Pollutant Removal Credits in Minnesota: The Sweeping Details
Mike Trojan, Hydrologist Emeritus from his 30+-year career with the Minnesota Pollution Control Agency, and Ranger Kidwell-Ross, Editor of WorldSweeper.com, discuss the effectiveness – and value derived – of street sweeping in removing pollutants, particularly in Minnesota.
Includes information about street sweeping-generated credits for municipalities in Minnesota. This Zoomcast interview was held in December 2024; interview was held when Trojan was at the Denver Airport traveling back home from the 2024 conference of the American Society for Testing and Materials (ASTM).
Mike Trojan's more than three decade career has largely been as an investigator of the best ways to remove stormwater runoff pollution. In a related @45-minute Zoomcast held in 2021, Mike discussed more details of how a comprehensive study by his workplace at the time (he's now retired), the Minnesota Pollution Control Agency, among other participants, has determined that removing phosphorus via street sweeping is orders of magnitude less expensive than any other method.
Mike Trojan: I retired about a year and a half ago from the Minnesota Pollution Control Agency. Before that, I worked with some cities, MS4s, and researchers at the University of Minnesota to conduct research on street sweeping. My role was to translate that research into credits for MS4s.
Our credit system is based on mass removed. You collect the material, weigh it, and measure the water content. We have established relationships between amount of phosphorus and nitrogen in the collected material. The credit varies depending on the time of year. We have two seasons in Minnesota: non-leaf drop and leaf drop. During leaf drop, nutrient concentrations are significantly higher in the collected material.
We initiated this crediting system because we believed that street sweeping could be very effective if done in the right place, at the right time, with the right equipment. Previous literature suggested that street sweeping wasn't very effective, but in my opinion, those studies weren't conducted properly.
Based on our data, if you're targeting nutrients (though street sweeping can accomplish more than just that), you can remove 60-70% of your annual phosphorus load through street sweeping in Minnesota when done correctly.
Ranger Kidwell-Ross: Did Minnesota's studies come up with better numbers than some of the previous ones?
Mike Trojan: Yes, much better. For example, the Chesapeake Bay, one of the first places to give credit for street sweeping, estimated around 5-10% annual load reduction. In Minnesota, if done correctly, we can achieve better than 50% annual load reduction for nutrients, and probably for solids too. We were primarily focused on nutrients rather than solids.
Bill Selbig's work in Wisconsin shows that the nutrient load peaks are in spring and fall. And of course sometimes people implement street sweeping not just because of regulatory requirements, but because they want to do the right thing and protect local water bodies. We've seen some of that in Minnesota.
Ranger Kidwell-Ross: That's commendable. It's important to inform them about best practices and enhanced sweeping techniques. Recent studies, like one from the City of Los Angeles, have shown a significant difference in pickup capability between broom sweepers (35%) and regenerative air sweepers (75%). This is a substantial difference that needs to be considered in future tests.
Mike Trojan: The issue of sweeping is complex and involves multiple factors:
- Technology used
- Disposal of collected material
- Sweeping techniques
- Timing of sweeping
There's an opportunity for those interested in different aspects of street sweeping to spread this message. People doing the sweeping need to know about new technologies, credits (if representing MS4s), and how to handle collected material.
Ranger Kidwell-Ross: There are challenges, particularly during heavy leaf fall seasons. Broom sweepers are necessary for thick, matted leaves, but they should receive fewer credits. Studies have shown that following a broom sweeper with an air sweeper can pick up as much as 500 pounds per curb mile.
Mike Trojan: In Minnesota, we've developed guidance that addresses this issue. During leaf fall, we recommend using a broom sweeper to collect the bulk of the leaves, followed by a regenerative air sweeper to pick up the remaining fine debris.
Ranger Kidwell-Ross: That's important because the smaller debris tends to contain more pollutants due to physics. Another hidden issue is the use of water for dust suppression. I recently estimated that sweepers pick up about 3 million gallons of perfectly good drinking water daily, which then becomes polluted and is disposed of.
My mantra for effective street sweeping is this: "in the right place, at the right time, with the right equipment." Target areas that actually need cleaning, rather than sweeping already clean streets. – Mike Trojan
The use of water in street sweeping presents a conundrum. While water is necessary for dust suppression and safety, a study has shown that dry sweeping could increase small micron pickup by about 20%. However, this approach might create dangerous dust clouds, negatively impacting air quality.
Mike Trojan: The importance of air quality concerns varies by location. In Minnesota, for example, air quality is not as significant an issue as it is in Southern California.
I am now working with the (National Municipal Stormwater Association) and its STEPP program. That is the National Center for Stormwater Testing and Evaluation for Products and Practices, to develop verified credits for street sweeping practices. While still in early stages, the goal is to establish credits that can be used to meet environmental requirements. This work aims to provide more accurate assessments of pollution removal effectiveness, as previous estimates for practices like retention ponds were found to be overly optimistic.
My mantra for effective street sweeping is this: "in the right place, at the right time, with the right equipment." Target areas that actually need cleaning, rather than sweeping already clean streets.
Future of Street Sweeping
Both Trojan and Kidwell-Ross agree that street sweeping offers a low-cost opportunity to address pollutant issues. Trojan expresses optimism about the future of street sweeping, noting that there's "a lot happening" in the field and that the work is "real promising."
We invite you to watch the 22-minute Zoomcast between Mike Trojan and Ranger Kidwell-Ross by clicking in the image below. Will open in YouTube in another tab:
If case you want to forward the URL or watch on another machine, the YouTube location is: https://youtu.be/qozIc9TwHAo.
Mike Trojan provided this link to Minnesota's street sweeping manual page, which may contain valuable information for those interested in the topic. Mike can be contacted via email sent to: mtrojan11@gmail.com.
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