Simulator of urban wastewater treatment plants set up under Windows, basically focused on personnel training and educational purposes. DESASS is designed and optimised for the simulation of the main physical, chemical and biological processes involved in a WWTP. In the design of DESASS a special emphasis has been placed on the speed of calculation, graphic presentation and user-friendliness. DESASS works in an intuitive environment and allows the evaluation of complete wastewater treatment schemes, allowing to represent from simple systems such as activated sludge for the removal of organic matter, nitrogen and phosphorus, to complete plants including anaerobic digesters, membrane reactors, etc. The programme has implemented a mathematical modelling algorithm based on the general biological model BNRM2 developed by CALAGUA.