Circularity of olive mill wastewater for soilless agriculture within the sustainable water-energy-food nexus

The study aims to evaluate the use of olive black water, which causes high organic pollution when discharged directly into the environment and is costly to treat, as a potential fertilizer source in the cultivation of arugula (Eruca sativa) plant with soilless agriculture system and to evaluate the water treatment efficiency of the plant. Olive mill wastewater was subjected to chemical and physical treatment and nutrient solution was prepared. The arugula were grown in a deep water culture system using raw olive mill wastewater (Raw-OMWW), pretreated olive mill wastewater (CT-OMWW, PT OMWW) and a control nutrient solution. The growth of arugula and olive mill wastewater parameters were evaluated after hydroponic farming at two initial electrical conductivity (EC) levels: 1200 and 600 µS/cm. 600 µS/cm EC level, PT-OMWW was the best growing group. As for the environmental parameters of the wastewater left over after soilless farming, according to Turkey's environmental legislation, only the chemical oxygen demand of CT-OMWW, PT-OMWW solutions was slightly above the environmental limit values set for discharge to surface waters. This situation enables the use and reutilization of pretreated olive mill wastewater in the cultivation of arugula in soilless agriculture. Comparison was made using 4 different machine learning methods; the Multilayer Perceptron (MLP) model demonstrated the highest prediction accuracy and the lowest error rate in estimating fresh leaf weight.