Germination and growth of horticultural crops irrigated with reclaimed water after biological treatment and ozonation

Integration of high water-volume and nutrient-rich industrial wastewater into the existing water management plan for agriculture could be a viable option to protect freshwater resources, mitigate water scarcity problems and support the agriculture sector. Hydroponics were set-up to study the effects of treated mixed industrial and domestic wastewater on physiological traits of lettuce and silverbeet. A sustainable hybrid biological-ozonation process was employed for the wastewater treatment. The studied process showed high treatment capacity by removing 96% COD, 92% diesel and 97% methylene blue compounds. Reclaimed streams were analysed for metals using ICP-MS instrument and the concentrations of all elements tested, exception of Cu, Mn, and Mo, fall within the permissible range for crop irrigation. The crops were grown using the effluents (with/without external nutrient) and compared to a recommended nutrient culture and tap water controls. Compared to the control, plants that received external nutrient had a comparable aerial biomass and vegetative growth, higher photosynthetic pigments but exhibited some root development impairments, probably due to high activity of H⁺, which promoted metal toxicities on plant roots. Under pH-controlled irrigation, an improvement in root growth was observed. The biomass of plants watered with only the treated wastewater were more than 50% higher than the yield in tap water control and plants exhibited high degree of root foraging. The root concentrations of the studied metals, particularly Al, Mn, Cu and Zn were higher than the corresponding shoot concentrations. Pearson correlation analysis showed a significant positive interaction (p<0.01) between K and other elements (Mg, Ca, Cu, Mn, Zn, Fe) in plant tissues, thus indicating its role in facilitating transportation of nutrients and photo-assimilates in plant. The results suggest a high suitability of the treated wastewater for growth of crops. Thus, the reuse of treated mixed industrial and domestic wastewater for irrigation can be considered a sustainable way to cope with the growing water stress across the globe and the water shortage in agriculture. The reuse potential for irrigation of crops depends on the quality of the reclaimed water and the treatment technologies used. With the limited research data available, further studies to elucidate the long-term implications of the irrigation is highly recommended.