Optimization of an Indoor DWC Hydroponic Lettuce Production System to Generate a Low N and P Content Wastewater

Hydroponic production raises economic and environmental issues related to the treatment, recovery or disposal of hydroponic wastewater, which can be rich in eutrophication-related nutrients, nitrogen (N) and phosphorus (P). Little focus has been put on the influence of the growth conditions on the N and P content in hydroponic wastewater, which is of uttermost importance when it is intended to reuse the wastewater for irrigation or other purposes with reduced impact on the environment. This study aimed to optimize an indoor non-recirculating deep-water culture (DWC) hydroponic system for lettuce (Lactuca sativa L. var. crispa) production, in terms of daily light integral (DLI) and volume of nutrient solution (NS) per plant, to maximize both the biomass production and the N and P removal, allowing for the wastewater to meet the criteria established for reusing in irrigation and minimizing the eutrophication impacts. A small-scale DWC hydroponic system with a fluorescent light fixture was built to study lettuce growth indoors for 35 days after transplanting (DAT). A first experiment was conducted under 14, 20 or 23 mol m−2 d−1 DLI and with 1.5 or 2 L of NS per plant. A pronounced inner leaf tip burn was observed, regardless of the volume of NS solution used, related to the unventilated conditions under high radiation. Total biomass was similar in all treatments and N and P removal was higher than 95% and 94%, respectively, at 35 DAT. Lettuces grown in 2 L of NS per plant exhibited higher average biomass. A second experiment was performed under 8, 10 or 12 mol m−2 d−1 DLI and with 2 or 3 L of NS per plant, making it possible to achieve healthy biomass at 35 DAT with higher water and light-use efficiency when compared to the first experiment. A DLI of 10 or 12 mol m−2 d−1 with 2 L of NS per plant and a DLI of 12 mol m−2 d−1 with 3 L of NS per plant made it possible to achieve both the best total biomass production and the highest N and P removal from water. Under those conditions, hydroponic wastewater complied with N and P criteria for reuse in irrigation, showing potential to be used as an alternative resource for agriculture and to minimize negative impacts on the environment.