Use of computational fluid dynamics (CFD) for aquaculture raceway design to increase settling effectiveness

Abstract

A computational fluid dynamics model was used to evaluate the impact of potential raceway design modifications on the in-raceway settling of solids. Settling effectiveness was evaluated on the basis of the percentage of solids removed by settling relative to the mass of solids introduced into the raceway, with solids settling primarily in the quiescent zone (QZ). The design modifications were applied to a simulated standard raceway (SSR). The SSR was a rectangular concrete raceway 30.2 m long, 3.0 m wide, 0.9 m deep, with a slope of 0.01. The raceway included a QZ of approximately 5.0 m (length), which was separated from the rearing area by a screen. The flow rate was 0.058 m 3/s. For simulation purposes, six groups of particles were used to account for the total suspended solids (TSS). The representative particle sizes were 692, 532, 350, 204, 61, and 35 μm, for Groups 1–6, respectively. The smallest particles (Groups 5 and 6) are the most difficult to settle because of their low settling velocity. After analyzing the velocity profiles obtained from the SSR and considering various design constraints, several raceway design alternatives were simulated. Among the alternatives tested, six designs were chosen based on their PSR values. The main feature in all the modifications presented is the addition of a baffle before the QZ or at the entrance of the QZ replacing the screen. The main purpose of adding these baffles was to increase the velocities under the baffle, which causes an increase in the accumulation of the solids after the baffle (in the QZ). According to the simulations, the highest PSR was obtained with the combination of a baffle and a screen under the baffle. The overall improvement of PSR with respect to the original system was small, with the most noticeable increases taking place for the smaller groups of particles, from 83 to 84%, 3 to 14%, and 1 to 5% for Groups 4–6, respectively. Further studies should include the evaluation of some of these alternatives in real raceways and the comparison of simulated and experimental PSR results.