From wastewater treatment plants to Oued Souhil agricultural experimentation unit: A study of treated wastewater quality reused for irrigation, Tunisia

This study evaluated the spatiotemporal variations in the physicochemical, bacteriological, and parasitological quality of treated wastewater (TWW) from the wastewater treatment plants (WWTPs) to the agricultural experimentation unit (AEU) of Oued Souhil, Tunisia. TWW samples were collected from nine sites, including the outlets of the WWTPs, pumping tanks (SP3,SP4), the regulation, storage and settling basins, the effluent used for irrigation after filtration and before injection into the drippers. The quality parameters for final effluent TWW ranged as follows: pH (6.94 - 9.64), EC (2.14 - 3.47 mS/cm), TSS (20.0 - 60.0 mg/L), BOD 5 (10.0 - 69.0 mg O 2 /L), COD (63.0 - 440.0 mg O 2 /L), and thermotolerant coliforms (0.3 - 3 × 10 3 MPN/100 mL). Results revealed that 54.5% of COD, 80% of BOD₅, 37.5% of TSS, and 45% of EC and pH values exceeded national and international TWW reuse standards. While Ascaris spp., Trichuris spp., and Ancylostomidés were absent, the presence of Taeniid eggs and protozoan cysts indicated potential sanitary risks. TWW quality varied along with the distribution network, reflecting intermittent inefficiencies in system performance and management. These findings underscore the need for continuous monitoring and optimization to ensure the safe agricultural reuse of TWW in downstream areas. • Holistic TWW spatiotemporal assessment identifies operational challenges at treatment stages • Variable degrees of water suitability for irrigation were recorded • Treatment systems design and management from WWTPs to the irrigated area need to be improved • Seasonal variations should be considered when evaluating the TWW quality in monitoring programs