Drones will take to the skies in some Minneapolis neighborhoods this fall to hunt for problems hidden in the city's sewers.
The city is experimenting with the technology as part of its annual smoke testing of the sewers, which helps crews spot trouble spots underground. It will be a first for the city's public works department, though engineers across the country are discovering public uses for drones — including inspecting bridges and ditches in Minnesota.
This year's smoke testing will focus largely on areas in Northeast and Southeast clustered around the central Minneapolis riverfront. Residents there recently received letters warning they may see a drone flying above the street taking pictures.
"We're not flying the drones over anyone's homes or over any private property," said Katrina Kessler, the city's director of surface water and sewers.
Smoke testing helps determine if connections remain between the city's sanitary and stormwater sewer systems, which the city has taken pains to separate to stop sewage spills during large rainfalls. Kessler said the system is now more than 99 percent separated, and after a decade, they are nearly three-quarters finished testing the entire city for leftover connections.
"One of the most effective ways that we know to ferret those out is through smoke testing," Kessler said.
Crews pump nontoxic smoke into the sanitary sewers and then watch rooftops of nearby homes to ensure it's coming out of the sanitary exhaust vents. If they don't see it, or if it emerges from a curbside storm sewer, there's likely a problem.
The task is fairly straightforward on a normal residential block, where the vents are easy to spot on rooftops visible from the ground, but becomes trickier in neighborhoods with industrial or large apartment buildings with flat roofs.
"In order to actually see if it's working you have to get up on the roof to see," Kessler said. "And that involves a lot of coordination, and it also can be a safety risk because often those areas are not meant for human habitation."
Enter the drones, which will be operated by contractor Houston Engineering.
Rather than people climbing atop flat-roofed buildings to keep an eye on the vents, the drones will hover above the street and rotate in a full-circle taking pictures. Then crews will examine the photographs to check for smoke.
"We can actually track how long it takes us to do the work and whether or not we're more efficient," Kessler said, adding they will be able to show risky situations crews were able to avoid.
"This could inform other uses going forward," Kessler said.
The drones won't be used on streets with just single-family homes. For residents interested in learning more about the tests, the city is holding a public meeting at 6:30 p.m. Aug. 23 at the Logan Park Recreation Center.
Other uses for drones
Drones have other helpful applications in public works. The Minnesota Department of Transportation (MnDOT) has been testing drones to inspect the state's bridges since 2015. They are significantly cheaper than traditional inspection methods, since drones reduce the need for traffic control and expensive "snooper" vehicles that reach underneath bridges.
Jennifer Wells, state bridge inspection engineer with MnDOT, said the goal is to employ the technology across the state.
"We're using it on a lot more bridges," Wells said. "But we're still gathering data to show more of an average of what the cost and time savings is over a number of different types of bridges."
Houston Engineering received permission from the Federal Aviation Administration to begin using drones last October. The North Dakota-based firm, with an office in Maple Grove, now uses them to create aerial maps and survey ditches for watershed districts.
Surveying a 10-mile ditch for blockages and water quality once took several days of walking and driving through sometimes difficult terrain, said Houston Engineering IT Technician Dave Olson. With a drone, the job can be done in six hours.
"To think we were going to use a drone for a ditch inspection four years ago was a pipe dream," said Olson, who will be the lead drone pilot on the Minneapolis project. "The technology has grown so fast."
Eric Roper • 612-673-1732
Twitter: @StribRoper
