Is Concentration of Dissolved Oxygen a Good Indicator of Processes in Filtration Beds of Horizontal-Flow Constructed Wetlands?
Filtration beds of subsurface horizontal-flow constructed wetlands (HF CWs) are generally considered as anoxic or anaerobic and, therefore it is assumed that the outflow concentration of dissolved oxygen (DO) is usually very low. During 2004 and 2005, nearly 60 HF CWs in the Czech Republic were visited and it was found that many systems really provided low DO outflow concentrations (<2 mg l–1). However, a substantial number of systems surprisingly provided relatively high concentration of DO (>5 mg l–1). We selected several systems where monitoring program included a broader selection of parameters, especially ammonia-N, nitrate-N, and sulfate, and set up more frequent measurements of dissolved oxygen. In our study, we also used some older data that were obtained during research projects in the past. We focused on nitrification and sulfate-reduction as processes occurring under strictly aerobic and anaerobic conditions, respectively. When evaluating the data, we found that in systems with very low outflow concentrations of DO, nitrification was frequently very limited but in some systems a substantial reduction of ammonia occurred. On the other hand, several systems with relatively high O2 outflow concentrations provided nearly zero removal of ammonia. The similar unanimous results were observed for sulfate, i.e., high O2 outflow concentrations were sometimes connected with high reduction of sulfate and on the other hand low O2 outflow concentrations were not connected with sulfate reduction. We concluded that DO concentration at the outflow from HF CWs does not provide good information about the processes occurring in the filtration beds.
KeywordsAmmonia dissolved oxygen nitrate subsurface horizontal flow sulfate
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