| Summary: | Constructed wetlands (CWs) are being introduced in many parts of the world to treat wastewater. CWs offer several advantages over conventional treatment, most notably to save costs and energy. By contrast there are several limitations associated with the use of CWs, such as variability and unpredictability in treatment performance. However the literature focuses largely on the advantages of the CWs with little attention being given to the limitations and impacts on the receiving environment. In South Africa, there are a few studies concerned with the application and performance of CWs, but as yet there are no guidelines for the design and construction of these systems. The aim of this research is to determine the performance of three CWs situated on the periphery of Cape Town, Western Cape, with the intention of contributing to knowledge on the South African CWs performance in general. The research interest was to purposely shift attention to an analysis of the performance of CW systems that could be measured in-situ as opposed to laboratory-based studies where certain variables could be contained or controlled. In this study the focus is on determining the impact that these systems might have on the surrounding environment by analysing the impact from these CWs on surrounding or receiving water bodies. Samples of influent and effluent were collected from various points within the CW and from the surrounding water bodies every two weeks during the winter season when biological activity is least productive. Performance was determined by considering the mean percentage change from influent to effluent, the significance of the difference between influent and effluent and by comparing resultant effluent quality to the Department of Water Affairs' discharge standards. The results of the study indicate a range of performance both within and between systems, but overall the performance was poor, with the exception of NH3 (96%) and E. coli (see below) that was removed at one of the sites, namely, at De Goede Hoop. While PO43- was adsorbed, it was very low at all three sites; 3.8%, 7% and 20% at De Goede Hoop, Wolwedans and Babylonstoren respectively. Furthermore, DWA's effluent standards of 10 mg/l for PO4 3- could not be met at all the sites. Poor PO4 3- removal can be explained either by low O2 concentrations or the choice of substrate that was used in the constuction. When O2 concentrations are low, solubilisation of minerals and subsequent release of dissolved of phosphorus occurs. Mean E. coli removal percentages were considerably lower compared to other studies undertaken elsewhere. E. coli removal was 85% at De Goede Hoop, 39% at Wolwedans and 65% at Babylonstoren. In general, the results indicate that more research on CW systems is required to improve our understanding of these systems. A better understanding of these systems will lead to enhanced design and thus assist in improved treatment performance so as to reduce the impact of CWs on the environment.
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