Abstract When ion exchange is considered for use as an ammonium removal process in wastewater treatment an extensive pilot plant testing program is usually required. The pilot study allows the design engineer to evaluate the influence of variables such as loading rates, regenerant volume and flowrate, etc. on ammonium performance. These pilot studies may be quite extensive and costly. In this paper a model is presented that may be used to assist the design of ammonium removal facilities following the collection of a limited amount of data to characterize the wastewater. The very simple computer model presented is capable of predicting the exchange behavior of natural clinoptilolite, an ammonium selective ion exchanger, during service and regeneration. The regeneration model and the service model are both filed in the computer. The model outputs are also stored in files and it is possible, for example, to use the output files from the regeneration model as input files for the service model. In this way an engineer may evaluate the impact of alternative regeneration conditions on ammonium removal performance during service, etc. Equilibrium relationships for ammonium distribution and physical column data are required for both the regeneration and service models. Experiments to evaluate this data are described. The predictive ability of the computer model is demonstrated in the paper for laboratory studies in which various regeneration and service schemes were utilized.