Asheville’s water turbidity improves but potable service likely still weeks out • Asheville Watchdog

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Asheville water customers are likely weeks away from potable water, despite water-stilling curtains placed in North Fork Reservoir, an ongoing second coagulation treatment and an improvement in turbidity.

Asheville Water Resources spokesperson Clay Chandler gave an update on the water system Thursday, noting that the critical turbidity measurement had dropped to 21.2 Nephelometric Turbidity Units (NTUs), down from 21.9 at the end of Tuesday. Despite the progress, turbidity is still far from the 1.5-2.0 level the city needs to be able to treat the water.

Chandler said the second treatment will wrap up this week and data should be available over the weekend.

Even when the reservoir reaches an acceptable turbidity level, it could be weeks before the city is pumping potable water into homes. The entire city system remains on a “boil water notice,” meaning the water is safe for flushing toilets and taking showers but not for consumption, because the city is essentially distributing chlorinated lake water to its 63,000 customers.

“Let’s say we’re able to start pushing potable water one day next week,” Chandler said, noting that would kick off a “flushing process” throughout the system. That plan has to be submitted to the EPA and the North Carolina Department of Environmental Quality, more for their awareness than approval.

Then flushing would begin.

“That process will take two and a half to three weeks to replace every single tank and primary transmission line and auxiliary transmission line with potable water,” Chandler said, marking the first time since Helene he has given a timeframe for restoration.

Chandler noted that North Fork Reservoir normally pushes out about 20 million gallons of water a day and the city’s Mills River treatment plant about 5 million. “That flushing plan that we will implement once the water is treatable is going to require a minimum of 27 million gallons a day. And so you’re talking about some pretty serious water pressure through our lines, pumping into our tanks.”

When that happens, residents will see a lot of open, flowing fire hydrants to accommodate the flushing.

“So that flushing plan and the back-end testing that we will have to do to make sure the water is potable in every part of the distribution system will take between two and a half to three weeks,” Chandler said.

That would put delivery of potable water well into November.

Chandler said contractors on Wednesday “dosed” two of the four zones near the North Fork Reservoir curtains with aluminum sulfate, the coagulant, and caustic soda for regulating the pH. They were to dose the other two zones outside the curtains and the area inside, on Thursday.

The contractors put in 1,200 gallons of the mineral solution Wednesday and planned to do the same amount Thursday. Workers finished anchoring the curtains Wednesday.

High winds disturbed a previous aluminum sulfate treatment in mid-October, but conditions were perfect this week for the second round, Chandler said. The weather was overcast and calm, and the reflection of the mountains in the water was still visible, which Chandler noted was “a very, very good sign.

“That’s a sign that the water is starting to clear up,” Chandler said, noting that turbidity is dropping six to seven tenths of a point every day. The city hopes “that the treatment process will speed that drop up.”

The minerals help the clay particles in the water coagulate into “chunks big enough to sink to the bottom,” Chandler said.

The EPA requires filter water to be less than .30 NTU for consumption. Normal filtered water from North Fork is between .03 and .05 NTU, meaning it’s “typically exponentially cleaner than required,” Chandler said previously.

Typical raw water coming into the treatment plant from North Fork has a NTU of 1 or less. As of Oct. 11, it stood at 30 NTU.

Asked if any other cities had tried the curtains/minerals treatment approach, Chandler said Centerville, Minnesota, is using it in its drinking water reservoir. That city has not yet posted results, he said.

Chandler noted that the aluminum sulfate/caustic soda coagulation process has been in use for 60 years.

Suspended clay particles in reservoirs can be extremely difficult to clear, according to two outside experts Asheville Watchdog consulted. Helene’s torrential rain, coupled with heavy downpours preceding that, caused North Fork to fill with sediment, giving the normally clear water a chocolate milk consistency.

Desmond Lawler, a professor in the Civil Engineering Department at the University of Texas, said the small size of the reservoir’s particles, as described by the city, along with their flatness, “makes them settle very slowly.”

“And yes, the negative charge on their surfaces makes them repel one another so that they don’t form flocs, get bigger and settle faster,” Lawler said.

With a “direct filtration” system at North Fork, the city cannot filter water with a high turbidity.

“If the reservoir usually had a turbidity of 10 NTU or higher, the plant would be designed with an extra few large tanks to do the alum treatment which would ‘destabilize’ the particles,” Lawler said. “That is, it would overcome the problem of the electrostatic repulsion between the negatively charged particles, allow them to flocculate and get much bigger, and then settle in another tank.”

That would drop the turbidity to around 1 to 2 NTU, “and the filters would do the rest,” he said.

“What the city is trying to do with the curtains and aluminum treatment in the reservoir is what would happen in the treatment plant that I describe above — the one that would exist if the reservoir turbidity was always in the range it is now,” Lawler continued. “But, nothing — the flow, the flow pattern, the ability to to mix the alum in, and probably several other things — is as controlled in the reservoir as it would be in an engineered system.”

The upshot, Lawler said, is that the city appears to be doing everything right, although the curtain system “is a little hit and miss.

“The only thing that perhaps could be questioned is how they are deciding how much alum to use, and how they are adding it to the water,” Lawler said. “They got a little unlucky with the wind.”

William Becker, a vice president with the Hazen & Sawyer engineering firm, works primarily in Colorado and New York state. With these kinds of clay particles, he said, coagulation treatments can work, but it’s still “a wait and let it settle” scenario.

“Clay is particularly difficult because it does depend on the type of clay, but the old glacial tills that were put down eons ago are often very finely divided materials, very small colloidal type particles,” Becker said. “And they just don’t settle, because they are so small and they need to be coagulated out in order to get them to settle.”

Becker cited one example that residents, now in month two without potable water, probably don’t want to hear.

“New York City has a reservoir that goes months, sometimes, before the turbidity comes back down again — several months, six months, eight months, 10 months — because it just sits there,” Becker added. “It won’t coagulate, because it is negatively charged, and so it won’t agglomerate into larger particles that can then settle out.”

The mineral treatments in Asheville, coupled with the curtain approach, could result in significant drops in turbidity, though, Becker said. The key is keeping the water’s acidity at the proper level so the aluminum sulfate can work most effectively, Becker said.

If the proper levels are hit, “it’ll work extremely fast,” Becker said, quickly adding that in a lake with the depth of North Fork (over 100 feet in place), “It’s hard to do.”

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