Ultra-low fluid velocity and nutrient mass transport in a coupled rainbow trout (Oncorhynchus mykiss) RAS and lettuce (Lactuca sativa) modified NFT aquaponic system: A mass transport modeling approach

Integrating hydroponics into Recirculating Aquaculture Systems (RAS) enhances nutrient reuse and eco-efficiency. This study evaluated a coupled RAS–NFT aquaponic system cultivating rainbow trout ( Oncorhynchus mykiss ) and lettuce ( Lactuca sativa ) under different NFT pipe diameters (75, 90, and 110 mm) and hydraulic flow rates (20, 30, 46, and 60 L h⁻¹), integrating a nutrient mass flow rate modeling approach. A 28-day trial compared a fish-only traditional RAS against a biointegrated RAS–NFT system. Physicochemical water quality parameters and dissolved nutrients were monitored regularly. To alter standard hydrodynamics, the discharge end of each main NFT pipeline was fitted with a 50 mm PVC reducer, creating a hydraulic backwater effect that modified the pipelines into a water-retaining, flooded-channel system operating under a full-pipe cross-sectional regime. To evaluate nutrient mass delivery under different hydraulic conditions, nutrient mass flow rate (Jm, mg min⁻¹) delivered to the rhizosphere were estimated using a mass transport model based on volumetric flow rates and mean dissolved nutrient concentrations. Weekly plant growth and final biomass production indices were evaluated. Biointegration maintained physicochemical water quality within similar ranges in both production systems. Among the dissolved elements, potassium exhibited higher mean concentrations in the coupled aquaponic system, due to routine alkalinity corrections, whereas the remaining nutrients showed comparable descriptive values. Mass transport modeling indicated that estimated nutrient mass flow rates (Jm) increased proportionally with volumetric inflow rates (Q), as expected from the modeled hydraulic scenarios. Under the 20 L h⁻¹ configuration, the mass transport model estimated nutrient mass flow rates of 39.33 mg min⁻¹ for NO₃⁻ and 25.67 mg min⁻¹ for PO₄³⁻ delivered to the hydroponic NFT channels. Variations in pipe diameters and flow rates yielded no significant differences in final shoot fresh weight, dry mass, or leaf count ( p > 0.05), suggesting that nutrient delivery was sufficient to support crop development across the tested operational range. The flooded-channel hydroponic NFT configuration operated under ultra-low fluid velocity conditions (4.53–4.84 m h⁻¹; 0.126-0.134 cm s -1 within the proportional core). Mass transport modeling indicated that estimated nutrient delivery remained sufficient across the evaluated hydraulic scenarios, and no reductions in lettuce growth were observed under the tested conditions. These findings suggest that, under the evaluated hydraulic conditions, engineering designs for coupled aquaponic systems may be optimized for structural, spatial, and economic considerations while maintaining plant growth.