Enhancing the Storm Water Treatment Performance of Constructed Wetlands and Bioretention Basins
Urbanisation leads to both quantitative and qualitative changes to storm water runoff. While the quantity changes have received much attention in the past, now the quality changes are beginning to receive significant attention. The quality changes are primarily due to a range of anthropogenic activities common to urban areas, which result in the generation of various types of pollutants. These pollutants accumulate on urban catchment surfaces and are eventually washed off by storm water runoff creating irreversible impacts on receiving water environments. In this context, structural storm water treatment measures are introduced, promoting pollutant removal through physical, chemical and biological processes. They also detain, retain and regulate storm water runoff to improve water quantity and quality characteristics. Bioretention basins and constructed wetlands are among the most common storm water treatment systems, and their treatment performance is closely dependent on hydrologic and hydraulic characteristics. Consequently, the in depth understanding of the role of hydrologic and hydraulic factors in bioretention basin and constructed wetland treatment performance is important for effective urban storm water design strategies. This research monograph presents the outcomes of a detailed investigation into the influence exerted by hydraulic and hydrologic factors on the treatment performance of bioretention basins and constructed wetlands. In relation to bioretention basins, the research outcomes confirmed that the antecedent dry period is an important factor influencing pollutant removal efficiency. A relatively long antecedent dry period will result in comparatively low moisture content in the filter media, which can enhance the runoff retention capacity and consequently improve treatment performance. This implies that planting of vegetation with high evapotranspiration capacity would enhance treatment efficiency. Additionally, it was found that pollutant leaching influences bioretention basin treatment performance, particularly reducing the ability for nutrient removal. This highlights the importance of the selection of appropriate filter media and its timely replacement. In the case of constructed wetlands, it was found that large and small rainfall events are subjected to different treatment. The pollutant load reductions in the vii initial sector of the runoff hydrograph from large rainfall events were relatively low due to the rapid mixing. This highlights the need to establish an inlet pond to initially intercept the flow entering the constructed wetland so that the inflow is stabilised. This is also supported by the fact that the initial sector of the runoff hydrograph generally carries higher pollutant loads, namely the first flush effect. Additionally, the provision of a bypass system is recommended to control the runoff to the constructed wetland. This will protect the treatment system from erosion damage resulting from high runoff rates. This research monograph further showcases an innovative approach for using conceptual models to analyse storm water treatment system performance. The approach adopted has the capability to generate key hydraulic data for individual rainfall events in relation to the treatment systems investigated. This is a significant advancement from conventional approaches for the analysis of treatment system performance, which is based on the use of lumped parameters. The knowledge presented provides practical guidance and recommendations for improved urban storm water management to assist researchers, design engineers, decision makers, urban planners and storm water quality model developers.