Seasonal variation in fecal indicators removal in constructed wetlands with horizontal subsurface flow

Wetlands are known to act as biofilters through a complex of physical, chemical and biological factors which all participate in the reduction of the number of bacteria. It has already been shown that the removal of pathogenic bacteria in constructed wetlands with horizontal subsurface flow is high but there is only little information about the seasonal effect. Six constructed wetlands with subsurface horizontal flow were monitored in order to evaluate the removal of potentially pathogenic bacteria (total coliforms, fecal coliforms and fecal streptococci) with respect to the seasonal variation and the distribution of these bacteria on macrophyte roots in constructed wetlands. The results indicate that the removal of potentially pathogenic bacteria is very high (>97% for total coliforms, >90% for fecal streptococci and >93% for fecal coliforms) and that there is only a little seasonality. In most cases the number of bacteria in the outflowing water was lower in winter as compared to summer. However, the differences were not statistically significant. Examination of bacteria counts on roots and rhizomes of Phalaris arundinacea and Phragmites australis revealed significantly higher numbers on Phragmites roots. It has also been found that the reduction of bacteria on roots takes place within the first few meters of the bed. There was no seasonal effect observed.