From 1 January 1995 in The Netherlands 75% of the phosphorus in the influent has to be removed in the waste water treatment plants (wwtp's). At this moment 40 – 50% is reduced by biological incorporation and some chemical dosing. By additional measures an extra 35% reduction has to be accomplished. A further reduction of total nitrogen to 75% has to be realised from 1 January 1998. Further extension of the activated sludge system is not always possible and in most cases rather expensive. Therefore optimization of the existing treatment plant is considered. For this purpose the advanced simulation model STREAM• is used. With STREAM• almost every waste water treatment plant can be simulated with the computer. In this way, for example, the effect of different control mechanisms and other measures can be studied without costly research. Computer simulation has been applied at the Kralingseveer wwtp of the HH van Schieland (Waterboard in the Netherlands) to optimize aeration control and to evaluate a number of technological process adjustments. Attempts are being made to reduce nitrogen drastically using equipment currently available, supplemented where possible with other limited measures. It can be concluded from the results of simulation, that the use of integrated aeration control, whereby influent flow, and nitrate and ammonium concentrations in the activated sludge are measured on-line, further improves the treatment process. Furthermore, when the sludge content in the aeration tank is raised and nitrogen is removed from the internal return flows, it seems that an average annual effluent quality of 10 mg Ntotal/1 is feasible. Verification of these factors will be carried out in the near future using practical measurements. The Kralingseveer project demonstrates that computer simulation is more than simply a ‘game’. It has become a real means of guiding or even limiting the need for extensive and costly (practical) investigation.