Pollutants collect on surfaces in between storm events as a result of atmospheric deposition, vehicle emissions, winter road maintenance, construction site debris, trash, road wear and tear, and litter from adjacent lawn maintenance (grass clippings). Sweeping of materials such as sand, salt, leaves and debris from city streets, parking lots and sidewalks prevents them from being washed into storm sewers and surface waters. Pollution Prevention and the MS4 Program 15
Timing, frequency and critical area targeting greatly influence the effectiveness of sweeping. This fact sheet provides an overview of studies assessing the benefits of street and parking lot sweeping and guidance on improving the pollution reduction benefits of sweeping programs applicable to MS4 SWPPPs.
Regular street sweeping reduces the amount of pollutants that get washed into the storm drain and ultimately discharge to lakes, rivers and wetlands. Targeted pollutants include sediment, trash and debris, leaves, organic matter and nutrients; metals and hydrocarbons. The following pollutant removal efficiencies for total solids (TS), total phosphorus (TP) and total nitrogen (TN) are from a conceptual model developed by the Center for Watershed Protection based on research findings from a variety of studies.
The lower removal efficiencies represent monthly street sweeping by a mechanical street sweeper. The upper efficiencies characterize the pollutant removal efficiencies using a regenerative air or vacuum street sweeper at weekly frequencies. Note that the relatively high frequencies of sweeping generate particularly low removal efficiencies, indicating that sweeping, although an effective aesthetic practice, does not necessarily translate into improved water quality. This is a similar finding of Selbig and Bannerman (2007) in their study of street sweeping in Madison, WI. Even so, every pound of trash and debris removed by sweeping is another pound not entering local waterbodies.
Pollutant removal efficiencies from street sweeping for total solids, total phosphorus, and total nitrogen. Source: Deriving Reliable Pollutant Removal Rates for Municipal Street Sweeping and Storm Drain Cleanout Programs in the Chesapeake Bay Basin. Center for Watershed Protection.
Link to this table
|Frequency||Technology||TS %||TP %||TN %|
The Center for Watershed Protection recommends considering the following questions in order to improve the efficiency and effectiveness of your surface sweeping program. Image Courtesy of Emmons & Olivier Resources, Inc.
An ideal surface sweeping program would answer yes to the following questions. Any missing program elements should be further considered to improve the surface sweeping program:
Sweeper technology and operations Water quality protection is dependent on the sweeper’s pick-up efficiency of fine-grained sediment because many pollutants are adsorbed to them. Street sweeping has historically been more effective at removing only large-sized particles providing little pollution prevention but, new technologies are emerging that will remove smaller, fine-grained particles.
Types of sweepers There are three main types of sweepers including mechanical broom, regenerative-air, and vacuum-assist. Mechanical broom sweepers are typically the least expensive but are better suited to pick up large-grained sediment particles and clean wet surfaces. They tend to create dust during operation, potentially increasing atmospheric loading of dust and/or increasing the amount of fine particles on the pavement that could ultimately wash through storm drains to surface waters. Regenerative air and vacuum-assist sweepers are better at removing fine-grained sediment particles, but are less effective on wet surfaces and are more expensive. Using a mechanical sweeper for large particles followed by a regenerative-air cleaner can be effective. No matter the equipment, tandem sweeping (when one sweeper follows another along the same route to pick up missed material) improves removal efficiency. A single sweeper that makes multiple passes on a surface has the same effect.
In early 2008, Minnesota Local Road Research Board’s Research Implementation Committee (LRRB-RIC) completed helpful guides to street sweeping. Specifically, the Resource for Implementing a Street Sweeping Practice includes information sheets that provide guidance for technical staff, policy and decision makers on: best practices overview, types of sweepers, reasons for sweeping and sweeping and roadway function.
Sweeper frequency Part of the LRRB-RIC research identified that Minnesota falls behind other states in terms of street sweeping frequency. Study surveys showed that Minnesota street sweeping frequency falls lower than nationwide averages. A typical Minnesota city sweeps two times annually, in spring and fall, while the national average was 10 times each year. At a minimum, sweeping should occur in early spring (before rainfall) and in the fall after most leaves