Title
EVALUATION OF TREATMENT TECHNOLOGIES FOR SILICA REMOVAL AT THE MINT FARM REGIONAL WATER TREATMENT PLANT
COUNCIL STRATEGIC INITIATIVE ADDRESSED:
Provide sustainable water quality and environmental infrastructure.
CITY ATTORNEY REVIEW: N/A
SUMMARY STATEMENT:
The decision to cease further investigation of alternate sources of drinking water supply was coupled with a commitment to optimize water quality from the Mint Farm water treatment plant. In November 2016, staff was directed to pursue dissolved oxygen (DO) addition to address taste and odor issues; and to evaluate treatment options for silica removal to address spotting issues.
For DO, an invitation to bid will be advertised this month to construct an air injection DO system at the Mint Farm water treatment plant. Pending final project approval from the Department of Health, construction is expected to be complete by the end of this year. After completion, DO levels will be incrementally increased over a period of up to 9 months. Introduced slowly, DO will help harden existing pipe scale, reduce iron release, inhibit hydrogen sulfide reversion and formation of disinfection byproducts, and eventually temper our dependency on chlorine to maintain chemical stability in the distribution system.
Silica removal remains the more complicated issue. The white spots left behind when water evaporates consist of hardness (calcium and magnesium) and silica. Hardness can be wiped off but silica deposits can be difficult if not impossible to remove. To reduce spotting, a 75% reduction of silica is recommended to approximate the concentration customers previously experienced from the Cowlitz River. With that goal in mind, six treatment technologies were evaluated at bench test level to determine their effectiveness, cost, impacts and benefits. Using a triple bottom line evaluation approach, the better options for silica removal were narrowed to electrocoagulation, precipitation, and reverse osmosis. ...
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