Because of increased effluent quality standards the central sewage treatment plant (STP) of Stolberg is upgraded into a two stage plant according to the AB-process. Special peculiarities of the rebuilding are several combined technologies for nitrogen and phosphorus removal and for the optimization of sludge treatment including energy recovery. Apart from special features of construction and machinery a remarkable process control system is installed, which operates with several linked personal computers instead of a main frame process computer. At the beginning of the reconstruction tests were carried out in a semitechnical pilot plant. Results of these tests as well as first operation results of the large STP allow a good adjustment of the especially high quality standards to be expected.