Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring
Fact Sheet - Street Sweepers
Streetsweeping is an effective ultra-urban best management practice for reducing total suspended solids and associated pollutant washoff from urban streets. Recent studies have found that streetsweeping programs using equipment based on new technologies can significantly reduce pollutant washoff from urban streets, with potential reductions of up to 80 percent in annual total suspended solids and associated pollutants (Sutherland and Jelen, 1996).
Streetsweeping is well suited to ultra-urban environments where little land is available for installation of structural controls. It should be considered in commercial business districts, industrial sites, and intensely developed areas in close proximity to receiving waters. For highway applications, streetsweeping may be considered for road shoulders, where safety permits, rest stop parking areas, or maintenance yards. The benefits of streetsweeping will be best realized by using the most sophisticated sweepers at a weekly to bimonthly frequency depending on local conditions, with a careful assessment of whether certain rules such as restricted street parking prior to and during sweeping can be enforced. Streetsweeping is not effective in removing oil and grease, and older conventional mechanical sweepers are limited in their ability to remove fine sediment.
Types of Street Sweepers
Mechanical sweepers employ a rotating gutter broom to remove particles from the street gutter area, with a water spray used to control dust. The particles removed are placed in the path of a cylindrical broom that rotates to carry the material onto a conveyor belt and into a storage hopper. This is the most widely used equipment for street cleaning in the United States.
Vacuum-assisted sweepers also use gutter brooms to remove particles from the street. However, the refuse is then placed in the path of a vacuum intake that transports the dirt to the hopper. The transported dirt is usually saturated with water. The overall efficiency of vacuum-assisted cleaners is generally higher than that of mechanical cleaners, especially for particles larger than the dust and dirt range (larger than about 3 mm).
Tandem sweeping operations involve two successive cleaning passes, first by a mechanical (broom and conveyor belt) sweeper, followed immediately by a vacuum-assisted sweeper.
Regenerative air sweepers blow air onto the pavement and immediately vacuum it back to entrain and capture accumulated sediments. Air is regenerated for blowing through a dust separation system. If the accumulated loading is not too great, regenerative air sweepers are generally considered effective for removing fine sediment (Sutherland and Jelen, 1996).
Vacuum-assisted dry sweepers combine the important elements of tandem sweeping into a single unit. These sweepers apply technology originally developed to remove spilled coal and coal dust from railroad tracks. The technology has also been applied to industrial sites where complete removal without leakage of particulate matter is important. The mechanical sweeping component in these sweepers is completely dry. A specialized rotating brush is used to scratch and loosen dirt and dust from impervious surfaces, allowing the vacuum system to recover practically all particulate matter. A continuous filtration system prevents very fine particulate matter from leaving the hopper, which prevents the formation of the dust trails typically seen with conventional mechanical sweepers.
The effectiveness of streetsweeping programs depends on several factors, including:
Type and operation of equipment used: Vacuum-assisted and regenerative air sweepers are generally more efficient than mechanical sweepers at removing finer sediments, which often bind a higher proportion of heavy metals (Table 18). The performance of sweepers can be enhanced by operating them at optimal speeds (6 to 8 mi/h), ensuring that brushes are properly adjusted, and ensuring that appropriate rotation rates and sweeping patterns are used. Tests conducted on the newer vacuum-assisted dry sweepers have shown they have significantly enhanced capabilities to remove sediment compared to conventional sweepers, with projected reductions of up to 79 percent in total suspended solids loadings from urban streets. In addition, these sweepers are extremely effective at removing respirable (PM-10) particulate matter (particles with an aerodynamic diameter less than or equal to 10 microns) compared to conventional sweepers (Table 19) and are designed to help meet National Ambient Air Quality standards.
Table 18. Efficiencies of mechanical (broom) and vacuum-assisted sweepers
||Mechanical sweeper efficiency (%)
||Vacuum-assisted sweeper efficiency (%)
|Source: NVPDC (1992), as cited in Young et al. (1996).
Sweeping frequency and number of passes: To achieve a 30 percent removal of street dirt, the sweeping interval should be less than two times the average interval between storms. To achieve 50 percent removal, sweeping must occur at least at least once between storms. Generally two passes per run should be conducted, which will result in the removal of up to 75 percent of total solids present before sweeping. Certain conditions may warrant increased sweeping frequencies. These include streets with high traffic volumes in industrial areas and streets with high litter or erosion zones. In addition, the sweeping frequency should be increased just before the wet season to remove sediments accumulated during the summer.
Climate: Sweeping appears most effective in areas with distinct wet and dry seasons (CDM et al., 1993).
Factors that limit the overall effectiveness of streetsweeping programs include:
- Presence of parked cars and traffic congestion during sweeping.
- Poor road surface and curb conditions.
- Presence of construction projects nearby.
Considerations for Equipment Selection
The selection of the type of sweeper will depend on specific conditions prevailing at sites targeted for sweeping. In general, mechanical sweepers are more effective at picking up large debris and cleaning wet streets and have lower capital and operating costs. However, mechanical sweepers can create large amounts of airborne dust. Vacuum-assisted and regenerative air sweepers are more effective at removing fine particles and associated heavy metals but tend to be ineffective at cleaning wet streets. They may also be noisier than mechanical sweepers, which can restrict the hours of operation in some areas. It may also be necessary to deploy a mechanical sweeper ahead of vacuum-assisted sweepers to remove large debris.
The somewhat larger capital costs associated with the newer vacuum-assisted dry sweepers may be warranted for areas where worker and public safety from respirable particulate matter is of concern. Vacuum-assisted sweepers are capable of providing close to 100 percent removal of PM-10 particulates and also provide better overall removal of sediment.
Maintenance and Operational Requirements
The overall maintenance requirements for mechanical sweepers are greater than those for vacuum-assisted and regenerative air sweepers since mechanical sweepers contain more moving parts that require periodic replacement. Vacuum-assisted dry sweepers have significantly less down time than water-based sweepers (less than 10 percent of total operating time compared to about 50 percent for water-based sweepers) because they require no water loading. In addition, clean-up and dumping times are shorter.
For an effective streetsweeping program, consideration should be given to the following operational requirements:
- Ensure there are adequately trained sweeper operators and maintenance personnel.
- Provide traffic control officers to enforce parking restrictions.
- Choose sweeping frequencies and cleaning routes to optimize overall sweeping efficiencies.
- Make appropriate arrangements for disposal of collected waste.
- Reduce source loadings through various measures such as public awareness of proper disposal procedures for used oil and yard waste, and enforcement of erosion control and stormwater pollution prevention practices at urban construction sites.
Conventional sweeper costs range from $69,000 to $127,000 (1995 dollars), with the higher end of this range associated with vacuum-assisted and regenerative air sweepers (CDM, 1993). The useful life span of these sweepers is generally four to seven years, and the operating cost associated with these sweepers about $70 per hour (1996 dollars; Finley, 1996). The capital cost of vacuum-assisted dry sweepers is on the order of $170,000 (1996 dollars; Enviro Whirl Technologies, personal communication, 1996) with a projected useful life span of about eight years and operating costs of approximately $35 per hour (Satterfield, 1996 dollars).
Camp Dresser & McKee (CDM), Larry Walker Associates, Uribe and Associates, and Resources Planning Associates. 1993. California Stormwater Best Management Practice Municipal Handbook. BMP: Street Cleaning, pp. 4-64 to 4-66.
Enviro Whirl Technologies, Inc. 1996. Product literature. Enviro Whirl Technologies, Inc., Centralia, IL.
Finley, S. 1996. Sweeping Works. Pavement Maintenance and Reconstruction. October/November, pp. 16-17.
NVPDC. 1992. Northern Virginia BMP Handbook: A Guide to Planning and Designing Best Management Practices in Northern Virginia. Prepared by Northern Virginia Planning District Commission (NVPDC) and Engineers and Surveyors Institute.
Satterfield, C. 1996. Enviro Whirl I PM-10 Efficiency Study Removing Reentrained Road Dust. Prepared for Enviro Whirl Technologies Inc. by Satterfield & Associates, June Lake, CA.
Sutherland, R.C., and S.L. Jelen. 1996. Studies Show Sweeping has Beneficial Impact on Stormwater Quality. APWA Reporter (November):8-23.
Young, G.K., S. Stein, P. Cole, T. Kammer, F. Graziano, and F. Bank. 1996. Evaluation and Management of Highway Runoff Water Quality. FHWA-PD-96-032. Federal Highway Administration, Office of Environment and Planning.