A Discussion of Street Sweeping and California's Increasing Requirements for Large and Small Micron Removal

This conversation includes the latest information on the increasing emphasis on removal of small micron (under 5mm) microplastics, as well as on the removal of large material known as 'trash.'

Each of the participants has significant experience in the topic areas; Roger Sutherland and Ranger Kidwell-Ross in the street sweeping arena and Jill Murray with California mandates and efforts to remove both smaller debris particles and larger trash components from the pavement's runoff stream.

Roger Sutherland is a Principal, as well as Senior Water Quality Engineer, with Cascade Water Resources, LLC. The exchanges begins by talking about something new to the street sweeping industry, which is trash Total Maximum Daily Loads (TMDLs), and how that is poised to affect the street sweeping industry in cities with that mandate.

To view the @33-minute Zoomcast hosted on YouTube, click on the graphic below. An overview of the conversation is provided below the video.

In the event you can't view the presentation in this venue, or would prefer to view it at its YouTube location, the YouTube link is: https://youtu.be/iyIvvgpNKgo

The trash amendments apply to what's considered priority land use areas and other hotspots. That's going to be mostly commercial, industrial, and multifamily residential. And then hotspots such as bus stops in places that have been identified as having a lot of trash. There are two main tracks.

Track one is achieved via the installation of full trash capture devices, generally underground. That would be screens put down inside catch basins to screen out the trash before it gets into the storm drain network. It would be larger devices that are called, for example, continuous deflection systems (CDS), for larger storm drain inputs and removing trash into a large vault underground, that will eventually get pumped out. And another method is putting nets on the ends of storm drains or even in a river system. Together, these are all called 'full trash capture devices.'

Track two is using a combination that may include some of the full trash capture devices. However, it can also include institutional controls such as street sweeping and trash pickups, as well as beach cleanups and creek cleanups, product bans on items like straws, disposable cutlery, plastic bags, and other things like that, along with outreach and education. Where this affects the street sweeping world is that communities that opt for track two are going to be using street sweeping to reduce the amount of trash that they are discharging to the storm drain.

The Phase One Stormwater Permit communities, or the larger communities that have gone before mine, have mostly planned to use track one. And I think that's been based on strong encouragement by the State Water Board, but I would want to hear from a Phase One stormwater manager before being quoted on that.

In any event, the Phase II general stormwater permit is about to be renewed and the trash amendments are going to be put into this newest permit for smaller communities such as mine in Santa Barbara. Phase II communities are those with less than 100,000 residents. We are going to have to achieve compliance quickly. I think that more of the smaller communities will use track two, the institutional controls; at least I'm hopeful that they will. Here's why: when you go track one and you put filters and such down in the storm drain or build something underground to remove the trash, you still see just as much trash on the surface. We're all going to look at just as much trash on our streets, on our sidewalks and on our roadways. I think we all want to live in a world that looks better than that.

Whereas, if we pick track two, and we're doing street sweeping and trash cleanups, we're going to live in a cleaner and cleaner world. I'm hopeful that eventually we'll change the culture toward less trash from littering. We're going to expect to see less and less trash in our environment. Thus, quantifying how much trash street sweepers remove is going to be incredibly important in determining whether communities feel they can go forward with track two.

WorldSweeper: What's the incentive to a city to have this trash picked up? Now we know that in the trash, there are a certain amount of pollutants, certainly microplastics, which is what your study is, most intriguingly, interested in. That also includes tire particles, which are really, I guess, micro rubber. (Here are two links with current information on this problem. This one at WorldSweeper and this one from a study by Yale University.)

Here in the Pacific Northwest those are being shown to have huge negative implications for salmon. The fisheries are getting drastically changed because of the pollutants that are in the rubber of tires. That's got to be addressed. Are there actually credits available to cities if they pick up more trash?

In Florida and more recently in Minnesota, credits are going back to cities for pollutants that are in the street sweeper pickup stream. What you seem to be talking about is getting credits for the trash component that's in the street sweeper amount. And when you separate out both the trash components and the pollution components in a street sweeper's debris hopper, you're not left with much else. That's basically what we're talking about that gets picked up off the street. So quantifying how important it is to get rid of both of those components and keep them out of the waterways, I think is what's next. Would you continue, please?

Jill Murray: I think it remains to be seen how the different California Regional Water Boards are going to credit street sweeping. But when it comes to trash removal we all have to show 100% removal of trash. So how much of that we can get credited from street sweeping is really going to depend on doing more research. I don't think there are a huge number of studies looking at street sweeping and trash in particular. However, I know there are some: I just saw a very interesting talk where the potential crediting went up from 1% of the street sweeper hopper by volume for trash pickup to above 25%. I don't know that the Regional Board has accepted that new info yet, but that's what the study showed.

I think more data needs to get collected on trash. Interestingly, the metric used by the water board is volume of trash. So, if people are thinking about planning these studies need to make sure they're looking at volume, not just mass of trash removed. Once we have a larger data set, I think that the water boards will be looking at this and deciding how much to credit street sweeping for track two trash removal.

Roger Sutherland: What's interesting to me is when you're talking about track one and track two, which is the first time I've really heard someone explain this. Track one has to be very capital intensive. If you have to go in and dig up things and put in huge vaults that's very costly. And then there's still a maintenance requirement because you eventually have to clean this material out and design it in such a way that it doesn't wash out during larger storm events, which has always been a concern.

Dr. Robert Pitt at the University of Alabama showed years ago how catch basins are ineffective because of that, unless you really have a mechanism that traps this material. My point is that larger communities should be seriously thinking about track two also.

The University of Florida and Florida's Stormwater Association spent three or four million over a 12-year period doing an analysis with the involvement of 14 MS4s that focused on the pollution content found in Particulate Material collected from various maintenance practices. That study demonstrated that street sweeping was up to 5X more effective when it came to something as difficult as picking up phosphorus and nitrogen. Trash is easy for a street sweeper to pick up; nitrogen and phosphorus not so much.

Both Minnesota and Florida study results concluded categorically that sweeping was the most cost-effective way to remove phosphorus and nitrogen. I feel like that can be easily transferred. Ranger and I will be involved in a portion of your project, Jill, which you just got from a NOAA Sea Grant, to look at microplastics. That study will involve a lot of players, not just us. What we're hoping we can do is demonstrate to the community the importance of using street sweeping as the first line of defense, in the fight against Microplastics being transported to receiving waters by stormwater.

Jill Murray: The definition of trash is five millimeters and greater; the definition of microplastic is particles five millimeters and smaller. In our study we will be trying to figure out how much street sweeping can also reduce microplastics, in addition to trash. We also have a section of the project that's about looking at the full capture devices that are inside the storm drains.

The concern we have with full capture devices is that they could actually generate microplastic because they're tumbling these larger plastics around: Are those larger pieces breaking down and then sending some microplastic particles downstream?

WorldSweeper: When we're talking about microplastics you're saying that that's five millimeters or under. How about the stuff that isn't plastic? For example, if sand is added to that, for example, or other things that aren't plastic or rubber, are those classified as a micro plastic?

Jill Murray: Rubber is a microplastic. And the reason is because virtually no rubber in our current industrial environment contains pure rubber from a rubber tree, it all has plasticizers, including 6PPD-q that has been shown to harm coho salmon and other fish. I've heard that a tire can have 1000 different compounds in it and most of them haven't even been identified yet. Thus we're a long way from understanding the impact of the tire wear particles. And in some studies, the tire wear particles make up half of the microplastics that can be separated from the storm water.

WorldSweeper: That's very good information. Let me share my screen for a minute (see graphic from the Florida study). These data are from the study that was done in Florida that Roger referenced. As you can see, the bottom line for nitrogen versus the next best BMP was $189 per pound by street sweeping and $1,016 for the next best, catch basin cleaning. Total phosphorus removal was $294 per pound by street sweeper versus $1,656 by the same next cheapest way to do it, catch basin cleaning.

Equally interesting for this talk today is the particulate matter portion. 'Stuff' that's out there, as Jill said, that is an eyesore and danger. Looking at the PM chart data, removal via a street sweeper costs 11 cents per pound; compare that to 70 cents per pound via catch basin cleaning. I encourage every reader of this to think about that. And on the upper size of PM we're now talking about overall trash removal. Then, on the small micron level, we're talking about microplastics less than five millimeters and much, much smaller. In Dr. Murray's upcoming microplastics' study, Roger Sutherland and I will be assisting in developing protocols designed to provide data on how well sweeping can do with both.

Jill Murray: For my community, street sweeping is done in combination with retractable screens over the catch basin inlets. The screens retract during a storm if they get a lot of trash or leaf material pushing up against them. These have been very successful for us. We've already studied whether material is getting into the catch basins. After the screens were installed the catch basins looked so much cleaner than they did before installation.

Roger Sutherland: I have been aware of catch basin screens and it makes a lot of sense. The action of the gutter broom on the street sweeper coming down the street along a curb, it's pushing some trash in front of it, you know, because it hasn't grabbed it yet. It'll just put it right into that catch basin if there isn't a screen there. It also should really solve a lot of flooding issues when you have a sweeper that's gone through. You never really know when it's going to rain even in you know in arid climates. When the sweeper has taken the material off the catch basin then that lets the water enter as planned.

Jill, could you say a few things about the microplastics project that you have been anxiously waiting to start?

Jill Murray: We submitted a proposal to NOAA's Marine Debris Competition. Part of the solicitation for that grant program included a lot of the goals of the California State Microplastic Strategy. NOAA and the California Ocean Protection Council worked together on getting some of the California goals into this solicitation. One of them was how can we intercept microplastics. That includes producer responsibility or at the site where plastics are being used, for example in an agricultural field where the plastic has been laid over the ground and then degrades; how can that get collected.

Infiltration projects infiltrate stormwater, but also capture trash and microplastic. A lot of those actions will take a long time, to be installed throughout a community. These might take decades, right? We also want to know what can we do right now? What are we already doing that might be removing microplastics. And that would be public works activities, such as street sweeping.

We know from an important study by the San Francisco Estuary Institute that most of the microplastic that's coming off of the land and getting into the ocean is from stormwater runoff. That's the microplastic on our impervious surfaces that are getting washed into the storm drains during storm events. How much can street sweeping intercept that?

We're getting at our information from three different angles. In Santa Barbara we're trying to estimate the total load of microplastic that's being discharged to the ocean. We already have a good handle on that and we'll get an even better handle on it. And then how much are we removing of that load? Of that, how much are we actually removing in our street sweeper hoppers?

This will be the challenging part because quantifying microplastic in a messy matrix, such as street sweeper hopper, is hard because we have a lot of organic material that is the same or similar density to microplastic. Separating it is tricky. We're going to have UCSB working hard to overcome some of the challenges with that quantification. Another method is to look at runoff concentrations in a very well controlled, swept versus and unswept study that's been conducted by the Southern California coastal water research program. That work has an excellent design of very realistic simulated rainfall.

The City of Santa Barbara has done some pilot work. We've swept the street by hand after the street sweeper came by and we have found more microplastics still on the street. So, we know there's room for improvement. Soon we'll be doing a study, ala Roger Sutherland's previous studies, where we're going out and testing the sweepers with simulated street dirt plus simulated microplastic. We'll be seeing if we can get better pickup rates and efficiencies by changing sweeper type and speed, etc. We want to put a number on what would be the best pickup we could get of the microplastics so that we have an upper bound.

We're approaching this from a very scientific perspective. I know that the street sweeper removed some microplastic, because we've done that pilot study, but we do not have quantitative numbers on this. And this is something we all need to know: we know we're removing some but are we removing 1% of the total load or 10% of the total load? The answer to that will make a real difference in how we relate street sweeping to our understanding of microplastic transport to the ocean.

WorldSweeper: Thank you; that's a very good summary. Let me weigh in from my editorial perspectives. If you think that what a sweeper is out there to do is to pick up gross material, cardboard boxes and castoff debris and other big stuff, then that's one thing. The difficult part that we've always had with Public Works directors is to make them realize the importance of removing small micron particles, which is what we're talking about with microplastics.

If you're at about 250 microns – your hair is about 72 microns wide – three times the width of our hair and smaller is only typically about 10% of what's on the street. Unfortunately or now that we understand this perhaps fortunately, we've learned that up to 60% of the pollutant load that the EPA is trying to get off the street, for reasons of physics, can be in that 10% of the smallest material.

We need the whole sweeping industry and the public works directors to force the feet of the sweeping industry to the fire, to make sure sweepers can pick up the small stuff. We can't have any more posts on social media like "I followed the sweeper in a cloud of dust." We can't afford that cloud of dust anymore.

First off, the sweepers have to be well maintained so that they don't create clouds of dust, and they need to move toward situations where we can maximize not only the trash that's picked up, but the small micron material.

Jill Murray: That is really interesting. I'm wondering if that is going to require brushless street sweepers because I've seen some results showing some of the fines are actually increased after the sweeper goes by. I think that's from the brush scouring the street surface.

WorldSweeper: We have learned that mechanical broom sweepers are the ones that tend to make larger particles into smaller particles without picking them up. Understand that even with regenerative air sweepers a curb broom is needed to push the material inboard from the curb because most of the material by far is within 36 inches of the curb. So you've got to do a good job scrubbing that area.

Maybe a suction on the brooms will be needed, which is possible. So any dust that's produced by this curb broom gets pulled up into the hopper. The material swept by the curb broom goes over into the big vacuum cleaner that is part of a regenerative air or vacuum sweeper.

The diagram, provided by TYMCO sweeper company, shows how a regenerative air sweeper works.

You may find it helpful to read this page the company provides that discusses why regenerative air sweepers are generally considered to be better for small micron debris pickup than are mechanical broom or vacuum sweepers.

What we can't do is continue to use broom sweepers that have much less small micron pickup, and there are so many cities in the snowbelt that need broom sweepers for their spring cleanup. Sometimes they need a front end loader as well as the broom sweepers. So they continue to use those throughout the year because they're so expensive to just let sit. However, the fact is, if water quality and air quality and pollution from trash to microplastic pickup matters, then you have to go with air sweepers that are well well equipped, well kept up and or find another technology or improvements in the technology.

We have a study online that talks about tandem sweeping. A broom sweeper went by followed directly by an air sweeper that then removed up to 500 pounds per curb mile. And all of that tends to be small micron degree. So the combination of the two is really important.

The broom sweeper, that's an acronym for mechanical sweeper, is where the the technology of getting the material into the hopper is a conveyor belt. A wide, transverse broom throws the material onto that conveyor belt and it's just churning, putting stuff in the hopper. That has the ability to remove much heavier material, more bulkier material down to maybe 2000 microns. It can obviously remove less than that. But the problem with that is it's exposing the fines.

Roger Sutherland: The National Urban Runoff Program testing in the early 1980s, when EPA spent millions of dollars throughout the nation, showed that street sweeping is largely ineffective in its ability to remove pollutants from stormwater. That was correct at that time because the only street sweepers out there were mechanical sweepers, i.e., broom sweepers. We now know that when the road surface is swept by a mechanical sweeper the smaller particles are not effectively picked up, they are instead exposed so when a rain occurs it's much easier for these contaminants to wash off.

When you run an air sweeper following a mechanical sweeper, which is called tandem sweeping, it will remove that smaller material. What we now know is that the technology has advanced so much that now we have air sweepers with their vacuum or regenerative air components, which appears to be the best choice for smaller micron pick-up. In most conditions, these machines can operate without a mechanical broom machine in front of them.

In certain parts of the country where you have huge loadings due to snow and ice control, you need to use a broom sweeper or even a front-end loader. I did some work in Illinois back in 1977/78 where I documented loadings up to 20,000 pounds per curb mile after the spring thaw in Champaign-Urbana, where the University of Illinois is located. A huge material loading on the street like that is a challenge.

WorldSweeper: Is there anything final that you'd like to say Jill? You're the microplastics' star and the small micron material being targeted for removal in your section of California?

Jill Murray: I think you've covered everything. I'm excited to see where our street based microplastics study goes. I have no preconceived ideas of what we're going to find. I hope that the street sweeping really helps, but we just don't know. I'm really looking forward to getting those first sets of data.

WorldSweeper: I'm looking forward to that as well. So thank you both for participating with this topic today. I think we've covered a lot of insightful points and hope to bring some of our watchers up to speed in ways that perhaps they weren't before.

In the event you have questions or comments for Dr. Jill Murray, you may reach her at JMurray@ SantaBarbaraCA.gov.

To contact Roger Sutherland, send email to sutherland.roger1@gmail.com.

If you have questions or comments for Ranger Kidwell-Ross, you may reach him at editor@ worldsweeper.com. To view the sweeping industry experience of each of the latter two presenters, click here.

If you have questions or comments about this article, please let us know. If appropriate, we will append them to the article.

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