Constructed wetlands for pollution control

Artificial wetlands are used in over 50 countries to sustainably treat wastewater. These constructed wetlands (CWs) make use of natural biogeochemical and physical processes to remove organic matter and nutrients, while providing co-benefits such as ecosystem services and recreation. However, their performance can be variable, as local weather conditions, wastewater composition and operation can affect contaminant removal. In this Review, we discuss the global application and distribution of CWs, and factors affecting their functioning. Based on data from 335 field-scale CWs, hybrid CWs (which use two or more types of CW during water treatment) are the most effective for water-quality improvement and greenhouse gas mitigation, removing 76%, 63% and 72% of organic matter, nitrogen and phosphorus, respectively (based on the median values). Plant species, substrate selection, and environmental and hydraulic conditions affect performance, with variations in contaminant removal performance mainly related to temperature, hydraulic residence time and pollutant loading rates. Pollutant removal and long-term performance of CWs can be enhanced through strategies such as artificial aeration, cold-resistant plant selection, electron donor supplementation and clogging repair. Engineers and scientists must work together to design and manage CWs, ideally maximizing their co-benefits alongside pollution control.