Large constructed treatment wetlands for decentralized wastewater treatment: an integrated decision-support framework for design, operation, and governance

Introduction Large treatment wetlands (TWs) can provide decentralized wastewater treatment at infrastructure scale, but successful implementation requires the integration of site conditions, wastewater characteristics, treatment-train design, operation, financing, and governance across the project lifecycle. Methods We developed an integrated decision-support framework based on a structured qualitative evidence synthesis and a comparative assessment of 64 large-scale TW projects. Project-level data were extracted on location, wastewater source, inflow, population equivalent, wetland area and type, treatment train, vegetation, pretreatment, post-treatment, conventional pollutant removal, data limitations, and special design or operational features. Projects were grouped into 18 configuration categories. Evidence on emerging contaminants was synthesized separately at the compound-group level. Results Staged vertical-flow/horizontal-flow systems and pretreated vertical-flow systems showed strong removal performance for BOD 5 , COD, TSS/SS, nitrogen indicators, and total phosphorus when supported by adequate pretreatment, hydraulic distribution, monitoring, and operation and maintenance. Performance varied with influent strength, treatment objective, system configuration, pollutant-specific data availability, and evidence quality. Micropollutant attenuation was promising but compound-specific and depended on media selection, redox zoning, hydraulic retention time, vegetation, sorption capacity, and multi-barrier design. The resulting matrix links six framework dimensions to key decisions, indicators, red flags, planning outputs, readiness levels, and no-go risks. Discussion Large TWs should be planned as governable, adaptive, and multifunctional public infrastructure. The framework provides implementation-oriented support for early project screening, treatment-train selection, governance design, and long-term decentralized water management.