Sustainable Cultivation of Aromatic Lamiaceae in Semi-Arid Palestine: Enhancing Biomass and Essential Oil Yields through Reclaimed Wastewater Irrigation

Water scarcity in arid and semi-arid regions calls for sustainable irrigation strategies such as the reuse of treated municipal wastewater. This study evaluated the effects of secondary-treated effluent (STE) versus potable water (PW) on the growth, essential oil production, and safety of five aromatic Lamiaceae species (Salvia fruticosa, Rosmarinus officinalis, Origanum syriacum, Mentha spicata, and Clinopodium serpyllifolium) under semi-arid conditions in West Bank over two growing seasons. Effluent irrigation significantly enhanced plant productivity, especially in S. fruticosa, which exhibited an 81% increase in fresh biomass yield (13.99 vs. 7.71 kg/m², p = 0.018) and a 93% increase in dry leaf yield (2.63 vs. 1.36 kg/m², p = 0.018) compared to PW. Total essential oil yield of S. fruticosa also rose markedly under STE, reaching 118.7 mL/m² versus 69.9 mL/m² under PW (p = 0.001). While other species showed modest, non-significant increases in biomass and oil yield under STE, no yield reductions were observed in any species. Essential oil concentrations (mL/kg fresh biomass) remained statistically unchanged across irrigation types for all species, indicating no compromise in oil quality. For instance, O. syriacum maintained ~12.5 mL/kg under both treatments (p = 0.24). HPLC analysis of carnosic acid and carnosol in R. officinalis and S. fruticosa confirmed no significant compositional differences between treatments. Soil EC increased moderately under effluent irrigation (2060–3620 µS/cm vs. 1170–1855 µS/cm under PW, p = 0.03), yet remained within acceptable ranges for salt-tolerant crops. No significant accumulation of heavy metals (e.g., Cd, Pb, Ni) was detected in soils or plant tissues, and levels remained below WHO safety thresholds. Post-distillation residue analysis confirmed essential oils were free from metal contaminants. Leaf tissue analysis showed nutrient levels (N, P, K, Ca, Mg, S) were within sufficiency ranges and similar between treatments, demonstrating that STE adequately met crop nutritional demands without requiring synthetic fertilizers. Overall, secondary-treated effluent proved to be a safe and effective irrigation source for aromatic herbs, enhancing yields and reducing reliance on freshwater and external fertilizers. These findings support the integration of effluent reuse in sustainable agriculture, particularly for non-food, high-value crops in water-limited regions.