Seasonal and Operational Drivers of Microbial and Organic-Load Removal in Municipal Wastewater Treatment Plants of the Peruvian Andes

This study evaluated the seasonal and operational factors influencing microbial and organic-load removal in four municipal wastewater treatment plants (WWTPs) located in the high-Andean Mantaro Valley, Peru. Monthly paired influent and effluent samples were collected from Huachac, Jauja, Quilcas, and San Jerónimo de Tunán during 2024, resulting in 48 independent WWTP–campaign pairs. Thermotolerant coliforms, Escherichia coli, biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total suspended solids (TSS), flow, rainfall, hydraulic conditions, and residual chlorine were evaluated. Bayesian hierarchical models were used to estimate microbial log-reduction values, pollutant-load removal, seasonal differences, and associations with operational covariates, while accounting for technical replication and right-censored microbial observations. Jauja achieved the highest microbial removal, with median reductions of 2.05 log units for thermotolerant coliforms and 2.06 log units for E. coli. Huachac showed the highest BOD5 and TSS load removal, reaching 54.7% and 77.2%, respectively. In contrast, Quilcas showed limited BOD5 and COD removal, with posterior medians of 6.0% and 0.3%. Effluent residual chlorine was the operational factor most clearly associated with increased E. coli removal. Seasonal effects were uncertain after adjustment for plant-specific conditions. None of the WWTPs achieved overall regulatory compliance because thermotolerant coliform concentrations exceeded the national limit in all monitoring campaigns. These findings demonstrate substantial heterogeneity among high-Andean WWTPs and support plant-specific improvements in disinfection, biological treatment, hydraulic management, and continuous load-based monitoring.