In this site plan prepared by CRW Engineering Group for the City of Wrangell, the municipal water treatment plant would see some new additions to its facility if converted. The dissolved air floatation-based facility being considered would require expansion of the roughing filter building and the addition of new tanks and storage. The current sand filtration units would be converted into additional storage for treated water, doubling the plant's current holding potential.
Wrangell appears to be closing in on a solution to its water filtration worries after a workshop held between the City and Borough Assembly, city staff and consultants on Monday.
Since its construction in 1999 Wrangell's water treatment plant has had a number of problems with its production. The plant's operation involves ozonation of water sourced from two open reservoirs, which then passes through a roughing filter and four slow-sand filtration bays before disinfection.
From the start the plant did not work as planned, with sediment found in the source water supply contributing to poor roughing filter performance. The plant has subsequently seen less effective water treatment than first hoped, as well as higher than expected operating costs, and struggles to meet elevated demand during the summer and winter.
With two seafood processors and a fleet of cruise liners making call during the summer, July and August end up making up the water plant's highest demand for the year. But depending on how low temperatures get during the winter, personal use by residents – in this case letting water run freely to prevent pipes from freezing – can nearly match summer demand. Speaking at Monday's meeting, water plant manager Wayne McHolland reported demand had last week reached 1,100,000 gallons a day.
Public Works director Amber Al-Haddad added that other factors can play into overall use, including unidentified leaks at homes and within the distribution system itself. While the plant is designed to produce up to 1.3 million gallons a day, keeping the plant's sand and roughing filters clear of sediment and other maintenance concerns can reduce that significantly.
Also worrisome is the plant's diminished ability to remove organics through its ozonation and filtration processes. Consistently high flow volumes are not allowing enough organics to be removed, on the one hand contributing to rapid clogging of the sand filters. CRW reported these then have to be scraped on a more regular basis than designed, which in turn inhibits development of a biofilter of the top layer of sand where primary biological treatment should occur. This effectively makes it more difficult for Wrangell's water to meet state and federal quality standards, and can increase the risk of health-related incidents with municipal drinking water.
For the past three years the city has been working with CRW Engineering Group to explore alternatives to the aging plant. After the Assembly approved a pilot study back in June 2015, the firm recommended an alternative plant model primarily based on dissolved air flotation (DAF), which removes unwanted sediment and impurities from the water using tiny bubbles.
At the start of this process, aluminum chlorohydrate is introduced into water drawn from the reservoir to act as a coagulant, bonding with dissolved organic and colloidal material in the water while also neutralizing its acidity. This water is then saturated with dissolved air in another tank, with bubbles floating impurities to the surface for skimming. Water would then be run through multimedia filtration before storage and distribution.
A package plant of this type was brought down by engineers in July 2016 for an eight-week pilot study. The method was found to be quite effective for Wrangell's water source.
Just prior to CRW installing the miniature DAF plant on site, an emergency water shortage prompted the city to issue a declaration of disaster. Residents were requested to cut back water consumption by 30 to 50 percent, and seafood production saw disruptions over the following month.
As a result, improving or else replacing the water treatment plant has taken the forefront among city projects, with capital items outside of Shoemaker Bay Harbor and Public Safety Building refurbishments being sidelined for the present. CRW was retained to consult with the city on its water treatment production issues and explore other rehabilitation options for the plant. The firm also prepared engineering reports necessary for financing applications.
While at first the Assembly was dismayed by the projected cost of building a new DAF plant – at an earlier stage estimated at around $13M – the cost of improving existing facilities has since been put at greater than $15.6M. CRW has estimated $10.9M of this cost toward upgrading treatment processes themselves, with $3.87M going to increase water storage capacity. Water storage is limited to just over 700,000 gallons at present, and limited capacity played a factor in 2016's production shortage.
CRW has since brought down construction estimates for a new DAF plant. Making use of some existing infrastructure and converting the current sand filtration bays into another 800,000 gallons of storage, a new plant is estimated to cost in the neighborhood of $8.3M with another $860,000 needed to capture backwash from the filtration process. The lower overall cost and greater usefulness the plant would offer over refurbishing the old led CRW and city staff to recommend its construction.
To fund these upgrades, the borough would plan to use a combination of sources. The Department of Agriculture (USDA) has offered $6.98M for the project, comprising a $3.82M loan and $3.16M grant, and is based on CRW's engineering report it submitted for Wrangell last March. Another $450,000 is being sought from the Alaska Department of Environmental Conservation through its drinking water fund, while $2.5M would be applied for from the Alaska Economic Development Administration. Money available through the borough water fund reserves stand at $458,000.
Participating in the meeting by phone was Jon Hermon, CRW operating officer in Palmer. He explained the DAF filtration method would be better suited to the sediment found in Wrangell's water, which has a propensity to float that makes gravity-dependent processing methods less effective. Though the technology behind the DAF plant has been around for half a century, Hermon said it was not in wide use domestically, though it has a good track record in Canada and Europe. For Alaska it would be downright novel.
"As far as we know this will be the first, at least for its size," he said. Hermon estimated the plant would have a life of 20 to 30 years, though some components would likely need to be replaced in the meantime.
City manager Lisa Von Bargen explained official action on the part of the Assembly would be needed to proceed from here. Financial particulars on loan repayment still need to be worked out, but a plan should be ready for members to consider by their February 20 meeting.
Intent is to get something before the Assembly no later than end of month
Looking ahead to the coming summer, Al-Haddad said Public Works would try to replicate its strategy last year, hiring on additional workers to expedite facility upkeep ahead of the summer peak.
"We had great success with that last year," she said. "Our plan would be to move forward, hire the additional help by May."
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