Long-Term Effects of Treated Wastewater Irrigation on Soil Quality and Pathogen Contamination in a Semi-Arid Region: A Case Study from El Hajeb, Sfax, Tunisia

The reuse of treated municipal wastewater (TWW) in agriculture represents a sustainable solution to water shortages and constraints in agricultural production. However, poor monitoring of this water in Tunisia poses health risks for humans and environment. In this context, the TWW from the Thyna treatment plant in Sfax, is transported to the El Hajeb farm for irrigation, after undergoing primary and secondary treatments. This study aims to assess the feasibility and risks of reusing treated wastewater for irrigation in semi-arid regions by evaluating its impact on soil chemistry and microbiology under real farming conditions. To achieve this, three plots were selected for comparison: the first was supplied with treated wastewater (STWW), the second with well water (SWW), and the third was a control plot irrigated solely with rainwater (SRW). All these plots share the same soil types: Arenosol. The results revealed that TWW contained high concentrations of essential fertilizing elements, including nitrogen and other nutrients that enhance soil fertility. However, it also has significant amounts of sodium and chloride ions, contributing to soil salinization. Low levels of trace elements and very low levels of heavy metals were also detected. TWW was rich in microbial content, including bacteria such as FMAT, FC, TC, and YM, as well as pathogens like Escherichia coli and Salmonella spp. Microbiological results indicated soil quality deterioration due to pathogenic contamination. These findings suggest that the current treatment at the Sfax Thyna wastewater plant is insufficient to fully eliminate microbiological risks. To ensure safer irrigation practices and reduce health hazards, an additional tertiary treatment step is necessary.