HANAHAN — When a raindrop falls north of Hickory, N.C., it could eventually end up in a glass filled from a Charleston faucet.

The most intense part of the drop's 250-mile journey would be the last few miles, when it passes through the vast water treatment plant here.

Most people will never venture beyond the Hanahan Water Treatment Plant's security gate to see its monumental array of settler basins, filters, clearwells and other support buildings.

And most have little clue about the $42 million upgrade that's underway, a project designed to ensure the Charleston Water System's supply of potable water can continue to be taken for granted.

The plant is rarely opened for tours, partly because it was never designed for that and partly because of safety issues, such as the potential hazard from having tons of chlorine stored on site.

Jane Byrne, director of the water treatment plant, notes a release of chlorine gas is possible but quickly adds, "It's never happened."

The multi-year project has remained mostly on schedule and on budget, though not without some twists, including one that almost led to Charleston Water System's first boil-water advisory in memory.

Getting from light brown to clear

The complicated treatment process begins just underneath a circular area smaller than a basketball court.

Deep below is where the raw water from the utility's different sources enters the plant. While the campus is dominated by assorted buildings and raised tanks, most of the network of pipes are underground, out of sight.

"It's like spaghetti under here," Byrne said. "There's a lot going on."

From this circular spot, raw water is pumped uphill to a 2-story-tall rapid mix basin where large paddles mix it with alum, a compound that binds the raw water's small particles into larger chunks.

That water then is sent to one of several flocculation and sedimentation basins where these solid chunks settle out and get removed. Replacing the oldest basins with newer ones is where most of the $42 million is being spent.

Two of these basins are more than a century old, dating back to before the city of Charleston took over this operation and created its Commissioners of Public Works — now the Charleston Water System. Each basin covers more than a football field, an area so large that white caps often appear on the water during high winds.

That choppy surface doesn't help solids settle out, but it's not necessarily the basins' biggest drawback. Each is difficult to clean and must be taken offline regularly so a worker driving a Bobcat can scrape the pluff mud-like accumulation off the bottom.

"You don't want to walk through there," Charleston Water System Assistant CEO Mark Cline said.

The newer basins essentially clean themselves. They feature metal plates resembling large metallic louvered shutters. The sediment forms on them, sloughs to the bottom and is removed by a sweep. Each basin can handle more flow in just a fraction of the footprint.

The water entering these basins is colored a light brown, but it's clear when it leaves, Cline said. It proceeds next to filters — basins with sand, gravel and anthracyte.

The current work also includes modifications to a clearwell, where treated water mingles with chlorine solution to completely disinfect it. The work also will upgrade the plant's chemical feed, new piping for treated water and improvements to the site where solids removed during treatment are dried out before being hauled off site.

For PC Construction Co. of Vermont, which is doing the work, a big challenge is ensuring that the plant can continue to operate successfully as large pieces of it are rebuilt or replaced.

"We're all very much aware that you don't turn the water off," Byrne said.

'The bottom of the hill' 

The treatment plant was first built at the dawn of the 20th century, when Charleston began replacing its artesian wells downtown with water piped down from the Goose Creek Reservoir.

Eventually, both the plant and the municipality that incorporated around it would be named after J. Ross Hanahan, a Charleston businessman who helped the city negotiate the purchase of the water system from a private company. 

Over time, the plant's freshwater supply would be augmented by a new tunnel that linked it to the Edisto River, around Givhans Ferry, then ultimately to the Bushy Park Reservoir, a mid-20th century engineering triumph that gave Charleston access to the Santee Watershed, the second largest one of the East Coast — behind only the Susquehanna in Pennsylvania.

The utility still draws about 10 percent of its water from the Edisto — enough to keep that source active should a problem ever develop with Bushy Park. As other states and metro areas increasingly wrestle with water shortages, Charleston has little to fret about it.

"We've never had an issue with droughts," said Charleston Water System CEO Kin Hill.

It didn't even call for voluntary water restrictions during the multi-year drought that began in 2007.

"We're at the bottom of the hill," he said.

The utility doesn't draw much of anything from the original Goose Creek Reservoir anymore, partly because of its relatively smaller size, said Andy Fairey, the system's chief operating officer.

If it did, Fairey said, "everybody who lives on that lake would be upset with us because they wouldn't live on a lake anymore."

Charleston's relatively rich glut of potable water, made possible by the creation of the Bushy Park Reservoir, even was featured in the Jan. 6, 1951, issue of The Saturday Evening Post.

"A lucky freak of geography near Charleston, S.C., is responsible for a new engineering project that's going to give our water-short land a huge reservoir — and incredibly small cost — yielding Ten Billion Gallons a Day," it declared. "Nothing even approximating it exists at any other deepwater Atlantic port."

More capacity? No problem

Just because Charleston has access to billions of gallons a day doesn't mean it can treat that amount and safely distribute it.

Most of the current work will increase the plant's reliability, which ultimately will make future expansion easier.

"While we're not increasing capacity today, this sets us up to increase capacity," said Capital Projects Officer Russell Huggins.

Hanahan's current capacity, as rated by the state, is 115.4 million gallons a day, but its average output is about half that, around 58 million to 60 million gallons a day. In the summer, daily demand reaches about 78 million gallons a day. Its record output was more than 102 gallons after a 1989 freeze burst pipes across the region.

A big hiccup

While the $42 million plant upgrade is designed to make the Hanahan plant more reliable, the recent construction actually led to a close call.

Minutes before several top officers with Charleston Water System were being interviewed for this story in late November, a contractor accidentally backed over a 14-inch PVC line and broke it. The line fed chlorine into the water at the end of the treatment process, a disinfectant step required by law.

A different dig in that same general area previously unearthed an old wooden waterline that utility officials had not known was there.

Some officers left the November interview to confer with state health officials. They knew it was possible that the state would require a boil-water advisory, urging the utility's customers to boil any tap water before drinking it or cooking with it.

"The question will be how quickly can we get it back in service," Hill said at the time. "Hopefully, they can get it hooked back quickly."

That turned out to be the case: The rupture happened around 8:30 a.m. and was repaired by 3:40 p.m. No advisory was issued.

Had it been, though, approximately 350,000 customers would have been alerted not only in the Charleston Water System's service area but also to other utilities — including Dorchester County, the Isle of Palms, Folly Beach and the St. John Water Co. — that buy water wholesale from CWS. (Mount Pleasant Waterworks cut off its feed from Charleston shortly after the problem arose and would have been in the clear.)

Less than 1 percent of the water leaving the treatment plant and passing through some of the system's 1,700 miles of pipes actually ends up in a drinking glass or a cooking pot, Byrne said.

The vast majority is used to flush toilets, wash clothes and dishes, fight fires, water lawns and the like.

But those running the plant never know which drop goes where.

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Reach Robert Behre at 843-937-5771. Follow him on Twitter @RobertFBehre.