s a naturalist, I know that filtering salt from seawater is not a novel idea. For hundreds of millions of years marine plants and animals have evolved unique methods of desalination. Salt glands discharge excess salt through the nostrils of marine iguanas, the eyes of sea turtles, and the tongues of crocodiles. The underside of the leaves of black mangrove trees exude pure salt crystals that glisten in the tropical sun; the spidery roots of red mangroves block salt from entering the tree. The gills of saltwater bony fish such as tuna or striped bass, the rectal glands of sharks and rays, and the super-kidneys of whales and seals perform a similar function.
I want to understand how desal works for humans, so I drop in on Ken Herd, 43, engineering and projects manager at Tampa Bay Water's Clearwater office complex. Tampa Bay uses a reverse osmosis (R.O.) membrane system, explains Herd, in which salt water is pushed at extreme pressure, up to a thousand pounds per square inch, through tiny pores, each 0.0001 micron in diameter -- approximately 1/1,000,000 the width of a human hair.
Osmosis, as you may recall from 10th-grade biology, is the tendency of a fluid to pass through a semipermeable membrane, such as the wall of a living cell, into a solution of higher concentration, to equalize concentrations on both sides of the membrane. Reverse osmosis is precisely... the reverse. The pores of the roughly 10,000 tightly rolled membranes are so small that ultratiny molecules of water pass through, but larger molecules of dissolved minerals like salt do not. Pressure forces out the salt, and the constant flow of water helps wash the outer membranes clean of concentrations of brine. R.O. membranes still clog, however, and have to be cleaned, every three weeks to six months or longer. The membranes last five to seven years, sometimes ten, and they are expensive to replace.
Herd shows me a model of a three-foot section of wound membrane. It looks like an oversize roll of paper towels, with the top cut away so that I can see inside. Salt water forced against the outside of the roll filters through the spiral until pure freshwater flows into the center port -- the equivalent of the cardboard tube inside the roll of paper towels -- and then out into a network of collecting pipes. The total surface area of the plant's 38-inch-wide membranes would cover nearly 65 football fields.
"However," says Herd, "R.O. is the simplest part." First, the bay water must be treated before it's forced across the R.O. membranes. Pretreatment filters out suspended solids -- such as scraps of seaweed, fish fry, aquatic larvae, sundry items of flotsam and jetsam. If this weren't done, the membranes would foul. "Pretreatment," says Herd, "is the challenging phase of desalination." Tampa Bay Water uses dual-stage sand filtration, in which incoming salt water flows up through two filtration cells, coarse- and fine-grained. Particulate matter larger than five microns in diameter that manages to pass through the sand filters gets caught in the cartridge filter -- a collection of long, thin filters, like those used in swimming pools, which act as the R.O. membranes' safety net.
In every performance test, both the pretreatment filters and the R.O. membranes clogged more frequently than expected, requiring additional cleaning. Increasing the strength of the cleaning solution for the membranes caused another unforeseen problem: Two million gallons of caustic, soapy cleaning fluid had to be transported to Tampa's wastewater treatment plant.
Asian green mussels turned out to be the culprit. The alien shellfish first appeared in Tampa Bay about eight years ago, having been transported in the ballast of tankers, and has thrived. Mussels love flowing, food-rich water, so the power plant's daily 1.4 billion gallons of effluent is bivalve utopia. Larvae pass through the power plant's intake screens, survive in the heated water, then clog the pretreatment filters, fouling the R.O. membranes with microscopic hairs.
The post-treatment phase also has its complications. Along with salt, alkaloids are stripped out of the water, leaving the desalinated water acidic and corrosive to pipes. So calcium carbonate (lime) is added during post-treatment, raising the pH level before the water is piped 14 miles to storage. All this trouble and delay has resulted in lost time and money. The desal plant has declared bankruptcy three times, most recently in October 2003. The plant is online only once a month, and Tampa Bay Water says it will not go into full production until 2006.
ontemplating the sprawl that surrounds the Apollo Beach plant, I find myself paraphrasing the line from the Shoeless Joe Jackson character in the movie Field of Dreams: "If you build it, they will come." Herd bristles a little at the phrase. "The government agency that allows growth supports its decision with electricity, drinking water, and waste removal. The water management district doesn't have the ability to limit growth; that's the job of the planning board. Tampa Bay Water just supports the growth that's already there."
Tampa Bay Water admittedly has taken significant steps to diversify its sources of potable water, and to do so in an environmentally responsible way. As of April 2004, the water authority was pumping only 74 million gallons a day from the ailing well fields, in hopes of restoring that corner of the Floridan Aquifer. As a result of these reductions, the surrounding wetlands have begun to recover -- lake levels are rising and marshland vegetation is looking fuller, more lush, a de Soto shade of green. "We didn't trade one environmental impact for another in Pasco County by shifting the burden to Tampa Bay," Herd says with justifiable pride.
Herd's optimism is refreshing. And he's right: It is not ultimately the water authority that determines the carrying capacity of a suburban landscape. Many of the 20 commercial seawater desalination projects under consideration for the Sun Belt are driven by planners who both forecast and encourage growth, often in ecologically sensitive coastal areas. Faced with lobbying by the U.S. Desalination Coalition, environmentalists will need to scrutinize each new project. For if new desal facilities mean that the wild hills become crowded with condos and the shorelines fill with sprawl, we may find ourselves echoing another line that's associated with the hero-turned-villain of the 1919 Black Sox scandal. We'll have built it, they'll have come, and like the distraught young fan, we'll be exclaiming, "Say it ain't so, Joe."