The removal of ammonium ion from wastewater is an important part of tertiary wastewater treatment and is increasingly necessary to meet drinking water and discharge standards being applied in much of the developed world. The use of nitrification filters is an established technology which involves the biological oxidation of ammonium ion to the more benign nitrate ion. An alternative technique for ammonia removal is the use of ion-exchange however the capital costs can be significant and the economics of chemical regeneration may be prohibitive for large volume applications. In this paper we examine the combination of nitrification and ion-exchange in a packed bed system using the natural zeolitic ion-exchangers, clinoptilolite, and mordenite on which colonies of nitrifying bacteria are cultivated. The combined system intensifies ammonia removal and offers the possibility in principle of a more robust technique less sensitive to rapid perturbations in ammonia concentration and less costly than in a single ion-exchange process. One limitation of this approach is the rapid consumption of oxygen in the biologically active column. The paper demonstrates a novel technique for enhancing the oxygen concentration in a combined ion-exchange and nitrification packed column. The performance of fixed beds of clinoptilolite and mordenite in the presence of nitrifying bacteria is compared to that in columns in which only ion-exchange is occurring. Two modes of aeration are compared: (1) external aeration in which the feedwater is aerated conventionally by gas bubbling; (2) in situ aeration in which a membrane-aeration unit is incorporated into the packed bed. The ammonia removal in the presence of the nitrifiers is significantly intensified. Both methods of aeration provided a comparable further enhancement. In the case of the membrane system the air consumption was reduced by a factor of 60. (c) 2007 Published by Elsevier B.V.