Swinging sky cranes are becoming a common site on the skyline of Bellevue, Wash., as the city undergoes major growth and redevelopment in the downtown core. An eight-story hotel being built by Ferguson Construction is one such construction project. Because the hotel is being built adjacent to a wetland bog and Sturtevant Creek, which supports endangered salmon species, the project has faced many challenges and required special environmental measures to protect storm water and groundwater.
Dealing with large volumes of shallow groundwater and storm water from consistent winter rains was one big challenge and further complicated by the small project footprint and its proximity to sensitive receiving waters. Conventional storm water management facilities such as detention basins could not be used during project construction due to space constraints, and the permanent facilities would not be available until midway through construction. Because of these challenges, Ferguson called on Clear Water Compliance Services to design a treatment system to meet its needs.
Storage, Flow, pH & Treatment
The company responded with a system based on the Washington State Department of Ecology’s BMP C250, a batch chemical treatment for storm water. BMP C250 was selected for the project because it was less equipment-intensive, meaning it would reduce costs and footprint. Typically, a batch treatment system requires a tremendous amount of water storage to manage severe rain events. Storage requirements for the project were initially met with above-ground storage tanks and later with the permanent underground vault and the hotel’s swimming pool. These facilities provided more than 80,000 gal of storage.
The batch treatment system operated with a discharge rate of up to 250 gal per minute. Treated water was filtered and tested for turbidity and pH before being discharged into a controlled dispersal system adjacent to an onsite wetland. This minimized the impact of concentrated flow on the wetland area and allowed the wetland to infiltrate at a more natural rate.
Storage requirements and flow rates were not the only issues; the treatment system required a robust pH-adjustment unit to neutralize low-pH water from the organic-rich peat substrate and high-pH runoff from freshly poured concrete. The pH varied widely from day to day depending on site activities and rainfall quantity. When routine monitoring indicated a need for pH adjustment, the treatment system neutralized the pH to the allowable discharge limit (pH of 6.5 to 8.5).
The water’s treatability was the final issue. The treatment procedure needed to handle the water’s high organic component and fluctuating pH and still produce a discharge of less than 10 ntu.
Clear Water used both polyaluminum chloride and chitosan to assure levels of suspended solids and turbidity were below required limits. Nearly 2.6 million gal (approximately 85,472 gal per week) of storm water and groundwater were treated without a single non-complying discharge. Over the eight months of operation, average pH was 7.09, and average discharge turbidity was 2.32 ntu. During that period, there were 27 in. of rainfall and multiple storms that required around-the-clock operations.