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Design criteria and performance of an intensive closed recirculating aquaculture system

The benefits of controlled-environment aquaculture have been clearly identified (Losordo et al, 1989, Timmons et al, 2001). By using various unit processes to recycle and reuse culture water, recirculating aquaculture systems can provide for the control of water quality under intensive feeding conditions. This requires the integration of equipment and technologies with specifically co-developed management techniques (Van Gorder, 1991). The present design demonstrates process engineering options which effectively and economically achieve the water quality control capabilities necessary to closed systems. A unique cross-flow tank design (Watten and Johnson, 1990) combines the self-cleaning capabilities of a round tank with the ease of maintenance and management of a rectangular raceway. Water quality control is provided by effectively co-engineered systems for clarification, biofiltration, de-gassing, and oxygen/ozone injection technologies. Materials and Methods The present design, operated for several years in Pennsylvania, USA, includes side-by-side “cross-flow” concrete tanks, each tank measuring 30 meters long x 1.2 meters wide, with a volume of 57, 000 liters. The two tanks share a central wall with a total volume of 114 m3 of water, and are integrated with a filtration/pumping/oxygenation/ozonation system. The cross-flow tanks have influent and effluent pipe manifolds at floor level running their entire length. Perpendicular influent jets distribute water flow uniformly across a u-shaped tank floor, providing the appropriate cross-flow pattern and velocity, effecting the movement of solids to adjacent effluent ports. Each tank is divided by partitions into four increasingly large sections, each thereby receiving a proportionally increased volume of oxygenated water. Fish are harvested from the largest end section after 24 weeks of culture. After each 6 week cycle fish in subsequent sections are moved into the larger vacated section, with the smallest section stocked successively with hybrid striped bass fingerlings. With eight populations of varying-sized cohorts, a stable biomass is maintained, thereby maximizing production capacity of each centralized system (Van Gorder and Jug-Dujaković, 1996). Water quality is maintained by integrated skid-mounted unit processes for the recirculationof the tank volume. Water returning from the tanks is mechanically filtered through a microscreen drum clarifier (60 micron). The flow then enters rotating biological contactors providing the required biofiltration (Van Gorder and Jug-Dujaković, 2005). A pump distributes the water to a carbon dioxide sparging chamber, and under pressure to an oxygen/ozone saturator, from which supersaturated levels of dissolved oxygen and ozone are distributed to the culture tanks. A chemical feed pump maintains pH, using NaOH dosing. Computer telemetry systems monitor electrical status, flow rates, and temperature, while controlling feeding, clarifier function, emergency oxygen activation, pH, and oxygen and ozone flow. Computer control functions also include emergency response and notification for disruption or variance in flow rate, dissolved oxygen levels, pH and temperature. Results Using the described system technologies, water quality was maintained at adequate levels for the continuous introduction of targeted feeding levels, with maintenance of unionized ammonia levels < 0.05 mg/l, carbon dioxide levels < 20 mg/l, and dissolved oxygen levels > 80% saturation. The use of oxygen saturators provided 100% efficiency with the dissolution of oxygen supplied to the recycle water flow. The intensive use of ozone provided a number of advantages, including the removal of residual nitrite levels, a significant level of water clarity, and the elimination of off-flavors. As a function of feeding levels, for every kilogram of feed added, this system used 0.5 kg of oxygen and 0.06 kg of ozone. Commercial operations of eleven of the described recirculating 2-tank aquaculture systems demonstrated an average capacity/tank of 22, 700 kg/year. At full loading, each 2-tank grow-out system was fed at a daily rate of 70 kg/day. The maintenance of eight varying-sized cohorts within each centralized system provides for extremely stable daily feeding levels, thereby providing an optimization of production capacity by continuous feed loading. Conclussion The recirculating aquaculture system described has been designed using a combination of high-efficiency unit processes engineered for optimal control of water quality and stock management. The use of a unique cross-flow tank design allows for efficient self-cleaning capabilities, with full access to multiple varying-sized cohorts, for frequent and continuous tank harvest capabilities of hybrid striped bass at densities averaging 50 kg/m3. The system maintains optimal water quality conditions under heavy loading using appropriately designed hardware integrated with co-developed management techniques. With maintenance of a stable biomass, each component operates at near threshold levels at all times, providing for continuous and uniform levels of production throughout the year.

design; performance; intensive closed recirculating aquaculture system

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