High Efficiency Sweeping as an Alternative to the Use of Wet Vaults for Stormwater Treatment
In May, 1996, the Port of Seattle contracted with Kurahashi & Associates, Inc. (KAI) to conduct an evaluation of selected stormwater treatment best management practices (BMPs) for the Port's marine terminals. The study's objective was to evaluate the stormwater pollutant removal effectiveness of new high efficiency pavement sweepers in combination with conventional sediment trapping catchbasins and to determine if this combination technology is essentially equivalent to the pollutant removal efficiencies of wet vaults. For new marine facilities, the only stormwater treatment BMP that is deemed technically feasible and currently approved by the Department of Ecology is the wet vault.
The type of Port facilities for which sweeping and sediment trapping catchbasins were evaluated is the cargo container yard. The Port currently owns five such facilities covering a total area of approximately 400 acres (160 ha). The Port is currently undertaking major expansion projects at two of the five yards.
The project of immediate interest is the Southwest Harbor Project. This project, currently under construction, will expand an existing container yard by 250 acres (100 ha). Wet vaults are estimated to have a life cycle cost of about $18 million for this expansion project due primarily to the structural requirements of the vault covers that need to fully support large, heavy container transport vehicles called top picks. In contrast, high efficiency weekly pavement sweeping with normal sediment trapping stormwater inlets cleaned annually will have a life cycle cost of approximately $2 million for the same expansion project.
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