Ammonium nitrogen was removed from pretreated domestic sewage using a natural zeolite-clinoptilolite, applying a column with downflow regime. The pretreated wastewater had an average total ammonium nitrogen (TAN) of 23 mg l(-1). Ion exchange was allowed to take place under normal sewage treatment plant conditions and tropical temperature setting. The sorption capacity of the natural clinoptilolite was studied with the column fitted to a chemically enhanced primary treatment (CEPT)-upflow anaerobic sludge blanket (UASB) system, operating at 33 degrees C. Optimal operational cation exchange capacity (CEC) of 14 g TAN kg(-1) zeolite was obtained for the zeolite granulometry of 1-2.5 mm in a reversible way. The treatment was done at 3.9 bed volumes (BV) per day, and breakthrough was attained after about 560 BV. Regeneration of exhausted zeolite was effected with a 25 g NaCl l(-1) salt solution, rendering the column amenable to reuse. Employing a 3-column test and bed contact times of 6, 2.6 and 1.1 minutes, the use of pretreated wastewater with average concentration of 38 mg NH4(+)-N l(-1) showed that the operational cation exchange capacity also depended on flow rate, with the removal efficiency decreasing from 95 to 47% with increase in the flow rate. Overall, the approach of removing TAN from digested sewage offers potential to recover the TAN from the domestic wastewater, thus minimizing the need for subsequent nitrification-denitrification of the whole water volume.