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

Data from 18 sampling wells in Kodijärve 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 Kodijärve 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 Kodijärve CW has improved significantly and there has been a slight increase in purification efficiencies of NH₄-N and N_tot. On the ohther 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.