Short-Term Reliability of Treated Sewage Effluent from Riyadh Wastewater Treatment Plants for Irrigation Reuse: Performance and Pathways to Unrestricted Irrigation

Treated sewage effluent (TSE) is central to Saudi Arabia’s plans to expand irrigation, but assessments of Riyadh’s TSE have relied on period averages. This short-term screening evaluated how reliably three municipal wastewater treatment plants (WWTPs) in Riyadh met the national irrigation limits, using one month of daily operator analyses (April 2026) and two years of daily residual chlorine. Reliability was assessed with the coefficient of reliability, empirical exceedance frequencies and block bootstrapping. Monthly mean five-day biochemical oxygen demand (BOD5) and total suspended solids (TSS) met the restricted limits at all plants, but no plant met all restricted limits consistently, because the ammonia, turbidity and residual chlorine limits apply to both classes. None met the unrestricted BOD5 limit: monthly means were 13.7, 23.1 and 18.0 mg/L against 10 mg/L, with ≤1% probability of compliance. One plant did not nitrify (ammonium nitrogen 43.7 ± 2.2 mg/L) and had residual chlorine below 0.2 mg/L on 96% of days since October 2025, whereas the others kept residuals within 0.2–0.5 mg/L on 82–99% of days. Salinity (electrical conductivity 1.7–2.2 dS/m) restricts crops to moderately tolerant species. Nitrification, solids removal and chlorine control are the priority upgrades, and a full year of data is needed to confirm these findings.