A large pilot scale trickling filter with an established nitrifying biofilm on a cross flow plastic material having a high specific surface area has been used to evaluate the short term effects of changes in hydraulic load and influent ammonium concentration. The experiments were conducted as 15 randomized 22 factorial design experiments where the hydraulic load was set at 5.7 or 11.3 m/h and the influent ammonium concentration was set at 8 or 16 gN/m3 independently of the hydraulic load. An evaluation of the factorial design experiments revealed no difference in nitrification rate between a high hydraulic load and a low ammonium concentration and vice versa for the two setups with the same ammonium load. The short term dynamics were investigated by pulse experiments and step changes in ammonium load. These investigations revealed that the amount of water in the trickling filter was approximately independent of the flow and that stable conditions after a drastic change in the influent conditions were achieved in the time order given by the residence time distribution. A mathematical model based on a general multispecies biofilm model was used for comparison with the experimental data. The model simulations showed good correlation with experimental data indicating a feasibility for multispecies biofilm models also for large scale processes.