Water plant manager Wayne McHolland explains how the dissolved air flotation filter works. Water is treated by floating impurities to the surface with compressed air, which is then skimmed for disposal.
In a side-by-side comparison of water quality, at the right, the brackish bucket of water from Wrangells reservoir has a UVT rating of about 40. The middle bucket contains the citys current tap water, which still has a slight coloration compared to the water at left, which was filtered through the DAF test plant.
A pilot study currently underway seems to be bearing good news for Wrangells water worries.
In mid-July the city declared a state of emergency as its water treatment plant struggled to meet local demand. An appeal to residents and local seafood processors to limit water usage followed, allowing Public Works time to replenish its reserve tanks. By August 18 City Hall declared the crisis over, but still encouraged people to conserve water.
The problem was primarily with the plants water filtration system, which pipes water from the public reservoir through roughing filters before being treated in slow sand filtration chambers. Since the plant was first installed nearly two decades ago, it was found to be ill-suited for the citys water sources.
Water comes from two open reservoirs, drawing from the lower of them. The water being used has considerable sedimentation, with organic material and other detritus having to be filtered out. The citys sand filters are designed for cleaner water coming in, so this material tends to build up more quickly. As a result the four filters need to be changed more frequently than designed – every four or five days instead of quarterly – slowing down production in the process.
Aware of these shortcomings, the city has already begun looking for an alternative to its plant. Last summer the Assembly awarded CRW Engineering Group with a $149,306 contract to undertake a pilot study, paid for through a Department of Commerce, Community, and Economic Development grant.
Borough Manager Jeff Jabusch explained a lot of preparatory work went into the project in order to match up Wrangells water supply with the most appropriate technology. CRW offered several alternatives, but settled on a plant utilizing dissolved air flotation (DAF). A package plant was brought down by engineers in mid-July to undergo a pilot study on site, which wraps up later this month.
The DAF method removes unwanted sediment and impurities from the water using tiny bubbles. In this system, aluminum chlorohydrate is introduced into water drawn from the reservoir. The highly-charged compound acts as a coagulant, bonding with dissolved organic and colloidal material in the water while also neutralizing its acidity.
This water then passes into another tank, where it is saturated with dissolved air. Bubbles float the foreign material to the surface, and a mechanized device skims these collected impurities from the top. The skimmed impurities would be collected and likely landfilled, McHolland added. The state Department of Environmental Conservation has protocols for proper disposal which the city would have to follow.
From there the water is run through another series of coal filters before heading to the slow sand filter. Water plant manager Wayne McHolland explained the water at this stage is significantly clearer than what the city currently has, which should greatly improve the latter systems efficiency.
"There would still be sand filtration, but on a much smaller scale," he said.
One way to measure the waters clarity is through an ultraviolet transmittance test. UV light is passed through a water sample, and the proportion of waves making it through the substance unhindered taken as a percentage. For example, McHolland explained at this time of year water from the reservoir has a UV transmittance (UVT) rating of 39 or 40 percent, as its high sedimentation impedes most light waves from traveling from one end of the cylinder to the other.
By contrast, water run through the DAF process is measuring at better than 93 percent, which is about a 16-percent improvement over what the water plants current output is rating.
"Right now it looks really good," McHolland said. These measurements are done on site, but official measurements of the processed water are awaiting examination in a laboratory. If the findings are good, a DAF plant may be a possible direction the city will move toward in the future.
A full-sized plant could cost between $5,000,000 and $5,500,000, and would take two or more years to design and install. Jabusch said the plant had some inherent benefits which would ease the transition, being modular in design. Examining the site, he said CRW reported a new plant could likely be installed using existing facilities, where the plants roughing filters are currently located.
In the meantime, Public Works is looking into ways to increase the current plants efficiency. The engineer with CRW has helped draw up a plan to improve the roughing filters, and is now awaiting approval from the DEC to implement. The contractor has also suggested ways to safely clean the sand used in the filtration process, which becomes a workplace hazard when dry. Replacing the sand in the filters – not done since the systems installation – would help it process water more quickly, but the city estimates the cost to do so could run into the $500,000 range.
Even with a new plant, the city could still run into supply problems moving forward. As with last year, water levels in the reservoirs could fall below production levels. The lower reservoir is also in need of a dredging, something McHolland said the city has been unable to do because the plant draws its water from it.
A bypass line from the upper reservoir had been proposed in the past, but plans have subsequently fallen by the wayside. Jabusch explained the city has currently been focused on sewer pump station replacement, but a reservoir bypass would still be a goal.
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