Sweeper waste was stored in separate dumpsters assigned to each test site. Each dumpster was weighed on an industrial scale after excess water was removed with a trash pump. Material greater than 2 centimeters in diameter was removed by hand and weighed separately to determine the proportion of debris in the waste material. Sediment accumulation in the catch basins was determined by measuring down from the rim of the maintenance hole to the surface of the sediment. A total of 12 catch basins were sampled at each test/control site.
In addition to collecting the amount of sediment from the streets, street dirt, sweeper waste, and catch basin sediment samples were collected for chemical analysis every four weeks and archived. At the end of each quarter, a single composite sample of each media type was prepared from the archived samples and submitted to the laboratory for analysis. Samples were analyzed for total suspended solids, metals, organic content (total volatile solids and total organic carbon), semi-volatile organic compounds, and PCBs.
Seattle to expand sweeping
Based on the results of the pilot study, Seattle is beginning to pursue an expanded street sweeping program. The study recommended that as the city expands its sweeping efforts that it:
- Prioritize areas within the city where water/sediment quality improvements are most needed and where street sweeping has the greatest potential to improve water and sediment quality. The study identified areas where sweeping could have the greatest impact as arterials and curbed roads.
- Establish a sweeping frequency of once every two weeks, which was the frequency shown to be effective in the study. The report also recommends that the sweeping frequency should be increased to once every two weeks in areas that are currently swept less frequently.
While the study did answer a number of questions, it also provided reason for additional study. The report suggests the city:
- Evaluate whether street sweeping can also be cost effective in areas without curbs. Many of the streets in the industrial sections of Seattle are not curbed and the study suggests additional research to develop a cost-effective strategy for dealing with these roadways.
- Continue to evaluate street sweeping performance by monitoring the volume of sweeper waste removed and tracking how removal rates vary with the type of streets swept (e.g., land use, traffic volume, number of parked cars, and the absence of curbs).
- Continue to evaluate the effectiveness of street sweeping at reducing drainage system maintenance costs. The study notes that though results indicate "that sweeping had a negligible effect on reducing catch basin sediment buildup, this result is counterintuitive and should continue to be studied as it has the potential to save the city considerable money if justified by further analysis."
Additional Resources that Build on the Topics of this Article
Seattle Sweeping SPU 2009 - Read the full study report.
Podcast interview with Terry Martin - The audio interview with Martin, one of the project specifiers, can be found at the end of the page.