Nutrient pollution can create dead zones — areas in water with little or no oxygen — where aquatic life cannot survive. Also known as hypoxia, these areas are caused by algal blooms that consume oxygen as it dies and decomposes.
According to the EPA, “Over 166 dead zones have been documented nationwide, affecting water bodies like the Chesapeake Bay and the Gulf of Mexico. The Gulf of Mexico dead zone is the largest in the United States, measured to be 5,840 square miles in 2013.” 1
Two of the largest contributors to hypoxia occur when total nitrogen (TN) and total phosphorus (TP) are allowed to enter waterways. Much of the TN and TP entering water bodies stems from impervious surfaces, primarily from pavement-based stormwater runoff.
As some readers of Pavement magazine may be aware, a variety of BMPs are currently being used in an attempt to mitigate such runoff. With the fiscal challenges posed by the pandemic, many U.S. communities find their organizations strapped for financial resources. That makes it more important than ever to operate all such programs as cost-effectively as possible.
Although street sweeping is listed as one of the available Best Management Practices (BMPs) for stormwater mitigation, due to lack of information on the relative costeffectiveness of current sweepers to remove TN and TP — as well as sweeping’s cost-effectiveness in removing particulate matter (PM) in general — many, if not most, professionals tasked with stormwater runoff pollution removal do not currently utilize sweepers at the frequency indicated by now-available data.
Historically, when budgets are tight many municipalities have reduced their street sweeping programs or put them on the chopping block. However, in communities that have a commitment to cleaning up their urban waterways in the most cost-effective way possible, the latest studies show that street sweeping frequencies should be increased, especially in communities that are currently sweeping less frequently than a minimum of monthly.
Thanks to the University of Florida, in conjunction with the Florida Stormwater Association and 14 MS4s (Municipal Separate Storm Sewer Systems) throughout Florida, we now have statistically determinant proof from that state that an effective street sweeping program provides the most cost-effective first line of defense against stormwater pollution. Over a 12-year study period from 2007 to 2019, sweeping was shown to be by far the singular BMP a community can implement to obtain the lowest cost per pound removal of three key materials: TP, TN and PM.
The study, estimated to have cost well over two million dollars to conduct, showed street sweeping to be, compared to the next cheapest practice, an average of 5.4 times less expensive for TN removal; 5.6 times less expensive for TP removal; and 6.4 times less expensive for PM removal. For all three pollutants the next cheapest practice was the periodic cleaning of sediment-trapping catch basins. 2
The exhaustive study found, as an example, the median cost of removing a pound of phosphorus via street sweeping was $257 — compared to a removal cost of $1,656/lb via catch basin cleaning, $7,450 for a baffled hydrodynamic separator, $9,210 for a screened hydrodynamic separator and $32,600 for wet basin sedimentation followed by granular media filtration. That means street sweeping was up to 127 times more costeffective than the other various end-of-the-pipe treatment practices currently being employed.
The study results are statistically defensible at a 95% confidence level (CL) when combining all 14 MS4s for PM, TN and TP. The study showed conclusively the vital importance of street particulate recovery — and that the more PM a MS4 recovers the more the pollutants are reduced in stormwater.
A 2021 presentation in conjunction with Minnesota Cities Stormwater Coalition (MCSC) and the National Municipal Stormwater Alliance (NMSA) also documented an astonishing cost-effectiveness for phosphorus removal via street sweeping: Minnesota study results found that at certain times of the year sweeping removes P for under $100/lb total cost! Randy Neprash, on staff at MCSC and vice-chair of NMSA, says that “makes street sweeping one of the most cost-effective, as well as just plain effective, stormwater BMPs available.” 3 He also emphasizes that cities with significant tree canopy cover, or ones that are increasing tree cover, should enhance their street sweeping programs.
As a direct result of the data collected, the State of Minnesota is enacting a system of credits — based directly on the amount of debris collected by street sweepers — back to its Municipal Separate Storm Sewer System (MS4) permittees. Neprash also recommends “MS4 cities around the US should work with their respective states to recognize the work that has been done by the State of Minnesota with the goal of having them enact a similar crediting system. As has long been advocated by the authors of this article, Neprash further suggests that city sweeping programs become part of their stormwater program so as to be eligible to use stormwater utility funds and other funding sources for buying and operating street sweepers.
Especially in the current tough financial times, the relatively costly end-of-the-pipe pollution removal practices are what should be examined for potential cuts, provided cuts are warranted. One caveat when discussing potential cuts to those practices is that we are referring to various publicly-funded retrofit projects that include end-of-the-pipe treatment practices, including low impact development (LID) or green infrastructure (GI) retrofit practices. The data from both Minnesota and Florida show conclusively that, measured on unit cost basis of dollars per pound of pollutant removed, end-of-pipe retrofit practices are orders of magnitude more expensive. That said, projects mandated by regulations and/or funded privately should be continued since their elimination will not result in any financial savings for community budgets.
Additionally, a range of actions are available to increase the amount of PM being captured by street sweeping programs, thus further improving the effectiveness of a street sweeping program’s ability to remove pollutants from stormwater. As examples, typical value-added measures include proper selection of the type of sweeper being used on particular routes (e.g., mechanical broom, regenerative air, vacuum); the forward operating speed of the sweeper; upkeep and repair; and, of course, the frequency of cleaning.
However, to maximize contaminated material captured, modeling studies have repeatedly shown mandatory vehicle removal during sweeping to be by far the most important single practice because it can increase PM and other pollutant pickup by 30- to-60% (depending upon parking density) without a significant increase in program cost. In fact, parking fines collected from violators will greatly offset or even pay for the cost of implementing and operating any such vehicle removal program.
The authors recognize that implementing and maintaining a mandatory car removal program can be a challenging task, one viewed by many in their communities as controversial and, so, politically difficult. When a program of this type is being considered, citizens often raise concerns about being reminded when to move their cars; being notified of last minute changes in the sweeping schedule; when to move their cars back to the curb; and, assurances that they won’t get a ticket if they move their cars back after the sweeper goes by but before the posted no parking timeframe expires.
Perhaps the greatest news on vehicle removal we can share with you is that emergent technologies have new, positive answers to the above. Mobile-based technologies are now available that can effectively address these types of concerns in a resident-friendly manner.
Such technologies can be used initially to identify where, specifically throughout a community, parked vehicles pose the biggest problem for efficient sweeper pickup. Once this information is known, the pollutant removal increase that a vehicle removal program would provide a community can be determined for each street sweeping route or, more specifically, for each street segment where parking significantly interferes with street sweeping effectiveness.
Today’s technology can also, when implemented, remind residents of scheduled sweeping events and any last-minute changes to those plans. Cities large and small have begun offering email notifications based upon their street sweeping schedule. The City of Los Angeles, CA, has developed a ‘Street Sweeping Near Me’ app that currently offers email message sweeping schedule reminders. New Haven, CT, calls its sweeping alert system ‘New Haven Alerts.’ Even relatively tiny Uwchlan Township, PA, offers a ‘Notify Me(reg) app that does the same.
A more widespread app called ‘SpotAngels’ automatically remembers someone’s parking location using the vehicle's Bluetooth. It then sends a notification when it's time to move the car for street cleaning or any other street parking rule.
Emerging technologies may now be developed to allow a resident to see where the sweepers are in real time, as well as where they are going along with an estimated time of arrival (ETA) at their residence. Technology can also be utilized to ensure only those who failed to move their vehicles prior to the sweeper going by are ticketed. Plus, the ticketing can now be done in a more cost-effective way that also ensures that those ticketed have proof their vehicles actually blocked the sweeper from doing its job.
As a result of these two exhaustive pollutant abatement studies in Florida and Minnesota, along with the emergent game-changing technologies, each and every urbanized community in America can make 2022 the year when the effectiveness of their efforts to remove contaminants from their stormwater can take a huge step forward. An effective, well-run street sweeping program — one that includes mandatory vehicle removal during sweeping, where warranted — is clearly the first line of defense in the abatement of pavement-based stormwater pollution. America’s sweeping and stormwater professionals now have proof positive that street sweeping should always be included as the first element when designing a designated treatment train.
1 According to the latest data available; see: https://www.epa.gov/nutrientpollution/effects-environment