In the planning phase of the extension of the Main Treatment Plant of Vienna, special effort and emphasis were put on the conception of the tender procedure. The project tender was divided into several tender units in order to achieve optimum quality standards by specialised workmanship. Important parameters for operation conditions, especially energy consumption and maintenance costs, were considered and valuated according to the tender guidelines. Suppliers were required to prove the guaranteed quality standards of the tender documents by means of preliminary installation units and extensive performance check procedures. Only after fulfilment of all requirements were suppliers allowed to apply the design of the preliminary installation works to the entire installation. If necessary, extensive optimization works were carried out. Thus favourable operation conditions were achieved and, in particular, a highly efficient aeration system was implemented. Process parameters were optimized by means of a follow-up project during the regular operation phase, taking into consideration the results of the performance check procedures. Further optimization of energy consumption was thereby achieved.