Lettuce Performance in a Modified RAS–NFT System Operating at Different Flow Rates

The integration of Recirculating Aquaculture Systems (RAS) and aquaponics represents a sustainable strategy to optimize water use and nutrient recycling in intensive aquaculture. This study evaluated, over 28 days, water quality and the growth of Lactuca sativa in a RAS coupled with an NFT hydroponic module for rainbow trout (Oncorhynchus mykiss) cultivation. The system used PVC pipes with internal diameters of 75, 90, and 110 mm, operating at flow rates of 20, 30, 46, and 60 L h⁻¹. Unlike conventional NFT systems, the system was configured as a partially flooded channel, using concentric reducers to promote water retention and decouple physical fluid velocity from nutrient availability. Hydraulic velocities remained extremely low (≈0.13 cm s⁻¹), while absolute nutrient input increased linearly with flow rate. Water quality parameters remained stable throughout the experimental period, with mean values of pH 6.81 ± 0.11, dissolved oxygen 8.76 ± 0.38 mg L⁻¹, temperature 17.58 ± 0.63 °C, and nitrate 118.00 ± 25.52 mg L⁻¹, demonstrating high chemical stability and adequate nitrification in the integrated system. Among dissolved macronutrients, Ca²⁺, SO₄²⁻, and NO₃⁻ represented the main nutrient mass flows delivered to the roots. No differences were observed for fresh biomass, dry biomass, shoot length, or leaf count across the evaluated pipe diameters and flow rates (p > 0.05). However, the highest flow rate (60 L h⁻¹) promoted greater root development, resulting in significantly greater root length in plants grown in 75 mm pipes at 21 days and in 110 mm pipes at 28 days of cultivation (p < 0.05). The results indicate that lower-cost pipes can be used without compromising plant performance, provided that adequate nutritional supply is maintained through volumetric flow.