An ion exchange process was assessed to improve ammonium removal by vertical flow filters. Six columns, filled with gravel and zeolite (30cm and 10cm, respectively), were compared with a standard column with 40 cm of gravel. Columns were fed for 3.5 days with semi-synthetic wastewater then rested for 7 days. Each column, filled with zeolite, had different inlet characteristics in order to study the effects of operational parameters pointed out as affecting the ion exchange process. Two columns assessed the impact of concentration (100mgNH4-N.L-1 versus 2000mgNH4-N.L-1) on exchange capacity and performance. The effect of competition with another cation on efficiency and saturation rate was studied at three different concentrations of sodium (0 mg.L-1, 85 mg.L-1 and 300 mg.L-1, respectively). Finally, regeneration of exchange capacity by nitrification as well as its effects on treatment efficiency was studied. Although zeolite showed promising results in the early stages of operation (>80%), performances quickly declined, until reaching similar efficiency as the standard (60%), suggesting insufficient regeneration. Moreover the concentration strongly affected the exchange capacity, the latter one being quite low for the range of ammonium concentrations usually observed in domestic wastewater. The sodium supply did not result in performance reductions for the studied conditions.