Sludge occurs in many waste water and drinking water treatment processes. The numeric modeling of sludge is therefore crucial for developing and optimizing water treatment processes. Numeric single-phase sludge models mainly include settling and viscoplastic behavior. Even though many investigators emphasize the importance of modeling the rheology of sludge for good simulation results, it is difficult to measure, because of settling and the viscoplastic behavior. In this thesis, a new method is developed that combines rheological measurements with a process-viscometer and numeric simulations, in order to produce flow curves that most probably display the real viscoplastic behavior of sludge below a concentration of 10 g/l. The settling velocity is determined with a hybrid approach combining standard measurements and numeric simulations of the measurements. This is called the hybrid sludge model, which finally demonstrates its power by simulating a complex lamella clarifier. The results of the simulation fit the measurements and observations of the testing plant well. This demonstrates that the hybrid sludge model contributes to the development of a numerical model that requires a minimum of measurements and computational effort, in order to simulate water treatment plants. Finally, sludge properties (settling and viscoplastic behavior) from different waste water treatment plants and drinking water treatment plants are compared, with remarkable results. This thesis provides efficient tools for including numeric modeling in the design and optimization processes of water treatment plants.