Removal of alkali metals and their sequestration in plants in constructed wetlands treating municipal sewage

This study is aimed at retention of K, Na, Mg, and Ca in two constructed wetlands (CWs) in the Czech Republic, and on the evaluation of particular standing stocks in both above- and belowground plant biomass. The study revealed that CWs with horizontal subsurface flow are not effective in retention of studied elements. Removal of K, Na, Mg, and Ca averaged only 10.6, 7.4, 6.1, and 1.4%, respectively. In general, concentrations of studied elements in various parts of Phragmites australis and Phalaris arundinacea were found within the range of concentrations reported from both natural and CWs. Aboveground standing stocks for K, Na and Mg were comparable with those reported from natural stands for both Phalaris and Phragmites, but Ca aboveground standing stocks found in our study were lower compared to those found in several natural Phragmites wetlands. Aboveground to belowground standing stock ratio was generally >1.0. However, this amount formed usually <1% of the annual inflow load of particular elements. The results of this study provide comprehensive information on retention and sequestration of K, Na, Mg, and Ca in vegetation during municipal wastewater treatment in CWs with subsurface horizontal flow.