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Like Growing Fish in a Barrel

By Michael Valenti

There's something fishy about two demonstration projects being sponsored by the Electric Power Research Institute (EPRI) in Palo Alto, Calif.--highly efficient water-recirculation systems for aquaculture, or fish farming. These recirculation systems promise to yield greater harvests than traditional fish-farming methods but use less water and require much less acreage.

The systems can make fish farming feasible virtually anywhere, from abandoned waterfronts to unused barns. In addition, these systems are enclosed, so they can prevent pollution and infection from damaging the fish.

Aquaculture involves raising fish in an enclosure, usually a pond or tank, where food, warmth, oxygen, and light are regulated to breed fish faster and bigger than in natural conditions. Electrical utilities have become interested in promoting aquaculture as a way to put underused land and buildings to work and aid local economies. EPRI formed the Agricultural Technology Alliance, a consortium of electric utilities and the farming industry, to develop aquaculture modules that can be built anywhere.

One of the alliance's projects involves creating a system to recirculate water efficiently. "While a nonrecirculating aquaculture system typically produces 3,000 pounds of catfish per acre, a recirculating system yields 100,000 pounds per acre," said Myron Jones, manager of EPRI's agricultural research.

One of two EPRI-sponsored recirculating aquaculture systems is a cooperative effort between the Public Service Company of New Hampshire (PSNH) in Manchester and GreatBay Aquafarms in Newington, N.H. GreatBay remodeled an unused warehouse at PSNH's 300-megawatt oil-fired generating station in Newington, converting it into a hatchery for summer flounder, or fluke.

A portion of the water later used to cool the power plant is piped to the hatchery's 39 tanks, where flukes are hatched and grown in successive tanks to about 6 inches, when they are shipped to other aquafarms.

GreatBay is building a pilot grow-out facility in another PSNH warehouse, scheduled for completion this month. It will consist of two raceways, each measuring about 20 by 50 feet, where the flukes will be grown to their full 3 pounds. Effluent water from each raceway will be fed to a drum filter to remove wastes down to 40 microns in size, then pumped through an ultraviolet-light unit to kill bacteria.

A recirculation system yields greater fish harvests yet uses less water

In the first system, water will run through a fluidized bed of sand to provide the large surface area to aid the conversion of ammonia into nitrate by naturally occurring bacteria. In the second, the ultraviolet-treated water will be sent through a wastewater filter. "The filter contains a plastic medium and an air manifold to provide the surface area for bacterial conversion," said Greg Beckman, a marine biologist and fish culturist for GreatBay. In both systems, 90 percent of the water will be returned to the tanks during the day, with new water added. GreatBay will test the tanks, filters, and oxygen and pH probes before designing a 200-ton-per-year grow-out unit.

The other EPRI project is a tilapia farm being built by Carolina Power & Light Co. (CP&L) in Raleigh, N.C. "Our purpose in this project is to support local agriculture as a profitable alternative to tobacco growing," said Alex Hobbs, a CP&L research engineer who is managing the project.

The Raleigh fish farm, to be built in a barn, will contain two small nursery tanks holding 2,000 and 4,000 gallons of water. Young tilapia, a hearty, warm-water fish, will be put into these tanks for six weeks each before being put into four 15,000-gallon hatchery tanks. After 210 days, the tilapia will reach full size.

Water from the tanks will be treated by a recirculation system in which the tank water is drained through a swirl separator drain that removes wastes, then sent through a screen drum filter and a packed column trickling filter that converts ammonia to harmless nitrates. An oxygenation column adds oxygen to the water at the trickling-filter discharge so that replenished water can be pumped back into the tanks. "Adding oxygen at the discharge point lets us take advantage of the higher pressures available from the pumps and get two to three times as much oxygen into the water," Hobbs said.

As a utility, CP&L is interested in making the recirculation system--developed by Tom Losordo, an associate professor of aquaculture at North Carolina State University at Raleigh--as energy-efficient as possible. Using only one set of pumps to bring water from the biological-filter sump to the tanks is an important feature of the design.

By recycling its water, the CP&L/EPRI fish farm will consume less than 5,000 gallons of water per day. The facility is due for completion in the summer, with an annual target harvest of 100,000 pounds of tilapia. Once the plant demonstrates its commercial viability, it will be turned over to North Carolina State.