Sustainability of a multi-stage free water surface constructed wetland in terms of water quality under changing conditions

Multi-stage constructed wetlands (CWs) are widely used for water quality improvement, especially in the treatment of wastewater. Many studies focus on their treatment efficiency under steady loading, but fewer studies consider their stability and sustainability under variable conditions. This study monitors the hydrology and water quality at the multi-stage CWs in the Hong Kong Wetland Park. Five wetland units along the flow path are examined for their long-term performance and sustainability in terms of water quality under seasonal changes, storm events, and shock loadings of pollutants. Time-series statistical analysis indicates that the multistage design well achieves stable performance. Each wetland unit has certain roles and they work together to achieve good performance. The reliability analysis shows that the CW system can largely buffer the fluctuations from most disturbances. While the resiliency analysis also shows that most water quality indicators could recover in a few days after the fluctuations. The water levels recover quickly but it was difficult to return to original water levels in multi-stage CWs. Besides, a numerical model is developed, calibrated, and utilized to predict future water quality changes. This will help evaluate measures to improve the sustainability of multi-stage CWs by simulating water quality changes under different influent concentrations and rainfall conditions. This study could provide appropriate recommendations and early warnings for wetland management and improvement.