Solids flushing, mixing, and water velocity profiles within large (10 and 150 m3) circular ‘Cornell-type’ dual-drain tanks

Abstract

The objectives of this research were to evaluate how bottom drain flow rate and water inlet structure affect solids flushing, water mixing, and water velocity profiles within large (i.e., 10 and 150m3) circular ‘Cornell-type’ dual-drain culture tanks. Results show relatively uniform water mixing was achieved in both ‘Cornell-type’ dual-drain culture tanks when tested at high fish densities (90–98kg/m3) and at hydraulic exchange rates of one tank volume every 20–32min. The results of the dissolved oxygen profiles showed that the minimum and maximum dissolved oxygen concentrations ranged from 10.0 to 10.6mg/L across the cross-section of the 10m3 tank and from 9.0 to 11.2mg/L across the cross-section of the 150m3 tank when inlet dissolved oxygen levels were 16–18mg/L. Mixing was not influenced by bottom-drain flow over the range tested, but was influenced by the orientation of the water injection nozzles projecting through the side-wall of the culture tanks.Adjustment of the orientation of the water inlet structure and of the water flow discharged through the bottom-center drain was shown to influence the water rotational velocities throughout the circular tank. Water rotational velocity profiles taken in a vertical plane bisecting the tank indicated that velocities were consistently higher at the tank’s perimeter and that these velocities increased nearly linearly with radial distance from the tank center. Results also show that higher rotational velocities were produced at larger bottom flow rates, i.e., a 12% bottom flow created higher rotational velocities than 6 or 0% bottom flows. Reorienting the direction of the water inlet nozzles also provided a simple method for adjusting rotational water velocities within the culture tank. For example, simply reversing the direction of one of the six 45° flow injection nozzles decreased the rotational velocity about the perimeter of the 10m3 tank from 17.8 to 13.4cm/s.Additional studies with arctic char or rainbow trout present at commercial culture densities showed that the rate that settleable solids are flushed from 10 and 150m3 circular ‘Cornell-type’ dual-drain culture tanks depended strongly on the water flow exiting the bottom-center drain and on the rotational period of the water within the tank. A rotational period of 1.3–1.7min produced optimal water velocities for flushing solids from the ‘Cornell-type’ dual-drain tank. Also, a water flow through the tank’s bottom-center drain of at least 5–6L/min for every 1m2 of tank plan area was found to flush settleable solids within 1–2min and 3–6min from 10 and 150m3 ‘Cornell-type’ dual-drain tanks, respectively.