The Erh-Ren River is located in southern Taiwan and is one of the most polluted rivers in Taiwan. A pilot-scale constructed wetlands (ERRCW) system containing a free surface flow wetland (FSF) followed by a subsurface flow wetland (SSF) was set up to treat the polluted river water. The system was designed to include solids removal. A removal efficiency of suspended solids (SS) in the ERRCW system that was greater than in a control system indicates that the existence of macrophytes in wetlands could enhance solids removal. The results of this study also showed that SS were removed mainly through the SSF in the ERRCW system. The poor removal of SS, or even an increase in SS, that occurred in the FSF was found to be caused by the growth of algae. Although the macrophytes reduced the penetration of sunlight into the water in the FSF portion of the ERRCW system, the residue of nutrients in water and a low flushing rate still induced the growth of algae on some open water surfaces. From this study, it may be concluded that the roles played by macrophytes in solids removal include providing barriers for filtration, reducing sunlight penetration, supplying oxygen to enhance biodegradation of organic solids, and absorbing nutrients to limit the growth of algae.

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