Optimizing decentralized wastewater allocation to urban green spaces in the water scarce megacity of Tehran

Abstract Water scarcity is among the most pressing global challenges, and conventional systems increasingly fail to secure reliable urban water supplies. Reusing municipal wastewater for irrigation offers a sustainable, low-cost option for sustaining urban green spaces, the largest non-potable water users in arid cities. This study develops a Mixed-Integer Linear Programming (MILP) model to evaluate a decentralized wastewater-reuse scheme for Tehran. The model minimizes the total cost of wastewater collection, conveyance, treatment and distribution, subject to hydraulic, capacity and demand constraints. It selects branch points on existing trunk sewers (diameter ≥ 1200 mm), routes flow by gravity to local treatment packages sited next to parks, and meets irrigation demand in full. The model was solved with CPLEX and verified on a reduced instance whose optimum was established by complete enumeration. All demands of 38 parks are met through 49 gravity-fed routes from 47 manholes. Charging local treatment at 10,000 tomans per cubic meter, the scheme repays its capital in 3.8 years and cuts the 15-year cost by 65.8% against the centralized alternative; the advantage holds below 58,333 tomans per cubic meter. The findings provide a scalable framework for embedding decentralized reuse within urban water planning for water-scarce megacities.