Design of water connections to improve the sustainability of an urban constructed wetland system

Abstract

The constructed wetland (CW) system is usually regarded as a cost-effective and eco-friendly technology, especially in urban drainage systems, where CWs attenuate flooding, purify pollutants, and support habitats. However, some challenges exist in modeling, evaluating, and optimizing CWs.

First, there are models available for CWs and urban drainage systems separately, but there is a lack of systematic framework for coupling wetland models for urban drainage system applications. Therefore, this thesis develops an integrated modeling framework to simulate various CWs in urban drainage systems. The model framework is based on the SWMM model to simulate simple cases of drainage systems and coupled with the OpenFOAM and modified EFDC models to cope with complex cases of hydrodynamic and biochemical processes, respectively. This modeling framework is more suitable for urban CW systems after simple case validation.

Second, hydrology is the primary driver of mass and energy in CWs and is vital to CWs’ design, operation, and maintenance. However, there is currently a lack of comprehensive understanding of CWs’ responses to changing hydrological conditions. Therefore, this thesis monitors a multistage surface flow CW, evaluates its performance and sustainability in water resources management and water quality improvement under changing hydrological conditions, and proposes recommendations for current urban CWs. A summary of previous cases based on a literature survey is also presented. Simple quantitative methods are used to investigate the feedback of the CWs under changing hydrological conditions and propose simple prediction equations. The insights generated can be used to deepen the understanding of changing hydrological conditions for urban CWs systems.

Third, hydrological connectivity is a critical factor for urban CW systems. Connecting different CWs can have synergistic effects that enhance overall performance and sustainability. However, currently there is a lack of index system to assess hydrological connectivity and how it impacts CW systems' sustainability. Therefore, this thesis proposes a CW management framework that uses an evaluation index system consisting of hydrological connectivity indexes and sustainability indexes. The index system is applied to a case study to analyze the improvement of hydrological connectivity design for urban sustainability. Results indicate that among the three connection modes of dendritic, parallel, and reticulate, the reticulate mode has the best effect in peak reduction. Each of these three connection modes has its focus, with the dendritic mode being more stable and the reticulate mode being more reliable and resilient. It was also found that the CWs located closer to the end of the drainage system are more sensitive and important, while CWs near the source can have a broader range and combination of design parameters. Finally, the optimal CW design parameters are derived based on the sustainability index.

These frameworks, methods, and tools developed to address individual challenges culminate in a comprehensive simulation, evaluation, and optimization framework that can be utilized to improve the sustainability of current CWs practices in urban areas.