Additionally, because many pollutants present in urban runoff tend to sorb into or cling to particulates, particularly the smaller size particles (such as medium sand, silt, and clay), street sweeping might also be able to reduce the pollutants running into area water bodies.
"We looked at everything and asked ourselves, 'What if we really ramped up street sweeping?'" says Martin. "Is it cheaper than cleaning catch basins? Is it cheaper than building downstream treatment facilities? The study was designed to answer those questions."
Results of the study showed that sweeping each side of the street every other week "is very effective in reducing the amount of sediment and associated pollutants discharged from city streets." According to the study, sweeping reduced the amount of dirt in all three study areas. "The median monthly street dirt yield at the swept sites was 48, 74, and 90 percent less than the control (unswept) sites....On an annual basis, sweeping removed approximately 2,200 to 3,100 pounds of material per acre of street swept."
The study also found that sweeping can reduce the amount of pollutants discharged from streets to bodies of water. "Contaminants found in street dirt, sweeper waste, and catch basin samples included metals, petroleum hydrocarbons, and phthalates." Of these chemicals in the 55 samples analyzed, motor oil (82%), carcinogenic PAHs (78%), zinc (18%), chromium (15%) and a host of other chemical concentrations were above the Washington state sediment/soil standards and guidelines.
"Street sweeping has the potential to be an effective source control strategy by preventing a significant amount of sediment and associated contaminants from being discharged to receiving waters, which not only impacts water/sediment quality but also impairs substrate quality," the study determined.
Catch basin cleaning
The study expected that street sweeping would reduce the amount of sediment that accumulated in area catch basins, which SPU hoped would reduce the frequency at which catch basins needed to be cleaned and reduce the money spent on catch basin cleaning. Test results, however, did not show that street sweeping affected the amount or rate of sediment accumulation in the test area catch basins.
"Differences in the amount of sediment that accumulated in the catch basins between the swept and unswept sites at the end of the study period were not statistically significant, either on the basis of total mass or mass per unit area of street draining to the catch basin," according to the study.
So study results indicate that street sweeping might not help to reduce SPU's catch basin maintenance costs, which the study termed "a serious disappointment." On the other hand, the study found that most of the catch basins in the study sites were less than 10% full of sediment at the end of the study period - well below the 60% threshold at which catch basin cleaning would be required.
Overall, the study determined that "street sweeping has the potential to be a cost-effective strategy for removing sediment and pollutants associated with sediment from roadways in the City of Seattle and is likely to be more cost-effective than annual catch basin cleaning or stormwater treatment."
Costs for a full-scale 2008 sweeping program were estimated at $43 per curb mile (based on SDOT's 2006 unit costs of $35 per curb mile, using a 3% inflation factor and a 15% contingency to convert to 2008 dollars). Solids handling and transportation costs were estimated at $34 per wet ton, and solids disposal was approximately $43.50 per wet ton.
Estimated life-cycle costs for a full-scale street sweeping program were $0.34 per wet kilogram of material removed and $0.62 per dry kilogram of material removed. In both wet and dry instances that's lower than the costs for the SPU city-wide catch basin cleaning program ($0.42 per wet kilogram and $0.74 per dry kilogram).
The study also determined that street sweeping is also cost effective compared to treating stormwater prior to discharge (which some communities opt for in lieu of upgrading street sweeping programs). The study estimated that street sweeping costs between 15% and 50% less than the cost of treating stormwater prior to discharge.