A novel vertical bed process for the removal of ammonium from secondary effluents, using a "passive air pump", has been developed. The process is based on convective aeration caused by a fill and draw operational sequence, and combines the advantages of the vertical wetlands concept with the high loading rates typically associated with trickling filters. Experiments were carried out in a 500-l reactor using simulative effluents and actual municipal secondary effluents. A maximal ammonium removal rate of 1100 g N/m2 reactor/d was achieved using simulative effluents and an effective gravel size of 0.96 mm. At all hydraulic loads applied, the nitrification rate was found to be limited by the oxygen transfer rate. The small-size medium used with simulative effluents clogged when using actual municipal secondary effluents. Two other media (2.46 mm and 4.31 mm) did not clog during the entire experimental period and a maximum removal load of 300 g N/m2 reactor/d was achieved. This value is still much higher than typical rates reported for conventional vertical beds.