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 treatment plants for nitrification. The influence of several process variables such as hydraulic load, media characteristics, NH 4-load, concentration 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. 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.