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Water solution or energy drain?

In the coming decade, water regulators aim to introduce desalination to the Tampa Bay area on a scale untried anywhere else in the United States.

If they succeed, the daily production of drinking water from the Gulf of Mexico could help revive lakes and wildlife areas drained by an urban system that demanded too much groundwater.

These environmental benefits would be offset by considerable costs and the prospect of greater air pollution.

A large desalination plant can burn trainloads of coal. At the scale planned for the Tampa Bay region, the conversion of seawater to drinking water would use enough electricity to light every home in a small city.

To the Southwest Florida Water Management District, known as Swiftmud, it is a question of balance. Is it better to continue draining rural wetlands, to the detriment of wildlife and local residents, or to develop an abundant source of water that costs more money and consumes more energy?

Mark Farrell, Swiftmud's assistant executive director, thinks that on balance, desalination makes sense for Tampa Bay residents. That's because the region's only major river and much of its local aquifer already are being tapped to supply 2-million consumers.

"If we had cheap groundwater and surface water which was plentiful, you would not give a high-energy proposal the time of day," he said.

But Swiftmud merely regulates water use. It doesn't deliver a drop. And when the region's major supplier weighs the benefits and drawbacks of desalination, the scale tilts the other way.

Bruce Kennedy, acting general manager of the West Coast Regional Water Supply Authority, says he doesn't consider desalination a "near-term" answer to regional water problems.

"I think it's something we've got to look at. I just don't think we're there yet," he said. "If it was so doable, you would see it operating."

For economic reasons, West Coast is reluctant to try tapping the Gulf of Mexico. "Desalination can easily be 25 times more expensive, principally because of the power used," Kennedy said.

Farrell says desalinated water clearly would cost more, but his price estimate is much lower. He figures it could be delivered at $6 to $8 per thousand gallons, compared to today's price of $1 to $2.

In its tentative plans for Tampa Bay's future, Swiftmud is depending on desalination to deliver a great deal of water at that price.

By 2005, it envisions coastal plants in Pinellas or Pasco County producing 20-million to 50-million gallons of drinking water daily from the Gulf of Mexico. The higher figure is almost half as much water as the entire West Coast system currently delivers to people in those counties.

Gallon for gallon, that production would use 15 to 20 times as much electricity as a conventional pumping and treatment system.

Salt is commonly separated from seawater by an energy-intensive method called reverse osmosis. Electricity is used to generate enough pressure to squeeze water molecules through a synthetic membrane, leaving the salt behind.

With today's technology, a reverse osmosis system delivering 20-million gallons of water daily would require a constant flow of at least 16 megawatts of electricity. For 50-million gallons, it would need 40 megawatts.

Forty megawatts is a lot of electricity _ less than the capacity of Florida Power Corp.'s largest plants, which generate hundreds of megawatts, but enough to meet the average needs of about 25,000 residential customers. In fossil fuel terms, it's enough to burn 11,000 tons of coal per month.

Farrell heads a national committee on desalination at the utility-sponsored Electric Power Research Institute. He says he is certain that seawater will look more attractive as other sources get scarce, and he expects California to join Florida on the list of desalinated water drinkers.

The timing will depend on "rainfall patterns, growth patterns and environmental issues," he said. "California is having some very pressing water issues."

Florida's energy office views desalination as a sensible water source for the future but would prefer to see it developed with alternative fuels.

Michael Ashworth, its conservation chief, said solar-powered desalination is a known technology that will decrease the electricity demands for coal and oil.

Worldwide, more than 5-billion gallons of desalinated water are consumed daily, mostly in the Middle East.

In the United States, hardly anyone drinks seawater.

Sanibel Island, Venice and several other coastal communities in Florida pump and desalinate brackish water from the ground. This process uses much less energy, because brackish water contains much less salt than seawater. It is also a limited source, and it contains low levels of radioactivity, which get concentrated in the byproduct.

Two U.S. cities have ocean desalination plants _ Key West and Santa Barbara, Calif. _ but neither is in use. Key West pipes its water from Dade County, and the Santa Barbara plant is on standby for droughts, Farrell said.

In the Tampa Bay area, water utilities will choose whether seawater is a source worth exploiting for future urban needs.

They may be forced to consider that choice as Swiftmud reduces their permits to pump groundwater from environmentally damaged areas. Other options: conservation, the treatment of sewer water for reuse and the collection of more rainwater.

Some local government officials have been pushing for another alternative: pipelines to water sources outside the Tampa Bay area. Swiftmud opposes that alternative until all local sources are exhausted.

If utilities do turn to the Gulf of Mexico, the first gallon of desalinated water is still years away from your faucet. Farrell estimates the permitting and construction of a large desalination plant would take three to four years.

Desalination, even on a large scale, would not affect construction plans at Florida Power Corp., spokesman Will Rodgers said.

"It wouldn't require us to build a new plant. It would increase our load," he said.

How the desalination process works

1 Wells 700 to 900 feet deep bring brackish (somewhat salty) water to the surface. A small portion is set aside; a scale inhibitor and sulfuric acid are added to the rest.

2 The water then passes through a carbon fiber micron filter, which removes any particles larger than a micron. A human hair is about 70 microns wide.

3 After passing through a high pressure pump the water is forced through a reverse osmosis membrane that draws out the salt. The potable water and remaining brine solution then separate and go different routes.

4 The brackish water that was set aside earlier is now combined with the potable water to make it a little less soft. The water then goes to clearwell holding tank where chlorine and sodium hydroxide are added and hydrogen sulfide is removed.

5 The water is pumped into storage tanks and then to customers.

A The solution that is left over from the reverse osmosis process is treated with oxygen and chemicals. The hydrogen sulfide is removed and the brine treated.

B The brine passes through a sump or holding tank before being pumped into the gulf.