The paper presents the effects of gradual upgrade of existing secondary clarifiers of the waste water treatment plant (WWTP) Asselbrunn. From existing two clarifiers at the beginning only one has been modified (clarifier no. 2), while the other remained unchanged (clarifier no. 1). In a first step the intake structure of clarifier no. 2 has been modified at flow proportional return activated sludge (RAS) control. In a second step RAS control has been changed according to sludge level control in clarifier no. 2. The comparison of the RAS flow, sludge level and TSS-concentration (total suspended solids) filterable substances [G1] have been made for various flow rates, from dry weather flow (100 l/s) to wet weather flow (300 l/s). The comparison of the operational performance of both clarifiers demonstrates, that modernization of the central inlet structure in combination with an improved RAS control considerably improved the capacity and separation efficiency of the modernized clarifier. It is therefore advisable to consider both constructive modification of the inlet structure and improvements of the automation before building a new secondary clarifier.