Numerical Investigation of Tank Geometry and Hydraulic Forcing to Increase Ammonia Water Renewal Efficiency in Stratified Aquaculture Earth Tanks

In aquaculture, efficient water renewal systems are critical to ensuring water quality and preventing ammonia buildup. The streamfunction-vorticity formulation is utilized in a two-dimensional vertical (2DV) finite element model to simulate buoyancy-driven water renewal and ammonia transport in laboratory-scale aquaculture tanks based on the Figueira semi-intensive fish ponds used in the SISdATA project. The model integrates incompressible flow with ammonia advection-diffusion using the Boussinesq approximation and employs Streamline Upwind/Petrov-Galerkin (SUPG) stabilization and an operator-splitting scheme to ensure numerical stability under advection-dominated transport conditions. To isolate the effect of geometry on renewal performance, five tank configurations with the same length (Lx=2.5 m) and progressively increasing water depths from 0.0227 to 0.350 m were investigated. The corresponding inlet opening heights ranged from 0.00454 to 0.070 m, while the Reynolds and Peclet numbers were maintained constant. Deeper tanks resulted in lower inlet velocities at a given discharge, delayed penetration of the polluted layer by the circulation, and increased the relative importance of circulation-controlled renewal compared with direct jet-driven flushing. Under baseline conditions, the 83% ammonia removal time ranged from 48 min for Tank1 to 208 min for Tank4, while the deepest configuration, Tank5, achieved 83% removal in 91 min because of enhanced late-stage recirculation. Hydraulic scaling further showed that maintaining a constant inlet velocity reduced the renewal time of Tank5 to 75 min, whereas fixed inlet and outlet opening conditions generally favoured the shallowest configuration. A constant-volume analysis demonstrated that decreasing the tank aspect ratio from 110.13 to 0.46 increased the 83% renewal time from 48 to 353 min. The results demonstrate that water renewal efficiency is governed by the combined effects of tank geometry, hydraulic boundary conditions, and aspect ratio. The proposed model provides a numerical tool for optimizing aquaculture tank design and hydraulic operation to improve ammonia removal and water quality.