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Environmental Information for Sweeping Professionals


This information first appeared in Vol. 6 No. 4 of Runoff Report,
a bi-monthly publication of The Terrene Institute, 4 Herbert Street, Alexandria, VA 22305. (Publication date was July/August 1998)

A Clean Sweep Now Possible

Clean streets mean clean water, right? Not always. Conventional street sweepers pick up trash, but not the tiniest pieces of dirt that contain most pollutants. So, left unprotected by larger materials, the contaminated fine particles are easily swept away by runoff.

Thus, EPA's NURP (National Urban Runoff Program) study in the '80s concluded that street sweeping did not prevent nonpoint pollution. "In Chicago in 1969, street dirt was identified as the primary source of nonpoint contamination," recalls Roger Sutherland, a consultant with Kurahashi Associates of Portland, Ore., who participated in the NURP study. "That was 30 years ago -- and nothing's changed!"

But, streets continue to be swept because people want clean streets, and today, virtually all cities use street sweepers as BMPs. The difference 30 years hence? Many of today's sweepers can now significantly reduce the amount of street dirt entering streams and rivers, one type by up to 80% annually. Cities use one or more of five basic kinds of sweepers:

* Mechanical: 90% of the street sweepers used in this country are traditional broom sweepers. A number of improvements have made these machines more effective than their NURP-era predecessors.

* Vacuum-assisted wet: These machines create a vacuum at the surface but use water to suppress dust from its gutter broom.

* Regenerative air: to capture sediments, this sweeper blows air onto the pavement and immediately vacuums it back.

* Tandem: a two-machine operation: a first pass by a mechanical sweeper, followed by a vacuum-assisted sweeper.

* Vacuum-assisted dry: these sweepers combine the essential elements of tandem sweeping into a single unit without using water. A sixth type, scrubbers, has more limited application. Scrubbers saturate the pavement with enough water to suspend the fine particles, then vacuum up the solution.

"Scrubbers are great when we in public works want a street very clean for aesthetic purposes," points out Gary Shawley, water resources engineer for Alameda County, Calif. "For example, intersections with granite or marble sidewalks -- but that's only about 1% of the thousands of curb miles in our cities." Two cities in Shawley's county, Oakland and Berkeley, are considering the purchase of Tennant scrubbers.

High-efficiency Key

Although any street sweeper's ability to reduce nonpoint pollution depends on many factors -- the condition of the street, its geographical location, the weather, the operator's skill, the presence of parked cars -- vacuum-assisted dry sweeping appears to be the most effective.

"We're talking high efficiency sweeping," emphasizes Sutherland, who is currently evaluating the new breed of street sweepers. "The ability to get those tiny particles [that may be loaded with cadmium, lead, phosphorus, mercury or zinc] --and thus, to dramatically reduce pollutant loads." Sutherland's studies show that conventional mechanical broom and vacuum-assisted wet sweepers reduce nonpoint pollution by 5 to 30%; its nutrient content by 0 to 15%. Dry vacuum sweepers reduce nonpoint pollution by 35 to 80%; nutrients by 15 to 40%.

Vacuum-assisted Dry

The word "dry" differentiates these sweepers from the conventional sweepers, which spray the pavement with water to facilitate pick-up. But "water only suppresses dust" [turns it into mud] -- and that lesson learned by EnviroWhirl developer Ken Wilkerson in picking up coal dust laid the foundation for dry sweepers' effectiveness as a nonpoint pollution BMP. As Shawley puts it: "Dry sweeping is the best street sweeping technology." Schwarze Industries' EV (formerly EnviroWhirl) and Tennant dominate the dry sweeper field.

EV is, in the words of one of its first users, Lewiston, Idaho's equipment services manager Jim Collins, "a clean air machine. Even when it's operating in a 10-mile-an-hour crosswind, it doesn't leave a particle of dust -- just puts out filtered air." A patented filtration system uses compressed air to sequentially purge the filters (8 on one unit, 12 on another) at a rate of three-plus a minute, "so there's no way for dust to get back in the air," says EV inventor Don Thomack, a physicist and engineer. EV2, which began life as a railroad track cleaner, is designed principally for dry use, so "wet stuff can plug the intake tube," Collins sees as its only limitation. "Right now, we're trying to figure how to put a vibrator on the tube to keep it from clogging." But EV can perform well in wet situations: When a deluge recently flooded the new NASCAR track at St. Louis, "EV picked up the standing water and within 15 minutes the track was dry," Thomack points out.

Tennant, one of the world's largest sweeper manufacturers, began making sweepers for industrial use more than a half century ago. "Dry-dust control is very important for industrial use -- and our sweepers are exceptionally good at this," says Mark Kinter, principal engineer with Tennant and, like Sutherland, a 30-year veteran of the sweeping business. Tennant sweepers also perform well in wet situations; airports use them to pick up de-icing fluids.

Kinter emphasizes the four-wheel steering and 60-inch turning radius that give Tennant the maneuverability to sweep around obstacles (such as parked cars) and corners: "This sweeper can get to where the dirt is." And it also uses a vacuum wand to reach inaccessible areas. As Shawley reminds: "90% of the materials to be picked up on the street are 18 inches from the curb." Tennant 830-II sweepers are designed with two independent systems: a broom system that puts trash on a conveyer system into a hopper, and a vacuum system that sucks up dust and dirt stirred up by the brushes into a polyester filter that removes dust and exhausts clean air.

It's All in the Fines

One dry vacuum-assisted sweeper picks up over 99.6% of 10-micron (PM-10) particles; EV2 can capture minute sediments as small as 2.5 microns (1/28th the width of human hair).

This past June, Los Angeles' Bureau of Street Maintenance tested EV's ability to pick up small particles: EV picked up 99.99%. "EV2 doesn't pick up the heavy material as well -- like the wiper blades and other unusual debris you find on city streets," observes Stu Finley, operations director for Lake Barcroft (VA) Watershed Improvement District, which tested EV this summer. Collins agrees "it won't pick up a 2x4." He uses two broom sweepers "as front-loaders for heavy stuff like rocks," and relies on the dry sweeper to pick up normal debris and the tiny dust particles. EV also picks up cinders and sand spread on icy streets, recycling them for future use.

Comparative Tests

EV, Tennant and TYMCO participated in a recent test on rough, dusty concrete near New Johnsonville, Tenn. "The dust was an inch thick," says the contractor running the test (he asked not to be identified). "We found that the TYMCO didn't work well in a real dusty area." Both the EV and the Tennant picked up well, but "the EV did a better job," the contractor found, largely because "its filter system cleaned continuously; the Tennant operator had to stop every 300 to 400 feet to clean the filter." The filters seem to be a major difference between the two machines. EV's filters continuously purge and "you can wash them with a hose," says the Tennessee contractor, who believes the Tennant 830-II "has a pretty primitive filter system." In a test by Lake Barcroft WID (see the last issue of Runoff Report) during July, the Virginia highway department operated their conven- tional broom sweeper (a Mobil) along one side of a 2.65-mile street; the EV2 cleaned the opposite side. The machines made two passes: in the first, the EV2 picked up 2,700 pounds; the Mobil, 2,160 (dry weight, water used in the cleaning process removed). After a drying period, the machines made a second sweep, switching sides of the street. On that pass, the EV2 picked up 1,080 pounds; the Mobil, 210 (dry weight). A lab is measuring sweep samples from the two machines and earlier Barcroft sweeping samples to determine pollution levels for 25 parameters, including heavy metals and phosphorus, a major problem for this Chesapeake Bay watershed.

A Perfect Machine?

Location, use -- these are only two of the factors that determine the type of machine municipalities and industries buy. Collins says EnviroWhirl (EV) was an excellent buy for his city of 30,000. "We used to run a water truck to wet the debris, followed by a broom sweeper to pick up all the heavy material and break matted material loose," Collins recalls. "Then, we ran a regenerative air sweeper behind the broom to clean up the trail left by the broom -- usually accompanied by a dump truck to haul away the material. "The EnviroWhirl made this a one-man operation, allowing us to devote more resources to other tasks."

Since Lewiston bought the machine in 1994, the city's 13-member crew has adapted it to Lewiston's hilly, often rough streets, most with no curbs and gutters, and "The company's incorporated some of our changes in its new models." Citing the fact that the city uses broom sweepers for heavy debris and wet conditions - - but only the EV2 in freezing weather -- Collins concludes "The perfect machine just hasn't been made yet."

Illinois contractor Vern Hines points to the 100,000 to 300,000 gallons of water a conventional sweeper uses ("none recycle") each year; and Kinter believes scrubbers may be "the state of the art." But those who have worked with street sweepers for three decades -- Shawley, Kinter and Sutherland -- agree with Hines that "all these sweepers have their niches in this world."

Cost/Benefits

Many factors weigh in a city's purchasing decision. EV sweepers cost $180,000; Tennant's Power 830-II, between $75,0000 and $100,000. The average conventional sweeper costs $60,000 to $120,000. Cost is the "biggest drawback," believes Don Waye, water resource engineer with the Northern Virginia District Planning Commission. "Obviously, whether it's cheaper to sweep or to dredge is a phony issue," observes Finley, whose urban watershed improvement district spent $222,000 last year -- over $2 million since 1960 -- to dredge road sand and toxic sediments from the lake. "The meaningful issue is water quality." For water and air quality alone, Sutherland believes "the benefits of the dry vacuum- assisted sweepers' performance far outweigh the costs." As the Tennessee contractor who tested the three sweepers puts it: "If there's a big difference in perfor-mance, it's worth the money."

The Next Step

"Let's establish a performance test for street sweeping equipment," advocates Sutherland in presentations to EPA and to the industry itself. "A Consumer's Guide," Sutherland calls this approach that would publish a report on how each sweeper performs each task.

Reinforcing the need for such a Guide, Shawley worries that numbers used to prove performance "can be very misleading."

As Kinter points out, "A sweeper's performance varies widely; you can get very different results from sweeping two different surfaces 100 yards apart." Kinter has devised his own "sand+paint additive" simulator for testing sweepers: "That way, they're expected to clean up the exact same composition." Answers may be en route from the Society of Automotive Engineers, where a committee (of which Kinder is a member) is writing test methodology for measuring sweeper performance, particularly as it relates to air quality.

For additional information, email Roger Sutherland or call him at 503-671-9709, ext. 24.

Gary Shawley, 951 Turner Court, Hayward, CA 94544; 510/670-5582.

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