Mass-balance process model of a decoupled aquaponics system

Aquaponics presents a viable solution to water pollution from aquaculture by utilizing nitrate- and phosphate-rich effluent for crop production. The objective of this study was to develop a mass-balanced process model based on a pilot-scale aquaponics facility growing Nile tilapia (Oreochromis niloticus) and cucumbers (Cucumis sativus) in Auburn, Alabama. This enabled a better understanding of how key elements partition among different downstream processes, ultimately affecting nutrients available to plants or discharged to the environment. Data were collected from a pilot scale decoupled aquaponics system for a full calendar year and included weekly water quality, direct GHG emissions, and water flows. Bio-solids, fish mass, and plant mass were also quantified and underwent elemental analysis. Together, these measurements were used to create stoichiometric equations for mass partitioning. The resulting stoichiometry was used to develop a mass-balanced process model constructed in SuperPro Designer software. Four separate variations of the model were developed, one for each season. The model showed that 21.6% of input nitrogen was assimilated by tilapia and only 2.81% by plants, while 33% of input phosphorus was assimilated by tilapia and 2.6% by plants. Modeled effluent concentrations of nitrate from the fish tank, clarifier, and plants averaged 440, 441, and 307 mg L-1, respectively, compared to average measured values of 442, 406, and 298 mg L-1. Modeled effluent phosphate concentrations from the fish tank, clarifier, and plants were 25, 27, and 20 mg L-1 of phosphate, respectively, over the course of one year, while average measured values were 30, 31, and 26 mg L-1. The model was not suitable for predicting short term system changes. The constructed model shows promise in predicting long-term changes in system outputs based on upstream operational changes and is effective for simulation and scenario analysis.