Suitability of Treated Domestic and Urban Wastewaters for the Hydroponic Cultivation of Rocket (Eruca vesicaria [L.] Cav.)

Reusing treated wastewater (TWW) in agriculture may reduce water use pressure. While TWW is often used for soil irrigation, its application in hydroponics remains limited. In these systems, TWW can serve as a source of nutrients for plants while also being further reclaimed. We evaluated two TWWs of different origin and composition for hydroponic rocket cultivation. Each TWW was tested in its native form (TWW1 and TWW2) and after dilution and supplementation with mineral salts (TWW1_DH and TWW2_DH), using a Hoagland nutrient solution as a control. Yield and qualitative aspects of the product, including health risk factors (nitrates and heavy metals), were assessed. Rocket grown in TWW1 reached the harvesting stage, but with a significant yield reduction compared to the control (−40%). In TWW2, plants reached only the cotyledon stage and were not harvested. Two harvests were obtained in TWW1_DH and TWW2_DH, with yields comparable to the control or even significantly higher (+25%) in the first harvest in TWW1_DH. No health concerns were detected, with values of Health Risk Index < 1 for all the heavy metals and nitrate levels (~3000 mg kg−1 FW) well below EU limits. The study highlights the potential of TWW for the hydroponic cultivation of rocket, but also highlights the need to tailor its use based on composition.