The effect of a vertical flow filter bed on a hybrid constructed wetland system

Data from 18 sampling wells in Kodijarve horizontal subsurface flow (HSSF) constructed wetland (CW) (South Estonia) is presented and differences in purification efficiencies inside the HSSF CW are calculated. Temporarily anaerobic conditions in the Kodijarve HSSF system did not allow efficient removal of BOD7, NH4-N, N-tot and P-tot. In 2002 a vertical subsurface flow filter was constructed to enhance aeration. The design of the system was based simply on the oxygen demand of the wastewater and on the aeration potential of vertical flow wetlands. The vertical flow system has shown satisfactory results. The purification efficiency of BOD7 in the Kodijarve CW has improved significantly and there has been a slight increase in purification efficiencies of NH4-N and N-tot. On the other hand, the removal efficiency of P-tot has decreased significantly. Although, the mass loading rates have increased, mass removal rates of all four parameters have improved significantly. Nevertheless, optimization of the constructed wetland system is essential in order to meet effluent standards during wintertime.