The ion exchanger loop stripping (ILS) is a novel, hybrid process to recover ammonium (NH4+) from the liquid phase of mechanical sludge dewatering (sludge liquor) at waste water treatment plants. The process combines ion exchange on fixed-bed columns containing natural zeolite (main mineral clinoptilolite) with simultaneous air stripping of ammonia to produce an industrial NOx removal agent. In the present work, the ILS process, which was first described in 2012, was developed from lab- to pilot plant scale to treat sludge liquor at a municipal wastewater treatment plant. The suitability of natural zeolite for the recovery of NH4+ from model solutions as well as samples of sludge liquor and the technical feasibility of the ILS was proven in laboratory experiments. Depending on the process conditions, NH4+ 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 the municipal wastewater treatment plant in Knittelfeld. Several weeks of experimental operation showed the technical feasibility of the process and the stability of the zeolite. The achievable NH4+ 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. Ammonia was recovered to obtain a highly concentrated ammonium sulfate solution (40 m%), which 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.