How can urban wastewater and reclaimed water systems tackle an uncertain future without regret? - Solutions based on adaptive pathway planning.

Urban wastewater and reclaimed water systems (UWRWSs) worldwide are increasingly facing deep uncertainty arising from rapid urban expansion, fluctuating service demands, and evolving regulatory expectations, challenging traditional deterministic planning approaches. This study introduces a generalizable adaptive pathway planning (APP) framework as a forward-looking alternative for long-term UWRWS planning. The six-step framework integrates uncertainty identification, multi-objective performance evaluation, adaptive pathway generation, and implementation monitoring into a unified decision process. Applied to the UWRWS planning of Xiong'an New Area, APP generated 48 Pareto-optimal initial solutions for 2025, from which six representative configurations produced 4860 adaptive pathways across 810 condition pathways. Results show that maintaining initial solutions without adaptation would lead to regret in at least 31% of future pathways. Compared with traditional planning, APP-derived solutions reduce the average lifecycle cost by 7-13% and increase resilience by 5-25%. Traditional planning fails to meet the 0.95 resilience target in two-thirds of pathways and incurs higher costs in up to 97% of pathways. Even under perturbed wastewater discharge scenarios beyond predefined pathways, APP demonstrates strong robustness. Beyond the case study, the APP framework is applicable to diverse UWRWS management contexts, offering a practical and transferable approach for cities seeking resilient, resource-efficient, and no-regret infrastructure strategies.