American Society of Civil Engineers Awards Green River Filtration Facility Outstanding Civil Engineering Achievement Award
MWH Global is the proud recipient of 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. These projects represent the successful combination of multiple engineering objectives, including design innovation and excellence, environmental sustainability, cost effectiveness, the effective use of materials, and aesthetics. Each year, the Seattle Section recognizes the award winners at a formal presentation in June.
The Green River Filtration Facility (Ravensdale, Washington)
The City of Tacoma, Washington assigned 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 over 300,000 direct customers in Pierce and south King Counties. The previous unfiltered system, originally built in 1913, sent as a daily average approximately 1,000 pounds of fine sediment into drinking water pipes; which had to be cleaned out, and occasionally became re-suspended in the water.
Now, coagulants are added to the raw water which causes silt suspended in the water to clump into larger particles that settle out in the new plant. Remaining particles are screened out through filters containing 50 inches of anthracite coal and 20 inches of sand.
The cleaner water meets new public health regulations, plus improves water quality and reliability. The removed solids are dewatered on site and pushed through 20-foot long, 6-foot tall screw presses that further separate the dirt from the water. Mechanical dewatering facilities were also constructed to handle solids taken out of the river so that they are dry enough to haul offsite. The process produces one to two large truckloads of residuals per week on average.
In the summer months, demand is at its peak (168mgd max). The river now serves as a high quality and very consistent source for consumers, where direct filtration is very effective and economical. Conversely, the winter months see the lowest demand (90mgd max). River turbidity can quickly rise above 300 NTU for long periods, requiring an additional robust sedimentation scheme, which is now fully operational.
In addition to meeting current and reasonably anticipated regulations, filtration improves the water by:
- Improves the taste and clarity of water;
- Minimizes natural organic material in water that forms disinfection byproducts when it reacts with chlorine;
- Makes river water available that is negatively impacted by seasonal water quality conditions;
- Adds flocculation/sedimentation basins, dual media filters, and clearwells.
All work products were completed on time and the treatment plant was successfully commissioned on schedule in December 2014.