Knowledges about reaction kinetics are a main factor for the construction of reactors. But also during the working process they are important for an effective control and regulation. Reaction rate constants and the removal degrees depend on the actual conditions, for instance, water flow rate, start soiling or water temperature. They depend also on constructional details of the reactor and his periphere systems. This paper proposes a simple kinetic for the removal of organic soil by trickling filters in a working clarification plant.