Questions & Answers
Can I read a summary of the Street Sweep Pilot Project Report?
Yes, the first part of the Street Sweep Pilot Project Report (pdf) includes an executive summary for those who want an overview of the Street Sweep Pilot Project.
What are the benefits of street sweeping?
Street sweeping, a source control Best Management Practice (BMP), removes pollutants from roadways before they wash off into our receiving waters. This approach has several benefits:
- Generally more cost-effective than trying to remove pollutants once they have entered the drainage system.
- Targets removal of pollutants from our roadways, Seattle Public Utilities’ stormwater management highest priority land use activity.
- Prevents a significant amount of sediment and associated contaminants from being discharged to receiving waters, which not only impacts water/sediment quality but also impairs substrate quality.
- Provides other benefits such as reduced flooding, better aesthetics due to less trash on roadways, and improved air quality due to reductions in street dirt.
Because most existing development and roadways in the City were constructed long before stormwater controls were implemented, runoff typically discharges directly to area receiving waters without treatment. Retrofitting these existing systems to improve stormwater quality is often difficult, and in many cases, retrofitting is not feasible due to physical site constraints (e.g., utility conflicts, grade restrictions, and tidal influence). Consequently, source control or nonstructural measures, such as street sweeping, have become increasingly important for improving the quality of runoff.
Why is transportation Seattle Public Utilities’ stormwater management priority land use activity?
Seattle Public Utilities (SPU) is responsible for managing stormwater runoff (quantity and quality) within the city service area. Street runoff is of particular concern to the City because the public right-of-way makes up approximately 26 percent of Seattle’s total land area and street runoff contains pollutants from cars, lawns, and business. Key sources of pollutants from roadways include:
- Vehicle exhaust emissions are complex and contain a very large number of organic and inorganic compounds. In terms of the road corridor, it is the particulate component of the emissions that are of importance. The particulates from diesel fueled vehicles are important contributors of polycyclic aromatic hydrocarbons (PAHs).
- Friction linings (brake pads) are an important source of a number of inorganic compounds, in particular copper and antimony. Friction linings also contain a number of organic compounds.
- Tires are an important source of zinc to the road corridor. Tires also contain a range of organic compounds. Each tire includes one percent zinc from the vulcanizing process.
- Road surface wear is a source of particulate matter and some organic compounds. Bitumen surfaces contain PAHs in low concentration.
How have changes in street sweeping technology improved street sweeping pollutant removal effectiveness?
The Seattle Department of Transportation (SDOT) is responsible for maintaining City roadways, which includes street sweeping and leaf pickup operations. The sweeping program is primarily conducted for road maintenance and aesthetic purposes, and historically has not been designed or funded to provide stormwater/sediment quality benefits. Sweeping activities generally focus on commercial roadways in downtown Seattle, the Ballard and University mixed-use neighborhoods, and select arterials throughout the City. Sweeping frequency varies from monthly or less on select arterials, to 6 nights per week in the commercial areas.
SDOT has recently upgraded its fleet to include regenerative-air sweepers, which were used in the pilot and are expected to remove significantly more pollutants than the old style broom sweepers.
The first mechanical broom sweeper, an Elgin motor sweeper, was launched in Boise Idaho in 1914.
In general, street sweepers are grouped into three categories, with the newer technologies (vacuum-assisted and generative-air) more effective at removing pollutants from the roadway:
- Mechanical - lifts dirt off the street by a rotating broom and feed it to a hopper by a conveyor system. A water spray is often used to control dust. This is the most common type of sweeper
- Vacuum-assisted - combines a mechanical sweeper (such as a rotating broom) with a high-power vacuum. Some vacuum-assisted sweepers use a water spray to control dust and others operate completely dry with a continuous filtration system.
- Regenerative-air combines a mechanical sweeper to loosen dirt with forced air to dislodge the remaining dirt. A high-power vacuum with a continuous filtration system captures the dirt and recycles the air.
However, in addition to the type of sweeper technology, pollutant removal effectiveness is highly dependent on site specific conditions, such as sweeping efficiency and sediment deposition rates:
- Sweeping efficiency is a function of sweeping frequency and sweeper availability, i.e. how often do you sweep the streets and how many times does the sweeper have to maneuver around parked cars.
- Sediment deposition rates are a function of pollutant build up and wash off, i.e. pollutant source and land use activity and rainfall intensity, rainfall frequency, and street condition.
Typical street sweeper pollutant removal effectiveness ranges
The table below summarizes pollutant removal effectiveness values from the literature for each street sweeping technology.
|Type||Solids removal (%) *||Particles <10um removal (%) *||Street Dirt Yield (lb/curb mile) **|
|Mechanical||14 to 62||-7 to 8||20|
|Vacuum-assisted||51 to 87||40 to 82||63|
|Regenerative-air||43 to 71||31||76|
* Zarriello, P.J., Breault, R.F., and Weiskel, P.K. (2002). Potential Effects of Structural controls and Street Sweeping on Stormwater Loads to the Lower Charles River, Massachusetts. U.S. Department of the Interior, U.S. Geological Survey Water-Resources Investigation Report 02-4220. 2002.
** Selbig, W.R., and Bannerman, R.T.,(2007). Evaluation of street sweeping as a stormwater-quality-management tool in three residential basins in Madison, Wisconsin: U.S. Geological Survey Scientific Investigations Report 2007–5156, 103 p.
What was the driver for the Street Sweep Pilot Project?
The 2005 Comprehensive Drainage Plan (CDP), which reflects emerging citywide perspectives on surface water management for Seattle, recommended the following maintenance strategy:
- Evaluate street sweeping and catch basin cleaning practices to develop the most effective combination for roadway maintenance.
The CDP envisioned that this strategy would be implemented with a pilot project:
- SPU intends to evaluate whether street sweeping practices (e.g., location, frequency, and street sweeping equipment) can be modified to improve stormwater quality and reduce the need for catch basin cleaning. By coordinating and optimizing street sweeping and catch basin practices, SPU hopes to develop a cost-effective SPU program that maximizes the water quality benefit in high priority watersheds. A pilot study would help to define the best combination of street sweeping and catch basin cleaning.
This pilot study is the result of the CDP recommended maintenance strategy. The pilot explored the potential stormwater quality and drainage maintenance benefits of street sweeping, leading to enhanced, drainage-targeted street sweeping by the City in the future.
What were the results of Seattle's Street Sweeping Pilot Study?
The pilot study answered the following:
- Street sweeping will reduce the rate of sediment accumulation in catch basins and thus will reduce the frequency of catch basin cleaning.
- Street sweeping will increase the total amount of sediment removed from a site compared to the amount removed by catch basin cleaning alone.
In addition, the study results indicated:
- Sweeping removed an average of 4,900 pounds of dry sediment per curb mile swept per year, and the highest yield was observed in the industrial Duwamish Diagonal study area:
- West Seattle (3,800 lb/curb mile/year)
- Southeast Seattle (4,700 lb/curb mile/year)
- Duwamish Diagonal (6,400 lb/curb mile/year)
- On an annual basis, street sweeping prevented between four times (in the residential areas) and 10 times (in the industrial area) more sediment from entering the storm drain system than catch basin cleaning alone.
- Contaminant concentrations were generally higher in the Duwamish Diagonal industrial study area than in the two residential study areas, and were generally higher in catch basin sediment than in street dirt and sweeper waste samples.
What recommendations does the Street Sweep Pilot Project Report make about next steps?
Seattle is looking into expanding its street sweeping program to provide greater benefits to receiving water quality.
The program will prioritize areas within the City where stormwater/sediment quality improvements are most needed and where street sweeping has the greatest potential to improve stormwater and sediment quality (e.g., arterials and curbed roads in separated or partially separated areas).
This information can be made available on request to accommodate people with disabilities and those who need language translation assistance. Call Seattle Public Utilities at (206) 684-3000. TDD telephone number is (206) 233-7241.
Para mayor información llame al teléfono 206-684-3000.