Recirculation system spares environment

Posted on Dec 18 2004

About seven month ago, Ines Deleon Guerrero’s pigpens in Fina Sisu were closed for construction so it could be converted into aquatic tanks that would be used for the production of tilapia.

Today, Deleon Guerrero is equipped with nine tanks, a recirculation aquaculture system, and hundreds of Tilapia brought in from Guam and the Northern Marianas College-Cooperative, Research, Extension and Education Services.

Of the nine tanks, two are used for breeding purposes, one as a nursery for baby tilapia, and the remainder used as homes for the fish until harvest time. Each tank can hold about 400-500 fish.

It takes about five to six months before the fish are at marketable size—about six to seven inches long, according to NMC-CREES aquaculture specialist Mike Ogo.

“In essence, [Deleon Guerrero] can have two crops of fish per year,” he said during an interview Friday afternoon.

Ogo cited that the significance of the recirculation system, saying this means that the over 10,000 gallons of water used during the period of raising the tilapia would not in any way harm the surrounding environment.

“Water that’s in the tank does not get dumped into the surrounding environment,” he said. “Anything such as fish that you grow in water would produce effluent, [which] is bad for the environment because you’re basically throwing out waste into the area. With this type of system, we have certain procedures that clean the water, and the water is returned back to the production tanks.”

“Water that gets put in when you first start is the same water that is being used six months later when you harvest the fish,” he added.

The water is changed for the next crop of fish, and the old water is used to irrigate plants “because it is rich in nutrients.”

“The contaminants are not just indiscriminately dumped into the environment,” Ogo said. “The plants will absorb the nutrients and nothing is polluting the surrounding areas.”

Ogo explained that in each tank, a pipe with holes at the bottom serving as drainage is placed in the center. Water flows into the holes to the solids filtration tank, which contains nets that are used to trap fecal matter or feed that was not eaten by the fish. Water then overflows to another tank, which contains a device called biofiltration.

“What’s happening is you have rolls of plates going across, and you have bacteria that attaches to the plates,” Ogo said. “Fecal matter in the water will create ammonia, so the bacteria turns the ammonia into nitrite, and if not kept in check, the water could be poisoned, so we have beneficial bacteria there that are cleaning the water before it returns back to the production tank. The water is cleaned and nitrified.”

Ogo said the main water valve would also be shut down at least once a week for cleansing of the nets.

Ogo said Deleon Guerrero’s project cost about $5,000.

“That’s because we didn’t start from scratch,” he said. “The foundation was already in place. Of course, if we start from scratch, it would be higher.”

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