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Wastewater from the first tank trickles through a gravity pipe into the second. There the nutrient-enriched phytoplankton are devoured by 300 inch-long blue tilapia fish, an African fresh-water species that can grow to twelve inches and survive in brackish water.
"As they graze and grow, the fish are accumulating nitrogen and phosphorus in their protein tissue," LaPointe notes. "On a large scale, we are potentially talking about a new industry for parts of South Florida. We're talking about transforming what are now thought of as pollutants into commercially valuable fish protein. Fish harvests around the world are being depleted due to overfishing and pollution, and at the same time demand for fish is soaring. Even if we couldn't eat them as humans, the fish could certainly be used for other sources of protein. Cat food is one. If there were some public health reason or a reason humans found it unpalatable to eat fish raised on sewage, there are other, international markets for high-quality protein, which is what these fish are."
Water trickles into the third tank, carrying with it what little nutrients haven't been eaten by the algae or the fish. Tank number three is covered over with thick, black plastic, shielding it from the sun. Protruding from the plastic are water lettuce plants, whose roots extend down into the tank. Beneath the surface of the water are several pounds of plastic substrate -- rings of plastic lattice material that act as a skeleton on which anaerobic microbes roost while they digest much of the remaining nutrients. More nitrogen and phosphorus are absorbed by the plants, which like the fish must be periodically harvested and could then be used for compost or to produce methane gas.
"The remaining amount of that redigested nutrient is then removed to the final, fourth stage, which is a 'polishing pond,'" says LaPointe. "We've tried a variety of different plants. You can use submerged plants such as hydrilla. We now have pennywort and duckweed growing in here. We've already achieved 90 percent removal by this point. When I say polishing, I mean that these plants are pulling out the last little five percent of nutrients -- virtually down to detection limit, which happen to be the levels in which coral reefs thrive. You can see that the water is crystal clear. It smells great."
LaPointe envisions a large version of his invention on each of the Florida Keys. Comparatively inexpensive networks of sewer pipes (smaller and shallower than conventional sewers) would be used to link the existing septic tanks of homes and businesses to the system. Instead of allowing septic tank effluent to percolate into the ground, small pumps in each septic tank would send the fluid into the first of a series of treatment ponds. From there the system would function essentially the same as the Pigeon Key prototype.
Besides removing nutrients from sewage, LaPointe says his treatment system will also eliminate most pathogens, such as fecal coliform bacteria. While conventional treatment plants process sewage in the course of 18 to 24 hours, the Pigeon Key system has a much longer detention time -- about ten days. During this time, the wastewater in the system goes through several diurnal temperature swings. The nocturnal cooling of the pools lowers the water's acidity, while the increased tempo of daytime chemical events raises it. What viruses and bacteria aren't killed off by this oscillation will be done in by the intense ultraviolet radiation beating down into all but the third tank.
Kevin Sherman, the HRS official who approved Florida's first commercial application of LaPointe's solar aquatic technology, says he isn't particularly concerned about the invention's ability to kill pathogens. Almost anything would be an improvement. "We know what the effectiveness of two feet of soil is in removing microbiological pathogens," he notes. "We feel that Dr. LaPointe's system will do at least as well as what exists now."
R.J. Helbling, branch office manager for the Florida Department of Environmental Regulation in Marathon, worries that LaPointe's system will take up too much land. He points out that about 70 cities and towns in the United States use some version of so-called constructed wetlands technology to treat wastewater that has already been partially treated by conventional sewage plants. (Another 400 household and small commercial arrangements are in use, according to the Environmental Protection Agency, including one at the Vermont corporate headquarters of Ben and Jerry's Ice Cream, and another at the home of Garfield cartoonist Jim Davis.) These systems, which often use low-maintenance marshes filled with cattails, water hyacinths, and other hearty weeds to remove nutrients, all depend on lots of acreage. LaPointe says Helbling's skepticism would be valid almost anywhere north of the Keys. But South Florida's near-tropical sunlight, LaPointe says, maintains temperatures, and hence reaction times for nutrient uptake, high enough to keep the land requirements reasonable.
Jim Kreissl, an environmental engineer at the EPA's research lab in Cincinnati, says the partnership between LaPointe and developer Bill Cobb represents the first time in America that a solar aquatic system has been built without government assistance, and in a real-world setting. (Cobb's project on Upper Sugarloaf Key calls for eight elegant Conch-style homes that will sell for between $400,000 and $2 million. LaPointe's innovative sewage system is just one element in an environmentally conscious design that includes a ban on power boats and a decision not to pave the development's roads.) While hopeful for the success of the project, Kreissl wonders if the sewage-treatment system will require perpetual monitoring by a Ph.D. in algal physiology.