Location: King County, Washington, United States
Status: In Progress
This project consisted of five tunnel drives under four main construction contracts (East, Central, West, BT3C) each from 11,100 to 21,000 feet in length, that connect to the new wastewater treatment plant and convey treated effluent to Puget Sound for discharge. The tunnel system was constructed in soft ground, utilizing earth pressure balance (EPB) and mixed shield slurry pressurized-face tunnel boring machine (TBM) techniques, with external groundwater heads ranging up to 240 feet (7.2 bars). Ground conditions range from dense glacially consolidated soils to loose glacial outwash and alluvial soils at depths from 50 to over 400 feet.
As a part of the tunneling work, five permanent and one temporary shaft structures were constructed, with depths ranging from 40 to 200 feet. The different final shaft structures are used for flow distribution, pump station foundation, tunnel access, and flow sampling and measurement. In addition to the tunnels, the Brightwater Conveyance Project also included approximately 5,700 feet of pipeline connecting the tunnel system to the existing interceptors. These pipelines included three microtunnels (approximately 2,400-, 3,300-, and 500-feet), 2,200 feet of open-cut pipeline, diversion and drop structures. Other facilities included odor control stations, chemical injection systems, and final site work to include park and public art designed to educate people about the importance of clean water.
The treatment plant currently has the capacity to treat an average of 36 million gallons of wastewater per day (mgd). By 2040, treatment capacity will be expanded to 54 million mgd. These capacities are for average wet-weather flows (AWWF). The plant must also be able to accommodate peak hourly flows of 130 mgd at start-up and 170 mgd by 2040.
Dating back to the early 2000’s, King County Department of Natural Resources and Parks, Wastewater Treatment Division determined that a new wastewater treatment system (treatment plant, conveyance system and outfall) was needed. In 2003, King County selected MWH and another joint venture firm to spearhead the design of this project. Construction started in 2006 with limited treatment plant start-up and operations that began in September 2011. The Brightwater Treatment System is currently under construction and scheduled for system operation in September 2012 and completion of wetland restoration and odor control facilities by 2014.