KILL DEVIL HILLS - With demand for water increasing as the drought and growth continue, some coastal counties in Eastern North Carolina are tapping a saltier source: rivers of brackish water that flow underground.

Pasquotank and Currituck counties awarded contracts last month to start construction of two water treatment works that will eventually produce a combined 6.5 million gallons per day -- enough to slake the thirst of a town the size of Goldsboro.

They'll produce water by filtering salt from brackish water drawn from deep wells. That will bolster existing supplies of fresh water and help meet the need for more water in the growing communities.

"We're having to go to lesser-quality source waters, including brackish water on our coastal areas," said Fred Hill, a regional supervisor with the state's Public Water Supply Section. "We're getting a lot of demand for development, and people are expecting higher-quality water."

North Carolina already has about a dozen water plants on the coast that remove salt from water using a process called reverse osmosis -- and at least five more are planned. Pumps force brackish water under pressure through a series of fine filters to remove salt.

The process produces high-quality water, experts say, but technology won't fill everybody's glass. The process is typically more costly than conventional treatment of fresh water, and there are environmental concerns about discharging concentrated salt into estuaries or fresh water.

Reverse osmosis can treat ocean water. But the plants in North Carolina typically start with brackish water, which is less salty and easier and cheaper to treat. The existing plants collectively can produce up to 14 million gallons a day -- still a small fraction of the roughly 50 million gallons a day purified by Raleigh's water plant.

Sorry, Raleigh

As the drought persists, some have asked why inland cities such as Raleigh don't look to the coastline as a water source. But that's not realistic, given the high cost of treating and piping the water.

"Reverse osmosis plants are a good option for affluent coastal communities where people have expensive homes, and paying $100 a month for water is not that big a deal," said Bill Holman, a visiting scholar at Duke University's Nicholas Institute for Environmental Policy Solutions who is studying state water resources. "The idea of treating large quantities of ocean water and pumping them 100 miles uphill does not seem economically feasible."

The new $17.5 million Pasquotank plant, which will initially produce 2 million gallons per day, will supplement an existing freshwater treatment plant that is fed by 30 freshwater wells. The brackish water is so much more plentiful than fresh water that one well will produce as much flow as eight to 10 wells in the shallow freshwater aquifer.

"We were having to put so many wells around the county that we started looking at other sources," said Randy Keaton, the Pasquotank County manager. "In our area of the state, if you drill very deep, you hit salt water."

Production and treatment costs vary greatly among water systems.

William Koros, a professor in the School of Chemical and Biomolecular Engineering at Georgia Tech, said the cost to consumers of conventional water treatment ranges from 90 cents to $2.50 per 1,000 gallons, citing a report by the National Research Council. By comparison, the report put the cost of treating brackish water at $1.50 to $3 per 1,000 gallons. The cost of treating sea water is $3 to $8 per 1,000 gallons.

The state's largest reverse osmosis plant, in Kill Devil Hills, can produce up to 5 million gallons per day. It's one of four Dare County desalination plants.

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