A Theoretical and Computational Model for Aquaponics Systems

Aquaponics, a sustainable farming technique combining the principles of aquaculture and hydroponics, suffers from uncertainty in its optimal operating conditions, preventing large-scale adoption of this technology. Relying on experimentation or full-scale testing to address these uncertainties is an approach that is material, labor, time, and cost intensive. As a result, aquaponics would benefit from a complete computational model of the system. This would allow for information to be gained without long and expensive experiments, but rather with knowledge of system operations and a modeling software package. In this study, the Activated Sludge Model 1 (ASM1) matrix framework, traditionally used in the biological wastewater treatment industry to model bacterial metabolic processes, was extended to model fish (specifically tilapia) and lettuce growth. Individual models for each of the three components in an aquaponics system (bioreactor, hydroponic bed, and aquaculture tank) were developed using the ASM1 matrix approach and were implemented in BioWin software. Success was seen in extending the ASM1 matrix framework to lettuce and tilapia growth, with the individual models of these components replicating the S-shaped growth curve expected of these types of biomass. The separate model components for the bioreactor, hydroponic bed, and aquaculture tank were then integrated into a comprehensive model of the aquaponics system. A procedure was developed to run simulations that accounted for the difference in growth times of plants and fish, yielding six lettuce crops per crop of fish. Both lettuce and tilapia grew successfully in the integrated model; however, the model revealed that when xi the fish are at the fingerling stage, they don’t produce enough nitrogen to provide lettuce with sufficient nitrogen for growth. A model simulation showed that supplemental nitrate for the first 60 days upon startup can increase the number of viable lettuce crops from 4 to 6 in the first 6 months of system operation. This model has future application in exploring aquaponics startup and optimization without the need for costly and time intensive physical experiments. Future work is needed to calibrate the kinetic and stoichiometric parameters for lettuce and tilapia growth using experimental data