Introduction: Energy recovery from excess head in urban water distribution networks using pump-as-turbines (PATs) can convert a portion of the wasted energy at pressure reduction stations into electricity; however, flow and head variations, along with operational effectiveness index requirements and economic considerations, make determining suitable configuration and operating conditions a complex problem.

Materials and Methods: In this research, a multi-objective optimization framework with two independent objectives maximizing net present value and maximizing the daily average operational effectiveness index was developed for PAT system design at a real pressure reduction station in northern Tehran. The energy recovery capability index (η_P) was applied as an explicit minimum performance constraint. Unlike conventional continuous approaches, decision variables were defined discretely based on a real pump catalog to ensure the final selection is close to commercially available products. The Pareto front was extracted through an exhaustive search in the discrete space, and the final solution was selected using the TOPSIS method. To assess decision stability, a Monte Carlo analysis with 10,000 iterations and ±20% variations in objective weights, as well as a sensitivity analysis to demand profile changes (±10% and ±20%), was performed.

Results and Discussion: The series configuration exhibited better energy and economic performance than the single configuration in all scenarios. At the reference design point, the η_P index was 0.676 for the series structure and 0.595 for the single structure (approximately 13.6% improvement), the NPV was €96,280 versus €87,814 (approximately 9.6% improvement), and the daily energy was 189.3 versus 166.6 kWh (approximately 13.6% improvement). Objective weight sensitivity analysis showed the series structure has a real trade-off between NPV and the operational effectiveness index, and a discrete jump occurs at wNPV≈0.1, reflecting the transition from a conservative configuration to an economic one. Monte Carlo analysis showed that the series structure has complete stability against objective weight uncertainty, while the single structure is sensitive to the exact weight value. Demand sensitivity analysis also maintained the series structure in all scenarios. The advantage of the series structure mainly stems from its ability to maintain energy recovery during low-flow hours.

Conclusion: Separating the objectives and applying an explicit minimum energy performance constraint within a multi-objective optimization framework, while clarifying the trade-off among energy, economics, and the operational effectiveness index, enables informed selection of the optimal configuration. Distributing the head between two PATs can improve the energy recovery capacity and economic justification of pressure reduction systems.