Seattle's 120-million-gallon-per-day Tolt treatment facility delivers high performance at reduced cost through an innovative public-private partnership. Tolt's unique and environmentally sensitive design combines ozone disinfection and filtration to achieve 5-log removal/inactivation of Cryptosporidium —even during periods of high source water turbidity—with minimum energy, chemical, and land consumption. Further, the ground-breaking design-build-operate (DBO) approach successfully delivered the project at a cost of $101 million—more than 40 percent below the city's benchmark cost estimate, a consumer savings of more than $70 million.
Water Treatment Innovations
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Pilot testing minimized ozonation costs and optimized flocculation by lengthening ozone contact (25 minutes) and coagulant detention (20 minutes) times.
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An innovative combination of flocculation and ozone contact basins optimizes treatment (detention time exceeds 25 minutes) and eliminates the need for an additional 2.5-million-gallon basin.
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High-rate (12 gpm/ft2), deep-bed, mono-medium filters achieve a 50-percent greater filtration rate than the benchmark design—up to twice that previously approved by the Washington State Department of Health—requiring fewer and smaller filters.
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Hydraulic flocculation provides an appropriate combination of mixing energy and time without added mechanical energy.
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Pumped injection flash mixing, using water jets supplied by the utility water pumps, avoids penetrating an ozone-containing basin with rotating equipment.
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The compact design on a 30-acre site—half the benchmark concept—reduced the potential wetland impact to less than 0.25 acre, and a 0.5-acre wetland was created to join existing wetlands and re-establish a continuous ecosystem.
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Hydraulically, Tolt is a closed system. Every drop of raw water is eventually delivered as treated water with no offsite discharges.
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Design was completed in less than 1 year to conform to critical seasonal construction windows.
Innovations in Project Delivery
While Tolt boasts numerous technical innovations, the ground-breaking DBO approach was the vehicle that delivered them. Synergistic relationships among the design, construction, and operations teams facilitated innovative design, ensured constructability and operability, and streamlined construction. CDM tightly managed a team of 12 subconsultants during concept development, design, and construction; produced a design document package of 430 drawings and three volumes of detailed specifications; and addressed more than 700 comments from Seattle Public Utilities. CDM revisited almost every standard approach to ensure that quality was defined by the ability to deliver performance over the long term, and not just by "the way it has always been done," thus raising the bar for the water treatment engineering profession.