After decades of spring street-cleaning with mechanical sweepers that remove pebbles and sand, metro-area cities are slowly acquiring "air" sweepers that pick up fine dust, too.

Research shows that metals, phosphorous and other pollutants ride with finer particles into lakes, ponds and streams when it rains.

And because cleaner streets mean cleaner water, cities such as Plymouth, Eden Prairie, Eagan, Brooklyn Park, Minneapolis, White Bear Lake and others are spending extra money to buy or contract for the use of cleaners -- called "regenerative air" sweepers -- that rely on streams of air to pick up the fine dust particles other models leave behind.

"Anybody using the regenerative air has the top of the line, the elite in sweepers," said Jim Eiler, assistant maintenance superintendent for Bloomington. Because the air sweepers sell for close to $200,000 compared with $140,000 for traditional sweepers, Bloomington will consider one when a mechanical sweeper wears out, he said.

The Minnesota Pollution Control Agency (MPCA) encourages all styles of street sweeping because anything that falls on the street can wind up in a lake, said Keith Cherryholmes, senior engineer in the MPCA's municipal storm water program.

The MPCA recognizes that the air sweeper achieves superior results in removing fine particles, "but we can't really dictate that people go to the deluxe model," he said.

Cities are striving for cleaner streets because the federal Clean Water Act has made them the guardians of local surface water, Cherryholmes said.

Managing storm water

A group of 234 cities, most in the Twin Cities metro area, have specific orders from the MPCA to "manage their storm water from A to Z" to reduce pollution washed into surface waters, he said. Cities also are using rain gardens, drainage basins, permeable pavements, manhole catch basins and shoreline plantings to clean storm water.

Air sweeping is a "best practice" to which more cities are aspiring, especially after its endorsement as a water protection tool by the Local Road Research Board last year, said Tom Colbert, public works director for Eagan, who serves on the board. The board, created in 1959, is made up of four county engineers and two city engineers with support from the Minnesota Department of Transportation.

In a report aimed at helping cities make wiser sweeper purchases, the board described how air sweepers use gutter brooms to move debris from the curb to the sweeper head; meanwhile, a tube blows air into the sweeper head and onto the pavement to dislodge material so a suction hose can vacuum it up and dump it into a hopper. The air is then recycled through the system.

For best water quality results, the air sweepers should follow a broom sweeper to pick up debris of all sizes, Colbert said. Using an air sweeper "in tandem with the old mechanical sweepers, we are really doing a one-two punch to knock out the pollution loading to our wetlands," Colbert said.

But the air sweepers "can also be efficiently used as a regular street maintenance sweeper," he said. Eagan has owned one for five years.

Eden Prairie bought its air sweeper in 2007 and found the benefits so clearcut that it's been in continual use, said Gene Dietz, public works director.

The city of White Bear Lake also cleans daily with its air sweeper from spring to fall to try to keep its signature lake in pristine condition, said Dan Pawlenty, public works superintendent. "We are getting very small particulate matter," he said.

Choosing not to buy an air sweeper of its own, Plymouth has a $131,895 contract with Carefree Services Inc. to sweep the city this spring, summer and fall. Carefree says air sweepers pick up 25 percent more debris than conventional ones.

Tons of fine dust captured

Plymouth measures its results. "We collect it right out of the hopper of the street sweeper and bag it up and send it to the University of Minnesota extension service" for testing, said Derek Asche, water resource manager for Plymouth.

Last year, the city estimated that air sweeping took up 1,500 tons of fine dust. The U's analysis of samples indicated that about 800 pounds of it was phosphorous that otherwise likely would have ended up in Medicine Lake. "Algae will use phosphorous to grow and reproduce and turn the lake entirely green," Asche said.

Because air sweepers have so recently been added to city fleets, "We know it's going to help, but we don't know how much," Cherryholmes said. "A lot of towns are not analyzing the material they pick up, so you really don't know how much nutrients you are keeping out of the water."

Plymouth is analyzing its street sweeping results because the city wants to know the value of the money spent on the practice, Asche said. But the information also is useful for a study of the condition of Medicine Lake, which began in 2000.

About 4,000 pounds of phosphorous a year drained into Medicine Lake from the entire watershed before the sweeping reduced that by 800 pounds, Asche said.

Through continued lake monitoring, Asche hopes to be able to quantify the benefit of air sweeping on water quality in about a year. "It's wait-and-see for now. We don't know how the lake is going to respond."

Laurie Blake • 612-673-1711