Introduction
One type of fish tank used in recirculating aquaculture systems (RAS) is the Cornell-style dual drain circular tank. Cornell-style dual drain fish tanks have two drains: a bottom center drain, and an elevated sidewall drain. Cornell-style dual-drain tanks are advantageous because the rotational velocity caused by the inlet water injection creates a self-cleaning effect. In operation, a Cornell-style dual drain tank functions as a swirl separator with waste solids collecting at the tank center and flowing out the bottom center drain. These fish tanks also provide a relatively uniform environment for the fish, with minimal gradients in dissolved oxygen (Davidson & Summerfelt, 2004).
In a typical Cornell-style dual drain circular tank, the injection of treated RAS water causes a rotational velocity that is highest at the tank perimeter and decreases toward the tank center. Recent research indicates increased water velocities provide performance benefits for Atlantic salmon post-smolts raised in fish tanks (Timmerhaus et al., 2021). To achieve these benefits, an additional inlet can be installed on separate pumped loop that removes water from the fish tank and re-injects it to provide increased water velocities. This internally pumped “velocity assist” inlet aims to increase water velocities available to larger post-smolt and harvest-size salmon in commercial-scale Cornell-style dual drain circular fish tanks. This study sought to evaluate the effect of the velocity assist technique by testing combinations of three different water flow rates and three different nozzle head loss configurations for a velocity assist inlet in a semi-commercial scale RAS used for land-based growout of Atlantic salmon with the intent of determining optimal combinations for increasing water velocities at minimum energy.
Materials and methods
The velocity assist technique was empirically tested by the addition of a pumped velocity assist loop with adjustable inlet in a 150 m3 Cornell-style dual drain circular tank that is part of the semi-commercial scale RAS at the Freshwater Institute (USA). Empirical water velocity sampling of the currents in the tank were carried out for all combinations of the following: ~30% of the main flow as velocity assist flow (1136 lpm), ~50% of the main flow as velocity assist flow (2271 lpm), ~60% of the main flow as velocity assist flow (2763 lpm) and low (0.14 bar), medium (0.33 bar), high (0.56 bar) velocity assist inlet nozzle headloss. Water flow was adjusted by changing the number of pumps operating and adjusting a control valve. Nozzle headloss was adjusted by changing the number of 2.54cm openings on the inlet. Water velocity measurements were collected using a SonTek Argonaut-ADV 3-axis Doppler velocity meter at three depths across two cross-sections once steady state conditions were achieved for each combination of flow and headloss (Figure 1). Water velocities were also characterized for operation of the tank without the velocity assist inlet in operation.
Results
Preliminary results indicate that operation of the velocity assist inlet improved velocity profiles in the 150 m3 Cornell-style dual drain circular tank over the base case without a velocity assist inlet. Higher velocities were observed throughout the tank with higher velocity assist inlet water flow and nozzle headloss. The complete results of the evaluation will be presented along with recommendations for optimizing the velocity assist technique in commercial-scale fish tanks for RAS.
Conclusions
The use of a velocity assist inlet effectively decouples the control of tank water velocities for improved fish welfare from the primary inlet flow and allows for more control as fish increase in size.
References
Davidson, J., & Summerfelt, S. (2004). Solids flushing, mixing, and water velocity profiles within large (10 and 150 m3) circular ‘Cornell-type’ dual-drain tanks. Aquacultural Engineering, 32, 245–271.
Timmerhaus, G., Lazado, C.C., Cabillon, N.A.R., Reiten, B.K.M., and Johansen, L.H. (2021). The optimum velocity for Atlantic salmon post-smolts in RAS is a compromise between muscle growth and fish welfare. Aquaculture, 532, 736076.