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Abstract
Retention times of treated water in a constructed wetland (CW) with horizontal subsurface flow were determined both in the vegetative and non-vegetative periods of 2005. Tracer experiments were performed using fluorescein, an organic compound detectable at extremely low concentrations. Nominal and tracer retention times were determined and compared. Winter tracer retention time (TRT 194 h) and nominal retention time (nHRT 190 h) were nearly exactly equal, while summer TRT (335 h) was approximately twofold higher than nHRT (158 h). Residence time distribution function (RTD) was used to compare retention times obtained for the vegetative and non-vegetative periods. The obtained results document a significant influence of dense common reed vegetation on retention characteristics of the studied system. Common reed can convert a significant volume of water from liquid to gas via evapotranspiration (ET) and thus prolong water retention times in the system. This is very important both technically and ecologically. Longer retention times mean a longer time for microbiological decay of wastewater. Water converted from liquid to gas causes cooling of the microclimate, which is very important, especially in intensively cultivated areas with a lack of water.
Acknowledgements
The study was supported by the Grant Agency of the Academy of Sciences of the Czech Republic (projects KJB 601410708 and KJB 601410502) and the Ministry of Education, Youth and Sports of the Czech Republic (research project MSM 6007665801). The authors are also much obliged to Assoc. Prof. Šárka Klementová for providing the fluorimeter. We also warmly thank Slavošovice and Libín village authority, the mayor Mr. Ludvík and constructed wetland manager Mr. Adamík for technical assistance and help with continuous measurement in the Slavošovice constructed wetland.