Urban pollutant removal by a constructed riparian wetland before typhoon damage and after reconstruction
Introduction
Constructed wetlands have been utilized for many years to improve the quality of wastewater and urban runoff. Such wetlands have been used to: (i) remove undesirable nutrients from domestic wastewater (Kadlec and Knight, 1996, Verhoeven and Meuleman, 1999, Öövel et al., 2007); (ii) decrease bacterial counts in wastewater (Vacca et al., 2005); and (iii) remove metals from urban runoff (Scholz and Xu, 2002, Scholz, 2006). Constructed wetlands are quite intricate systems, removing contaminants by physical, chemical, and biological mechanisms (US EPA, 1999). Microbial diversity (Baptista et al., 2008, Nikolausz et al., 2008) and nutrient removal mechanisms (Tietz et al., 2008) are constantly being investigated for constructed wetland systems. In seeking to achieve these objectives, free-water-surface (FWS) wetlands generally cost less and have greater environmental value than other alternatives (because they offer additional ecological habitats), but they have the disadvantage of being more sensitive to temperature fluctuation (Kadlec, 2007), especially in tropical or subtropical regions such as Taiwan.
Experimental or pilot-scale systems utilizing FWS wetlands in tropical and subtropical regions have been shown to remove undesirable nutrients from: (i) campus residential runoff (Kao et al., 2001); (ii) industrial wastewater (Chen et al., 2006); (iii) oil-refinery and steel-mill wastewater (Yang and Hu, 2005); and (iv) polluted river water (Jing and Lin, 2004). In addition, nitrates in groundwater have been successfully removed by FWS wetland cells planted with Typha, Pennisetum, and Ipomoea vegetation (Lin et al., 2007).
In Taiwan, the high population density and the still-developing sewage-treatment systems have meant that constructed wetland systems are an attractive alternative for treating urban drainage and agricultural runoff. As shown in Table 1, several systems of this type have been brought into operation in the past four years in Taiwan (Taiwan EPA, 2007). It is apparent from the table that the sizes of these projects have varied greatly, depending on land availability; however, most of the projects have been set up in riparian areas between dikes and rivers because most human settlements have historically developed along these rivers and there is therefore a need to treat their pollutant loading to protect the quality of river water. Moreover, in addition to improving water quality, restoring wetlands reclaims lost habitats (Meier et al., 2005) and protects coastlines (Mitsch, 2007).
However, an inevitable feature of these systems in tropical and subtropical areas is natural disturbance brought about by the occurrence of hurricanes (or typhoons, as they known in the north-west Pacific area). Massive amounts of sediment can be accumulated as a result of flooding after a single tropical storm (Turner et al., 2006), and an Australian study has shown that the function of wetlands in retaining phosphorus can be significantly compromised by such storms (Novak et al., 2007).
Against this background, the present study assesses the performance of a constructed riparian FWS wetland in removing urban pollutants before and after a typhoon in a subtropical climate. The efficiency of the system before the typhoon and after reconstruction is analysed by Wilcoxon matched pairs test and Mann–Whitney U-test with respect to biochemical oxygen demand (BOD), ammonia-nitrogen (NH4-N), and total phosphorus (TP).
Section snippets
Site description
The Hsin-Hai Bridge FWS artificial wetland, which was completed in May 2004, was treating 1000 cubic metres/day (CMD) of domestic wastewater from a channel that receives a combination of rainfall runoff and residential septic tank effluents. The artificial wetland is located on the right bank of the Da-han Creek riparian, which is part of the Tan-shui River basin in metropolitan Taipei (25°03′121″N; 121°45′714″E). Flow rates of the drainage channel ranged from 31,000 CMD to 42,000 CMD. The
BOD removal
The findings with regard to BOD removal are shown in Table 4 and Fig. 2. For the whole system, removal efficiencies before the typhoon and after reconstruction were 86.9 ± 6.7% and 81.5 ± 18.1%, respectively. The slightly reduced removal efficiencies might have been caused by the increased hydraulic loading after reconstruction (as noted above). The mean influent BOD values before the typhoon and after reconstruction were 41.0 ± 11.2 mg/L and 42.9 ± 17.5 mg/L, respectively (Fig. 2c). The mean effluent
Conclusions
Although flooding as a result of an extreme weather event such as a typhoon is always a potential threat to constructed riparian wetlands in subtropical regions, the present study has demonstrated that the pollutant-removal performance of the Hsin-Hai Bridge FWS flow wetlands in Taiwan was capable of being largely restored by reconstruction of the wetland after Typhoon Aere at a fraction of the original cost of constructing the wetland system.
Although increased hydraulic retention time and the
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2010, Science of the Total EnvironmentCitation Excerpt :Therefore, this finding confirmed that a limited restoration could not fully recover the pollutant removal capacity of the FWS wetland from the impacts of Typhoon Krosa. Comparing our results with Fan et al. (2009), this study suggests that more aggressive reconstruction is required to fully restore pollutant removal efficiencies. Nevertheless, conservative restoration by removing the vegetation that was washed ashore might be considered an alternative for riparian constructed wetland management strategies in subtropical weather regions.