Based on 20 months of pilot experiments, tertiary plastic media trickling filters have shown to be a feasible and cost-saving solution for the enlargement of existing waste-water treatment plants for nitrification. The influence of serveral process variables as hydraulic load, media characteristics. NH4-load and fluctuations, suspended solids, organic nutrients and biomass grazing on nitrification activity was evaluated. From performance analysis along the trickling filter depth, basic design informations were obtained. Besides Ammonia-oxydation, special emphasis was put on the formation of Nitrite. In addition to suspended solids removal, subsequent deep-bed filtration proved to be an excellent final treatment to further reduce and stabilize effluent Ammonia and Nitrite residuals.