Multi-Criteria Optimization of Sanitation Systems for the Transition to a Circular Urban Metabolism in Peri-Urban Ecosystems in Mexico

Abstract

Effluent discharges released without adequate treatment and containing pollutant loads exceeding the maximum permissible limits contribute significantly to the degradation of receiving ecosystems, including rivers and streams. Simultaneously, such contamination reduces the potential for the indirect reuse of water resources, thereby exacerbating water stress within the affected region. The migration towards a circular urban metabolism merits the modernization of the infrastructure for wastewater treatment, essentially in peri-urban ecosystems that receive mixed organic loads and hydraulic variability. This research proposes a multi-criteria optimization in the design of a Wastewater Treatment System (WTS) in the town of Etzatlán, belonging to the Valles de Jalisco Region, Mexico. Methodologically, a retrospective-prospective demographic modelling (horizon 2020-2045) and a direct in situ flow measurement were used to define the hydraulic regime. The results revealed an atypical hourly coefficient of variation (k2 = 2.19) and organic load with high concentration (Biochemical Oxygen Demand (BOD5) of 652.3 mg/l, with fats and oils that exceed the regulations) related to agro-industrial pig discharges. Through a qualitative-quantitative decision matrix, five technological alternatives were evaluated, determining that the configuration of an Imhoff Tank coupled to Biological Filters provides greater operational resilience. The system absorbs toxic shocks through physical phase separation, minimizes the carbon footprint and offers a 43% saving in operating costs, compared to conventional activated sludge systems. It is concluded that the modular design facilitates the closure of the hydrological cycle through the revaluation of nutrients for agricultural fertigation, strengthening the water-energy-food-ecosystem (WEFE) nexus. The research demonstrates that the transition from conventional sanitation systems to resilient, decentralised approaches aligned with circular economy principles represents a technically feasible and environmentally sustainable alternative for peri-urban municipalities in western Mexico exhibiting similar hydraulic and socio-environmental characteristics.