Ensuring water quality in recirculating aquaculture systems (RAS) is complex due to numerous interdependent factors that can influence water quality. Mathematical models have proven to be powerful tools for assessing the interactions between RAS design, operations, and system dynamics. However, few models include pH and carbonate species as dynamic variables.
This study introduces a dynamic model for RAS (dynRAS) that integrates fish growth, CO2 and Total Ammonia Nitrogen (TAN) excretion and removal rates with a pH and carbonate system reaction model formulated according to the law of mass action. Additionally, the model incorporates a dosing system for and which framework is based on a structure similar to a Hill function. We use empirical data recorded during the experimental trial of Jafari et al., (2024) as a benchmark to validate our model.
Model simulations were consistent with empirical data, reproducing observed patterns and interdependencies between pH, alkalinity, and CO2 (Fig.1). Despite some quantitative disparities, we could use the model to explore scenarios where we simulate the addition of NaOH or to control different alkalinity levels and investigate the changes in the dynamic of the system over long periods (Fig. 2). The simulation results highlight that optimal alkalinity control is not immutable and should be adjusted in accordance with the system state. With future refinements, the dynRAS model presented here could act as a potent resource for the development of tools to work toward an adaptive control system for alkalinity and pH.
Reference
Jafari, L., Montjouridès, M.A., Hosfeld, C.D., Attramadal, K., Fivelstad, S., Dahle, H., 2024. Biofilter and degasser performance at different alkalinity levels in a brackish water pilot scale recirculating aquaculture system (RAS) for post-smolt Atlantic salmon. Aquaculture engineering 102407