The City of Tacoma, Washington selected MWH Global to design a new water treatment facility that combines direct filtration and conventional treatment at the same location, to better manage the challenges in dealing with a mountain river source and vastly differing water qualities between summer and winter.
In 2006, the EPA required that, to meet new national standards for public health protection, water utilities provide treatment by 2014 for the human pathogen Cryptosporidium. The Green River Filtration Facility added a physical barrier between the Green River and the tap for Tacoma Water’s approximately 316,000 direct customers in Pierce and King Counties. Water from the plant can reach as many as 500,000 direct, partner and wholesale customers. The previous unfiltered system, originally built in 1913, served as a suitable water source for more than 100 years, but would occasionally experience high turbidity and other water quality issues that are now fully resolved with the new treatment facility.
Coagulants are now added to the raw water to bring suspended material together for removal in downstream processes, either through high rate gravity sedimentation or by deep bed granular media filtration. Treatment is state-of-the-art and complements existing ozonation and chlorination procedures which Tacoma Water had already implemented at the site. The facility is the largest filtration plant in the Pacific Northwest region.
The cleaner water meets new public health regulations, plus improves water quality and reliability. The removed solids are thickened and dewatered on site and pushed through screw presses that further separate residuals and water, producing a material sufficiently dry so that it may be hauled off site. All water is recycled at the plant, making this a zero discharge facility.
In the summer months, when customer demand is at its peak (168mgd max), the river is of high quality and direct filtration is very effective and economical. Conversely, the winter months see the lowest demand (90mgd max), but river turbidity can quickly rise above 300 NTU. An additional robust sedimentation scheme is required to supplement the granular media filters.
In addition to meeting current and reasonably anticipated regulations, filtration improves the water by:
- Improving the taste and clarity of water;
- Minimizing natural organic material in water that forms disinfection byproducts when it reacts with chlorine;
- Making river water available that is negatively impacted by seasonal water quality conditions;
- Adding flocculation/sedimentation basins, dual media filters, and clearwells.
Tacoma Water and MWH worked effectively together to produce all work products on time. The treatment plant was successfully commissioned on schedule in December 2014. The original project budget of $217 million was reduced by more than $30 million through the efforts of Tacoma Water, MWH and the highly qualified contractor, Hoffman Construction Company. Project innovations included use of ground modification to address unusual subsurface conditions and potential for high seismic activity. Based on detailed physical investigations, rammed aggregate piers were used for selected water-holding structures to achieve superior resistance to seismic forces while optimizing capital costs. Additionally, the plant is a first-of-its-kind hybrid, in which sedimentation basins are taken off-line during the summer. Benefits include smaller concrete basins, less sludge production, reduced chemical and energy use, and substantial cost savings.
- Planning and layout;
- Basis of Design Report;
- 38 technical memoranda;
- Cost evaluations and OPCC preparation;
- Treatment plant and intake improvement design;
- 1,000 construction sheets prepared;
- Construction phase assistance;
- Startup and commissioning assistance.
Tacoma Water in collaboration with MWH Global received a 2015 Outstanding Civil Engineering Achievement Award granted by the American Society of Civil Engineers (ASCE) Seattle Section, for design of the Green River Filtration Facility. The annual Outstanding Civil Engineering Achievement Awards recognize projects that have improved the quality of life and contributed to the economic development of the local community, area or region
The Green River Filtration Facility was recognized as best in class in the ‘Water Resources Engineering’ category by ASCE.
Fly-over video of the facility:
- 36932 SE Green River Headworks Road, Ravensdale, WA 98051
- 47.3037244, -121.84993659999998