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City of Seattle Doubling its Sweeping Program to Address Stormwater Pollution

by Ranger Kidwell-Ross

Seattle's Challenge

Because of Seattle, Washington's location on Puget Sound, the city is planning to increase its street sweeping program significantly in 2016 in an effort to reduce stormwater runoff pollution.

Brake dust, remnants of auto exhaust and the zinc from vehicle tires are just a few of the substances commonly found in the toxin-laden residue on streets in the City of Seattle. All are among the major pollutants being targeted by an initiative that is being spearheaded by Seattle Public Utilities (SPU). Seattle's current tab for street sweeping to reduce stormwater pollutants is $1 million. The planned expansion of the city's street sweeping program will almost double that figure, to $1.8 million.

SPU will be partnering with Seattle Department of Transportation (SDOT) in the project, continuing a partnership that has already created a drop in the cost per mile of sweeping from $75/mile to $70/mile.

The expansion will increase the number of sweeping routes from 33 to 43, and the annual miles swept from 10,000 to 20,000. The goal for pollutant removal is to increase from the current 100 tons of pollutants taken off the streets currently to 140 tons once the program is fully implemented. The sweeping window will also be increased from the current 40 weeks per year to 50 weeks per year. Sweeping frequency will be adjusted as needed.

Load Reduction Chart As is shown in the chart to the right, in 2011, the first year of enacting the sweeping program as an adjunct to existing end-of-the-pipe solutions, the reduction in Seattle's stormwater pollutants entering Puget Sound was a whopping 300%.

Shelly Basketfield, Seattle's Street Cleaning for Water Quality program manager, says "When it rains, all of that [pollutant load] ends up in the waterways. It's nasty stuff." In a city known for abundant rainfall, and where the surrounding waters are home to species like salmon, orca whales and harbor seals, the runoff of road grime presents a serious problem.

Basketfield also says their team realizes it is significantly less expensive to remove pollutants from arterials via street cleaning than by any other method. In addition to Puget Sound, Seattle is also the home to a number of other waterways, including Lake Washington and the Duwamish and Green River systems. All eventually flow into Puget Sound. The utility has collected performance data that shows street sweeping is effective at helping to keep pollutants out of stormwater. At the same time, sweeping is a more cost-effective way of doing so than building end-of-the-pipe infrastructure.

Right now, Basketfield says their life cycle unit rate per pound of pollutant removed is about $5.30. When they double their sweeping, that cost is projected to become $5.60. To determine the pollutant load that SPU will count for water quality benefits, every two weeks the organization will sample the material removed by the sweepers. Based on the sampling information and the total load removed SPU will then estimate the total water quality benefits obtained. Of the total material being picked up, about 21% is less than 250 microns.

However, SPU is being very conservative in its calculations, by classifying as pollutant-laden only the particle size fraction that is about 12% of the total dry load that's removed. Even though using such a conservative estimate, SPU estimates the life cycle unit cost [per pound of pollutant removed] for sweeping will be four-to-eight times less than employing conventional end-of-pipe strategies. That becomes an even better value when compared to the cost of treating streets directly since streets are more expensive to treat due to their long and linear nature.

Basketfield also made it clear that her organization recognizes the added value of removing larger material from the roadways via sweeping, even though those may not currently contain a large pollutant component. The reason: Larger particles, when left on the road, will eventually become smaller via being run over by vehicles, through wind and rain events, etc. At that point, as the material becomes smaller, it becomes both more accessible to pollutant attachment and, due to the smaller size, more available to becoming storm water runoff due to wind and rain events.

Sweeping also offers a high level of nutrient runoff reduction, much of which is caused by leaf drop from deciduous trees. Nutrients contribute heavily to algal blooms, which results in low oxygen levels that are dangerous to any animal or plant life that depends upon breathing. "All that material [above 250 microns]," said Basketfield, "if it had remained on the street, would have broken down to a smaller size and then either washed off or blown off."

In about two-thirds of Seattle, the rainwater that hits city streets goes down storm drains and through pipes that feed directly into bodies of water. About 13 billion gallons of this discharged water is released in an average year, according to Seattle Public Utilities.

The other approximately one-third of the city is served by a system where stormwater and sewage flow through the same pipes to treatment plants. However, during heavy rains this system can overflow and dump into waterways. SPU estimates these overflows totaled 154 million gallons in 2012.

Seattle Waterfront "If you can remove the pollutant before it gets into water, it's cheaper," said Basketfield. "Once the pollutant is in the water, you have to treat all the water." Basketfield says the planned start for the enhanced sweeping project is January 2016. This time frame will allow the two partners on the program, Seattle Public Utilities and the Seattle Department of Transportation, time to plan out the new routes, add more sweeper operators and purchase new sweepers.

Basketfield also says that a major source of contaminated runoff are Seattle's higher-capacity streets, known as arterials. Although these roadways make up just four percent of the city's surface area, they contribute an estimated 16-percent of the pollutant load in stormwater. She also stressed that, as new routes are developed, they will be emphasizing finding ways to sweep each route in the most cost-effective way possible, along with focusing cleanup efforts on the city's higher volume arterial streets. Via the use of more frequent sweeping – done with regenerative air sweeper models at an increased frequency – the city intends to remove a larger amount of pollutants prior to them running off into the storm drain.

"We have been able to show definitively that street sweeping is one of the most cost-effective measures we can use to protect our waterways from street runoff," Basketfield said. The city has developed data that show street sweeping costs 4-to-10 times less than treating polluted water with conventional stormwater treatment. These findings correlate well with the information in the WorldSweeper white paper on the topic of the effectiveness of street sweeping versus other, end of the pipe, solutions.

When SPU got involved in this process, SDOT owned four Schwarze A8000 regenerative air sweepers and three mechanical machines. The A8000 (now A8 Twister) models had been chosen because of their high-dump/side-dump capability, which SDOT managers thought would make leaf season offloading into rolloffs more cost-effective to operate. However, due to that model's smallish, 6 cu. yd. capacity, for water quality sweeping they are now seen to require longer trips to the landfill and so not as overall cost-effective.

Schwarze A9 Monsoon Primarily for this reason, in 2013/2014 the entire fleet was replaced with seven Schwarze A9 Monsoon models, which have a 9.6 cu. yd. volumetric (8 cu. yd. usable) capacity and one mechanical broom sweeper. In 2016, when the sweeping miles are doubled, the SDOT/SPU management teams calculate they will only have to add one additional sweeper to their current fleet.

All of the regenerative air sweepers are outfitted with onboard scales and tracking systems, in order to measure the amount of material removed from the streets as well as log the miles swept across the city's current 33 sweeping routes. This data, as summarized in the chart below, are critical for assessing the long-term outcome of the project.


Pickup by Route

SPU is also gathering information about what's in the sweepings. The material is sampled every other week to determine the weight of the pollutants, as well as the level of specific contaminants such as metals, PCBs and fecal coliform . For example, based on the Seattle Public Utilities figures in 2014 the sweepers collected a total of 1,110 tons of debris off the city's arterials.

Of that total, 130 tons were composed of very fine-grained particles that the utility classifies as pollutants. As an example, that amount is roughly equivalent to how much a 53-foot railroad gondola can carry when fully-loaded. In 2012, the total amount of fine-grained solids that street sweepers working under the program removed from the street was 120 tons and in 2013 it was 100. Within the 130 tons collected in 2014, a few hundred pounds were especially problematic substances.

Previous Wisdom For instance, there were 170 pounds of total phosphorous, known to cause algae blooms that upset aquatic habitats, and 43 pounds of copper, which can affect salmon's sense of smell, leaving them more prone to getting eaten by predators. Further, the data on the program to date illustrate how cost-effective the Seattle sweeping program is: When the dollars spent are compared to the pounds of pollutants removed by the sweepers, the cost per pound worked out to about $4.80. This may be contrasted with an early 2000s CalTrans study that estimated that end-of-the-pipe abatement was about $35 per pound of pollutant removed without considering the cost of real estate. Plus, in addition to the relatively small-micron pollutants, sweeping removes a large quantity of larger debris, as well.

The CalTrans figures align with the City of Seattle's figures: Basketfield said they have tracked the cost-effectiveness of eight stormwater projects designed to capture end-of-the-pipe pollution and mitigate it, including ponds and swales. With the exception of one pond, the cost-effectiveness figures for these facilities checked in between $8.20 per pound and $53 per pound, far higher than the figure for the sweepers.

Seattle Sweeper Operator To become even more cost-effective, SPU and SDOT have moved to 10-hour sweeping shifts for the stormwater routes, which provides more actual sweeping time since the relative amount of prep time and machine cleanup as a percentage of total sweeping time has been reduced. Because changing the schedule was a union issue, volunteers were requested and two sweeper operators volunteered. Both continue as operators today. Instead of having water quality routes and general sweeping routes, for the next phase the organization is also developing routes with the least travel time in order to become even more efficient.

The entire sweeping program is automated to increase data for better decisions. Each sweeper has a GPS unit that tracks where each sweeper goes on its route. Data on each sweeping route include a calculation about what part of the route drains to a water body and what portion to the publicly-owned treatment works. SPU pays for the water quality part and SDOT pays for the rest. Each sweeper's onboard scale also allows them to calculate a sweeper's pickup pounds per mile.

Seattle CSO Illustration The near doubling of Seattle's street cleaning program is just one component of what is called the 'Protect Seattle's Waterways Plan." This is a $600 million, 15-year plan that is designed to reduce the amount of pollutants flowing into Seattle's various bodies of water, in order to comply with state regulations and the federal Clean Water Act. The Seattle Public Utility, which is the lead agency in the project, is proposing to fund the Plan via an increase in both drainage and sewer rates. Although the proposed rate structure has not yet been approved, it is expected that average monthly residential rates would increase by about 6.4% next year.

PSP Logo The expansion of Seattle's street sweeping program supports the analysis by the Puget Sound Partnership (PSP), which is the Washington State agency charged with cleaning up and restoring Puget Sound. In a 2011 report the PSP reported that it believed surface runoff, not sewage overflow, was the biggest source of pollution to Puget Sound.

Based on the positive results from a Pilot Study completed in 2009 (see World Sweeper article), SPU embarked on a full-scale street cleaning program with an emphasis on stormwater quality in 2011.

Basketfield said her organization clearly recognizes the need to keep Puget Sound from receiving pollutants. "In the long run," she said, "if we keep dumping all this stuff in the waterways potentially we're going to have to go in and clean it up, which would be really expensive. Our goal is to get ahead of the water quality, stormwater pollutant game.

"In terms of all the pollutants that are washing off our streets, it's much more effective for us to remove them now than it will be at some future point in time trying to clean them out of the water once the pollutants are already there. Plus, if they end up in the water body they will be negatively impacting water quality and aquatic life.

"The other aspect is stormwater regulations: They are becoming more more rigorous because it is being recognized nationally how big a problem these pollutants have become. What we are doing is developing the best pollution reduction program for us. Regardless of what the water quality benefits are, street sweeping is removing a lot of pollutants from our environment. That is part of the big picture we should all be focused on. The more we can remove those pollutant loads, the better for our entire environment. In general, this is just a great story. It's good for the city, good for our residents; clean water and clean air, you just can't beat it."


We are pleased to provide a 26-minute podcast discussion between Shelly Basketfield, PE, Street Sweeping for Water Quality Program Manager for Seattle Public Utilities, and Ranger Kidwell-Ross, Editor of WorldSweeper.com and Executive Director of the World Sweeping Association. To access the podcast, activate the sound bar shown below.


City of Seattle Logo Many thanks to Shelly Basketfield, PE, Street Sweeping for Water Quality Program Manager for Seattle Public Utilities. Shelly invited any interested parties with questions to feel free to call her at 206.386.1127. Thanks also to Ingrid Goodwin of Seattle Public Utilities' Communications Division

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