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The Changing Emphasis of Municipal Sweeping...

....May Be Tandem

...Is Clean Water

by Wendlyn Alter

In the past, appearance has been the primary motivation in urban street sweeping programs. However, that picture may be changing. With budget cuts forced by taxpayer revolts, clean neighborhoods are likely to become a lower priority than essential services such as law enforcement. Municipal sweeping is among the services which may be targeted for cutbacks.

Litter-free streets are not the only benefit of sweeping, however. A new urgency for municipal sweeping is evolving: there is now a mandate to reduce pollutants entering the runoff stream. In 1972 the United States Congress passed the first Clean Water Act. Before then, many rivers and streams were essentially open sewers.

Waste water treatment facilities were built, resulting in dramatic improvements in river and lake water quality. In 1987, Congress reaffirmed our commitment as a nation to water quality. The focus now is on the nonpoint sources of pollutants. Urban storm water runoff is a nonpoint source that is a significant contaminator of our resources.

Research shows that sediment accumulated on paved surfaces is the major source of nonpoint pollutants found in urban runoff. Urban storm water carries sediment-borne pollutants that have accumulated on pavement – streets, parking lots and driveways – after eroding from adjoining plots or wearing from pavement by heavy traffic or studded tires. Oil and greases, heavy metals from car exhaust and tire wear: all may contaminate sediment. In addition, decaying organic litter can release nutrients which deplete the oxygen levels in receiving waters.

To comply with federal regulations concerning water quality, municipal sweeping programs may become more important than ever before. But in order to meet this challenge, the methods of sweeping will have to change, as well.

Municipal managers have been discouraged by studies conducted in the early '80s by the Nationwide Urban Runoff Program (NURP). These studies appeared to show that sweeping did not significantly reduce the concentration of pollutants in urban runoff.

However, Portland's Bureau of Environmental Services has demonstrated just the opposite: sweeping can significantly decrease contaminated sediment in the runoff stream when sweeping methods are tailored to the problem of pollution rather than to just the collection of litter.

....May Be Tandem

The proven best method for getting up fines.

The City of Portland takes the problem of runoff contamination seriously. When heavy rainfall overloads their combined sewers, exceeding the capacity of the system to transport the runoff to the treatment plant, the untreated overflow goes straight into the Willamette River.

During the past four years, the city has sponsored a set of scientific studies. Their goal is to determine exactly how and when contaminants enter the runoff stream, which contaminants are present, and how they can most effectively be removed through various controlled methods, including the city's municipal sweeping program. A specifically designed computer model was used to analyze the experimental data.

Just as in the National Urban Runoff Program's (NURP) studies, Portland engineers have found that standard sweeping practices do little to remove contaminants from runoff sediment (7-8 percent reduction). The reason? Broom sweepers are effective at picking up litter and large dirt particles, but harmful contaminants are concentrated primarily in the fines – the particles less than 63 microns.

Not only are these left behind on the pavement after broom sweeping, but once the heavy covering sediment is gone, fines and their contaminants are even more likely to wash into storm drains during the next rain.

The researchers had suspected this was the reason for NURP's discouraging results. They proposed two variations on standard sweeping operations, tailored specifically to get at those fine particles usually missed. (Standard sweeping practice assigns a single flusher truck to each sweeper, using a light spray to wet dust before sweeping and to erase the sweeper trail afterwards.)

In a second runoff basin, an extra flusher truck was assigned to follow each broom sweeper with a heavy flush to wash the exposed fines into the combined sewers during dry weather. There they would enter the treatment plant under controlled conditions instead of risking an overflow into the river during heavy rain. In a third runoff basin, two sweepers were run in tandem: the light-spray flusher truck and broom sweeper cleaned as usual, but then a vacuum-type sweeper followed behind to draw off the exposed fines.

An industrial vacuum was used to sample pavement both before and after sweeping with each of these methods, and the material collected was analyzed by a laboratory. The results were dramatic. Average pickup efficiencies were 39.8 percent with standard operations, 45.0 percent when following with a heavy flush, and 74.2 percent when broom and vacuum sweepers were operated in tandem.

More importantly, the tandem operation removed both fine and coarse material equally well. The heavy flush alternative removed coarse particles slightly better than fines, while the standard alternative only poorly removed fine material. Chemical analysis of the sampled particles showed that most pollutants did, indeed, concentrate in the fines – notably aluminum, chromium, copper, iron, magnesium, lead and zinc.

The research engineers applied the data from this study, along with detailed information about rainfall and other local factors, into a computer simulation program called the Simplified Particulate Transport Model, (SIMPTM). The model projected that although tandem sweeping is the most productive alternative for residential streets, benefits would level off if sweeping were scheduled more than seven times per year.

For arterials, the model predicts that benefits are maximized by a schedule frequency of fifteen times per year. [See accompanying charts.] But, says Roger Sutherland, P.E. and President of Cascade Water Resources LLC, who conducted the study, "Let's not forget that we met with the street sweeping operators when this program kicked off and we told them, 'The way you operate your equipment is probably more important than anything we can do.'

They were given the right amount of time to cover the miles they had to sweep at 4 to 6 miles per hour, being very careful. The point is that if you don't have the money to sweep the whole town six times a year, then sweep it good three times a year."

These alternatives, especially tandem sweeping, are clearly environmentally effective, but are they cost-effective? A marginal cost analysis was conducted to find out. In 1993, records showed that the standard sweeping practice (one flusher per regular sweeper) cost the city $25.28 per curb mile swept.

Heavy flush sweeping (two flushers per regular sweeper) cost $35.18 per curb mile swept, and tandem sweeping (one flusher and one vacuum-type sweeper per regular sweeper) cost $46.01 per curb mile swept, These costs did not include travel time, which could increase the overall costs by up to 20 percent.

The cost analysis showed that for residential streets, the higher cost of tandem sweeping is more than countered by its greater removal benefit. Observes Sutherland, "The tandem operation can cost-effectively get you almost twice as much removal."

For Portland, the optimal residential street cleaning program of five tandem cleanings per year could reduce annual sediment washoff by about 300,000 pounds at a marginal cost of $1.40 per pound and total cost of $350,000. The other two alternatives had higher total and marginal costs, yet removed less than 200,000 pounds. [See charts on page 30]

Treating contaminated storm water overflow is expensive and often provides very low removal rates. "When you are dealing with urban areas," emphasizes Sutherland, "it is extremely difficult to retrofit them with effective treatment facilities. Removing 30% of metals, or even 20%, through sweeping would be worth the investment.

If street sweeping operations could be maximized more towards the objective of water quality and less toward aesthetics, then I think there would be multiple benefits achieved. Programs not only should continue but should be strengthened. Instead, what I believe has occurred throughout the country is that most of these programs have been reduced or even eliminated."

To achieve compliance with the Clean Water Act, municipalities must begin to realize that sweeping operations are more important than ever. But as the urgency shifts from appearance to pollution control issues, standard sweeping practices may need to be reevaluated – and a whole new approach to street sweeping may result.


Combined Sewer Overflow SFO Compliance: Interim Control Measures Study, City of Portland, Bureau of Environmental Services. Final Report, May 1993.

Characterization of Portland's Storm Water Quality Using SIMPTM, Roger C. Sutherland and Seth L. Jelen, the American Water Resources Association's National Symposium on Water Quality, Chicago, Illinois, November 6-10, 1994.

Thanks to Roger Sutherland, P.E. and President of Cascade Water Resources, for the information in this article. Sutherland is a Water Resources Engineer with over 45 years of consulting experience solving the problems associated with urban storm water, in both its quality and quantity aspects. He may be reached at 503-704-0522 or via email sent to

This article is reprinted from American Sweeper magazine, v4 n1.

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