Past and Present Converge in Charleston's Capital Improvement Program

With steady population growth as its driving force, the Charleston (South Carolina) Water System forges ahead on its many capital improvement projects

Past and Present Converge in Charleston's Capital Improvement Program

Work being done to replace a 24-inch water line on a capital improvement project. (Photo by Ken Osburn)

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The Charleston (South Carolina) Water System (CWS) has an historic past. The first cement-lined grey iron pipe was installed there by American Cast Iron Pipe in 1922. Its Hanahan Water Treatment Plant is built on the site of a steam pumping station that dates back to 1903. Its first water supply was an 1879 artesian well.

Today, a well planned capital improvements program is helping the utility keep up with growth in the Charleston area, and rehabilitate critical infrastructure.

Planning for the future

Kin Hill, utility CEO, says the influx of people moving to the service area has averaged as high as 38 people a day. “It’s been unprecedented growth.”

The major share of it is coming in the outlying areas of Dorcester and Berkeley counties, with current populations of 156,000 and 217,000 respectively. About 410,000 people live in Charleston County.

“We added 2,000 water taps and 500 sewer taps last year,” he says. “Our water service is two to three times larger than it used to be, and we’re now providing wholesale water to nine different accounts.”

Capital projects officer Mark Cline points out that the rising population has resulted in high-density building and many high-rise buildings, which have put additional strain on the utility’s older water and wastewater infrastructure.

Capital projects

In order to ensure available capacity to meet water demands associated with growth and fire protection, as well as projected growth-related wastewater flows, CWS is committed to making improvements to its treatment plants and the distribution and collections systems.

The current CIP investments total $98 million for drinking water improvements and $162 million for wastewater.

While many of the capital projects are associated with roadway improvements and pipeline and facility replacements, a number of them are extensive and involve significant funding and advance planning. For instance:

• At the Hanahan Water Treatment Plant, Charleston Water is replacing older, space-consuming sedimentation basins with new high-rate plate settlers with integrated flocculation cells. The supplier is MRI, and the cost is $50.4 million.

• The water plant is switching from vertical mixers to paddle wheel mixers, and also changing its chemical feed to improve flocculation.

• Twenty filters in the Storey Filtration Plant are being rehabilitated in an $11.3 million project.  Granular media has been changed out, new underdrains installed (Phoenix underdrains from AWI) and the old air-scour system has been replaced with a badly needed air- and water-scour system.

• The Ashley River Transmission Line Crossing is another major project, replacing a deteriorating 24-inch water line that had been abandoned. Nearly 4,700 feet of new 36-inch prestressed concrete pipe was laid under the river, using horizontal directional drilling by the Mears Group.

• In addition to running new lines to service areas to improve service, Charleston water also cleans and rehabilitates older, existing piping. In the downtown area, some of the cast iron pipes date to the late 1800s. “Many of these lines are structurally OK,” says Hill. “We scrape the inside walls clean and the reline them with cementitious grout, using rotary spray head and trowel technology.

• At the Plum Island wastewater plant, a $70 million project is improving process controls and the SCADA system, rebuilding the old primary and secondary sedimentation basins, and adding an anoxic selector zone to cope with increased ammonia flows into the plant.

• The network of deep tunnels carrying wastewater to the Plum Island plant is being replaced. Costing over $200 million, the project is now in its fifth of six phases, which involved digging a 1.6-mile long, 120-foot-deep tunnel, 8 feet in diameter, using the caisson method of construction to maintain a dry environment for workers. The new tunnel will have a capacity of 60 mgd.

• A new 75 gpm pump station is also part of the project. 430-hp Flygt dry submersible pumps rated at 15 mgd are being used to lift tunnel contents to the plant.

Work being done to replace a 24-inch water line on a capital improvement project. (Photo By Ken Osburn)
Work being done to replace a 24-inch water line on a capital improvement project. (Photo By Ken Osburn)

The planning behind these projects is comprehensive.  “We master-plan our entire system,” says capital projects officer Cline. “We operate on a 25-year cycle, with four- or five-year phases. We work with our staff and our consultants, and look at needs based on historical growth projections as well as replacement of existing assets.

“Then we vet and prioritize each project — core projects as well as priority repair projects.”

The CIP plan and its projected funding is then taken to the city council for approval.

“We use revenue bonds, rate revenue including retained earnings, and impact fees to fund the projects,” Cline says.

CWS is an independent public utility responsible for supplying drinking water and treating wastewater for the city of Charleston and portions of neighboring Berkeley and Dorchester counties. The water service area covers 450 square miles and includes 1,800 miles of water mains, ranging in size from 1 and 2 inches, to 72 inches in diameter, and also includes 9,000 fire hydrants.

The Bushy Park Reservoir — owned by the utility and located in Berkeley County — and the Edisto River are the raw water sources. Treatment takes place at the Hanahan Water Plant, a 115.6 mgd-rated conventional surface water treatment facility. The plant has been in operation since 1917.

The utility treats wastewater from an area of 250 square miles and 56,000 accounts at its main 36 mgd treatment plant at Plum Island. Wastewater moves through a system of 900 miles of sewers boosted by 200 pumping stations. It enters the treatment plant through either the West Ashley Tunnel or the Harbor Tunnel — deep underground interceptors 100 to 125 feet below the surface.



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