The operators of modern wastewater treatment plants (WWTPs) are faced with increasing demands. Beyond the requirement to consistently meet discharge limits for pollutant loads, the cost efficiency of plant operation is becoming more and more important. This results in new challenges for automatic control and human control action. On-line simulation opens up interesting perspectives to provide comprehensive process information, serving as a base for optimised operation of WWTPs. This paper describes the development and application of a computer analysis and support tool for the large-scale municipal WWTP of the city of Magdeburg, Germany. It will show that by linking a simulation server to the Distributed Control System (DCS) relevant additional features for plant control arise. A good cost-benefit ratio of the system is achieved by using web techniques for implementing the software.