Performance Evaluation of an Urban Wastewater Treatment Plant (WWTP): Statistical and Comparative Approach to Estimate Water Quality and Pollution Indices, a Case Study of Maghnia WWTP, Algeria

Abstract Algeria faces severe water stress, marked by a decrease in water availability per inhabitant from 1,500 m 3 /year in 1962 to 430 m 3 in 2020, with a projection of 300 m 3 by 2050. In this context, optimizing wastewater treatment plants (WWTPs) becomes essential to reduce the water deficit. This study evaluates the performance of the urban wastewater treatment plant of Maghnia (Northwestern Algeria) by carrying out a statistical analysis of physicochemical parameters and comparing the Weighted Arithmetic Water Quality Index (WAWQI) and Water Pollution Index (WPI) based on data collected between January 7, 2020, and March 22, 2021. Reliable detection of outliers was achieved through the combined application of Grubbs and Tukey tests, while data normality was verified by Shapiro–Wilk, Anderson–Darling, and the P-P plot, thus strengthening analytical robustness and minimizing bias. The results show stability of physical parameters (temperature and pH) and moderate variability of chemical parameters, including chemical oxygen demand (COD), total suspended solids (TSS), and biochemical oxygen demand (BOD₅), at the plant inlet. However, notable exceedances of standards at the outlet were observed, notably for ammonium (7.92 mg/l, standard limit: 5 mg/l) and phosphates (10.01 mg/l, standard limit: 2 mg/l), indicating deficiencies in biological treatment despite high organic matter removal efficiency. Bland–Altman analysis confirms the complementarity of the WAWQI and WPI indices, highlighting low quality of treated water and chronic system overload. These results underscore the need for targeted process optimization, particularly for nutrient removal, to ensure safe reuse of treated wastewater.