Street and Municipal Sweeping:
The Effectiveness of Sweepers as an Urban Stormwater BMP in Maryland
Study released in June of 2013 and this article posted at WorldSweeper.com in July of 2013
by Ranger Kidwell-Ross
The goal of this study was to evaluate which urban stormwater practices provide the greatest nutrient and sediment reductions – for the lowest investment – to help localities in the James River watershed more cost-effectively achieve the pollutant load reductions required by the Chesapeake Bay TMDL.
The values presented in this study provide a snapshot of urban BMP cost-effectiveness based on the data available at the time, and are likely to change as new BMPs are approved and/or pollutant removal efficiencies are refined.
As is the case in many other areas of the U.S. and elsewhere, urban stormwater is the fastest growing source of pollution to Viriginia's James River and its runoff basin. If not controlled, runoff pollution threatens to undermine the progress that has been made towards restoring the health of the river.
Moreover, achieving needed pollution reductions from existing developed areas through improved stormwater management has become the most difficult element of the Virginia watershed implementation plan (WIP) to meet the Chesapeake Bay total maximum daily load (TMDL). Local governments have raised significant concerns about being able to meet the pollutant reduction goals for total nitrogen (TN), total phosphorus (TP) and total suspended solids (TSS).
Street sweeping is a BMP component of pollutant removal. However, historically speaking, a relatively low pollutant removal has been documented as occurring via street sweeping. This is perplexing, since the total volume of material being removed is large, now in the 90-percentile range with air sweepers. Nonetheless, the pollutant reduction at the outfall has historically not been in line with what might be expected.
The terminology: "pickup efficiency" is the amount of debris picked up by the street sweeper; i.e., the total load picked up by the sweeper. This is separate from "pollutant removal efficiency," which measures the reduction in pollutants from the outflow of a particular BMP, such as an outfall for street sweeping. The Center for Watershed Protection led a study in the Chespeake Bay watershed to measure the pollutant removal efficiency based on what was found at the outfall. The results of the study were used to develop a BMP efficiency for the Chesapeake Bay Program used in the James River Study.
The pollutant removal efficiency that was ultimately used for street sweeping, which is classified as a non-structural Best Management Practice (BMP), was calculated for total solids (TS), phosphorus (TP) and for total nitrogen (TN). The data were further broken down into removal efficiency for weekly sweeping and for monthly sweeping, as well as via regenerative air and by mechanical broom sweepers. This chart is reproduced below; for contrast, the current removal efficiency allocated to catch basin cleanouts is also shown below that.
In addition to the information in this article, we offer a 20-minute audio interview with Dr. Neely Law, Senior Research Analyst for the Center for Watershed Protection and Editor-in-Chief, Watershed Science Bulletin. In the audio interview, linked at the bottom of this article, Law explains the methodology used to compile the pollutant removal efficiency numbers for sweepers. She also discusses the dilemma faced by assessment personnel, since sweepers remove such a large amount of 'gross debris,' yet the ensuing percentage of pollutant removal, when measured at outfall, is surprisingly low.
The study Law headed was conducted in two of what are termed 'ultra urban watersheds' in the City of Baltimore with 68-77% impervious cover throughout the area. The sweeper routes were changed to allow the study to gather data on sweeping effectiveness. To determine pollutant removal efficiency, the outfall of the catchments of the two areas were outfitted with water quality sampling equipment. Since previous data were available showing the runoff concentrations prior to the sweeping taking place, this provided data designed to indicate the impact of sweeping.
The City of Baltimore used a regenerative air vacuum sweeper for the study. The assessment team also used current literature to factor into the final recommendations. One item the team observed was the difference between the debris pick-up efficiency of street sweepers, which approaches or exceeds 90%, and the pollutant removal efficiency, which is based on reduction at the outfall.
To recap, the former is the total load of debris collected by the sweeper. In contrast, the latter is defined as the pollutant load coming into a BMP and the pollutant load coming out of the BMP. For street sweeping, this means assessing the pollutant runoff load measured prior to sweeping, as compared to the load seen after sweeping occurred. For purposes of the study, what was being calculated was the reduction in pollutant load, not total debris removal.
"We were perplexed there was such a difference between the two," said Law. "The amount of debris removed was very large, compared to the pollutant outfall reduction. For a given set of assumptions and sweeping frequencies," writes Dr. Law, [As shown in the chart] "the range in pollutant removal rates from street sweeping for total solids (TS), total phosphorus (TP) and total nitrogen (TN) are: 9-31%, 3-8% and 3-7%, respectively.
"The lower end represents monthly street sweeping by a mechanical street sweeper, while the upper end characterizes the pollutant removal efficiencies using a regenerative air/vacuum street sweeper at weekly frequencies."
In the resulting CWP study, which provided recommendations about the cost-effectiveness of a variety of BMPs for pollutant removal, a single baseline value was chosen to represent the pollutant removal efficiency of street sweeping. This was an average of the overall results, and so did not take into account broom sweeping efficiency vs. air sweeping efficiency, frequency of sweeping, or other factors.
However, an expert panel is being convened to reevaluate the impact of sweeping, given that the impact of the large amount of gross solids removed by sweepers is not currently being taken into account. For example, these include removal of leaf litter which, if not removed, would potentially provide a significant pollutant load runoff.
Even so, the positive impact of sweeping's removal of this material has not been credited, to date. The panel will try to account for the large debris removal aspect of sweeping, which is not accounted for via catch basin monitoring.
As part of the audio discussion with Dr. Law, she talks about that effort in more detail. Ultimately, this would tend to advance the role of sweepers in the overall master plan of pollutant removal capability.
Two of the 'Programmatic Recommendations' made in Dr. Law's analysis include recommendations that pertain to sweeping:
We have co-located on the WorldSweeper.com website two informational documents by the Center for Watershed Protection in PDF format. Use the links below to access these. For more information, go to the Center for Watershed Protection website.
Click here to download the Final Report: "Cost-Effectiveness Study of Urban Stormwater BMPs in the James River Basin."
Click here to download "Deriving Reliable Pollutant Removal Rates for Municipal Street Sweeping and Storm Drain Cleanout Programs in the Chesapeake Bay Basin."
Click here to download "Research in Support of an Interim Pollutant Removal Rate for Street Sweeping and Storm Drain Cleanout Activities." This is a 2006 literature review of published information about the effectiveness of street sweeping to remove stormwater runoff pollutants.
Click here to access the articles search link at the CWP website.
We invite you to listen to the approximately 20-minute audio interview with Dr. Neely Law. To play the interview, click this link or on the small triangle inside the circle you see to the left. If you have any trouble accessing this audio, please let us know.
You may reach Dr. Law via email sent to: firstname.lastname@example.org.
The Center for Watershed Protection, Inc. is a 501(c)(3) non-profit organization dedicated to fostering responsible land and water management through applied research, direct assistance to communities, award-winning training, and access to a network of experienced professionals.
The Center is your first source for best practices in stormwater and watershed management. The Center was founded in 1992 and is headquartered in Ellicott City, Maryland. As national experts in stormwater and watersheds, our strength lies in translating science into practice and policy, and providing leadership across disciplines and professions. To learn more about the Center’s commitment to protect and restore our streams, rivers, lakes, wetlands and bays, go to www.cwp.org.
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