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. Options eliminated from consideration include lime softening, ion exchange, and adsorption using activated alumina.
A 20-year life cycle cost was developed for each of the alternatives based on annualized capital investment and annual expenses for operation and maintenance. The potential rate impact to customers was determined by the life cycle cost spread amongst all customers, and is reported as monthly cost per Equivalent Residential Units (ERU) where one ERU is defined as the average amount of water consumed by a single family residence in one day. With capital costs ranging from approximately $20-28 million, and annual O&M cost on the order of $1-1.5 million, the estimated monthly cost per ERU for the three short listed options is as follows:
$12.23/mo for Precipitation
$12.32/mo for Electrocoagulation
$15.41/mo for Reverse Osmosis
Any of these monthly costs would be in addition to the existing Mint Farm water treatment plant costs included in a customer’s water bill.
Any selected silica removal option will require additional testing for regulatory approval and to prevent unintended consequences. Staff projects it will take 24-30 months to pilot, design, permit, construct and implement any of the options. Each treatment option presents inherent advantages and disadvantages which translate to differences in water quality, environmental, operability, and distribution system impacts. In addition to cost and rate impacts, the decision making process should carefully consider all of these factors, as well as the no-action alternative not to install silica removal treatment.
In accordance with the terms of the Participant’s Agreement between the City and Beacon Hill Water and Sewer District (BHWSD), BHWSD own approximately 15% of the Mint Farm Regional Water Treatment Plant and BHWSD board of commissioner concurrent is necessary to construct additional treatment processes. The BHWSD board is scheduled to consider silica removal options at their meeting on June 21. Depending on BHWSD board action at that meeting, a joint meeting between the city council and BHWSD board may be prudent to mutually discuss this treatment decision.
FINANCIAL SUMMARY:
No funds have been allocated for further evaluation or constructing and operating silica removal treatment. Because the Mint Farm water treatment plant produces potable water meeting all federal and state regulations, it is unlikely this project would be awarded grants or low interest loans. All costs would be paid by City and BHWSD water utility customers.
RECOMMENDED ACTION:
Provide direction to staff regarding silica removal; some options require BHWSD concurrence. Options include:
Discontinue further action - do not remove silica
Schedule joint meeting with BHWSD board to discuss silica removal
Proof one or more alternatives
Select alternative and begin design/construction
Solicit customer opinions
Other
STAFF CONTACT:
Jeff Cameron, Public Works Director