lines. City Municipal Utilities Director Art Krueger agreed the smaller lateral lines are probably a big source of excess flow in the system at a recent meeting where Commissioner Tim Werner presented estimates of how much excess there is — and what it costs to treat it.
Werner used hourly flow rates from the city’s last sewage pumping station before the plant from 2013 to October 2020 and compared the data with Lake Michigan water levels. He used a statistical analysis method called linear regression to show how lake levels above 579.2 feet above the International Great Lakes Datum — analogous to sea level — correlate with sewage flows above a 2.3-million-gallon baseline.
Lake levels haven’t been that low since March 2017, as previously reported.
He then estimated the total excess flow to October 2020, and used some more integral calculus to do the same for sewage flows tied to massive rains that hit that year.
The resulting estimate was 541 million gallons from January to October 2020, 457 million of which was from groundwater. It cost about $2.8 million to treat the total amount of excess flow.
The numbers likely weren’t perfect, Werner said — the cost estimates, for example, were based on the average cost to treat 1 million gallons of water, but economies of scale would mean the price of treating excess flow on top of the first million would drop.
Yet even estimates that aren’t perfect illustrate the scale of the problem, and keep the focus on a crucial subject, Werner said.
“And I hope that’s the takeaway tonight, that we’re spending as a community quite a bit of money, so could we be better off directing that money to fix that collection system so going forward we’re not spending so much on groundwater and rain water at our wastewater treatment plant,” he said.
Werner and other commissioners wanted to know what the city could do to address the problem, and city Manager Marty Colburn said plans are in the works to add major projects to the city’s six-year Capital Improvement Program, and to find funding sources.
Krueger said data from a Stormwater, Asset Management and Wastewater Grant that paid to run cameras through half of the city’s sanitary mains suggested infiltration and inflow wasn’t a major problem. But that data was from 2015 when lake levels were nearly four feet lower.
Lake Michigan levels dropped 18 inches from record highs and forecasts call for a lower summer peak than in 2020, but more unprecedented water levels are possible in the future, he said.
City crews are using four portable flow meters to watch how storms impact flow, and the city already knows where some of the worst submerged sections of sewer main are, Krueger said. That includes along Bay Street and Grandview Parkway and in parts of Slabtown. It’s also likely the service lines along a stretch near the Boardman River could be letting in groundwater.
Commissioner Christie Minervini asked if the city could incentivize people to replace their failing service lines — Krueger said it’s already city policy to replace its portion from the water main to the edge of the city right-of-way, but the privately owned portion they’re connecting to stays in place.
Werner said he wants to know what kind of legal options city has if a service line is known to be failing — Mayor Jim Carruthers noted the city in past decades replaced sidewalks and put adjacent homeowners’ portions of the costs on their tax bills whether they wanted to pay it or not.
The city could coordinate around road reconstruction projects so sewer customers could replace their portion when much of the digging already is done and the cost could be considerably lower, McGillivary said.
He also suggested a slew of options the city could consider to nudge property owners toward fixing old, failing sewer service lines, from seeking grants to waiving permitting fees for working in city rights-of-way, to paying to replace any sidewalks dug up as a result.
“It’s not just what’s flowing into these lines, it’s what’s flowing out,” he said.
