For the recovery of ammonium (NH4+) from the liquid phase of mechanical sludge dewatering (sludge liquor) at municipal waste water treatment plants, a hybrid process, the “ion exchanger loop stripping” (ILS), has been developed by the chair for Process Technology and Industrial Environmental Protection. The ILS process combines ion exchange and air stripping in a hybrid process and works with fixed bed columns containing natural zeolite (main mineral clinoptilolite), alternately loaded and regenerated. Clinoptilolite exhibits high selectivity for NH4+ ion-exchange. For the regeneration process a sodium hydroxide solution at a pH > 11 and a temperature of about 60 °C is used. The regeneration fluid is simultaneously air-stripped and the resulting gaseous ammonia is captured as concentrated ammonium sulfate in solution via an acid scrubber. The product can be used as an industrial NOx-removal agent, for example in cement works providing an all year round utilization of recovered nitrogen. The technical feasibility of the ILS-process was proven in laboratory experiments, treating model solutions as well as sludge liquor (20-30 L h-1). Depending on the process conditions, ammonium removal rates of 60–80 % were achieved for input concentrations of about 1000 mg NH4+ L-1. Based on these results, a mobile pilot plant with a treatment capacity of approximately 500 L sludge liquor h 1 was designed and operated at a local municipal waste water treatment plant. In this study, we present results from the pilot plant operation. Several weeks of experimental operation showed the technical feasibility of the ILS process. The achievable ammonium removal and recovery rates in consecutive loading and regeneration cycles were measured and revealed ammonium removal rates of 69–94 %, when sludge liquor with typical input concentrations from 500–1500 mg NH4+ L-1 was treated. Removal rates increased after the first NaOH regeneration of the zeolite and remained constant over more than sixty consecutive loading and regeneration cycles. Ammonia was recovered to obtain a highly concentrated ammonium sulfate solution (40 m%). The solution was successfully tested as an industrial NOx removal agent in a cement work in Styria. The site-specific results show, that the implementation of an ILS production plant at municipal wastewater treatment plants for the recovery of ammonium from sludge liquor is ecologically and economically viable under certain conditions.