Applications of computational fluid dynamics to modeling hydrodynamics in tilapia rearing tank of Recirculating Biofloc Technology system

Abstract

Biofloc Technology (BFT) has been widely used in tilapia rearing. However, the Total Suspended Solids (TSS) at the bottom of rearing tank will increase to high concentration due to biofloc sedimentation and accumulation. Therefore, it will be have negative effects on the feeding enthusiasm, gill and even survival of cultured species, and especially in the initial stages of larvae culture. The Recirculating Biofloc Technology (RBFT) would be one of the potential solution for the above concerns. To achieve an accurate TSS regulation, the biofloc distribution should be as homogenous as possible. The aim of this work is to specify the optimal bubble size and Hydraulic Retention Time (HRT) to the uniformity of biofloc distribution in a RBFT system through Computational Fluid Dynamics (CFD) method. For this, a three-dimensional and three-phase unsteady transient model was developed to simulate the hydrodynamics in a gas-liquid-solid tilapia rearing tank based on the commercial software Workbenching 15.0. Firstly, to achieve a more reliable prediction, the influence of the grid types and the computation turbulence model (Standard, Renormalization-Group (RNG), Realizable) to the simulation result were discussed by contrasting the simulations and experiments results of solid holdup at dimensionless radial position. Secondly, appropriate mesh size (181395 elements) and mixture standard k-e model were implemented to study the effect of bubble size and HRT on solid distribution uniformity. Simulations were performed by using three different mean bubble sizes (diameter=1, 2, 3mm) and three different HRTs (0.56, 1.13, and 2.25h) to study the sensitivity of the results to the uniformity of the biofloc distribution, respectively. The results showed that to get a better biofloc distribution, the bubble size should be range in 1mm and 2mm, and as the HRT decreasing, the distribution of biofloc distribution will be more homogeneous (0.45≤HRT≤0.56h). This paper provides an essential data set for determining the bubble size and HRT in production, as well as evaluating the accuracy of various CFD models for capturing the complex flow field in a BFT rearing tank.