Method for determining output data during a filtration process in which a first solution comprising at least one product in the form of a biopharmaceutical active ingredient as one solute is processed by a filter system comprising a filtration module, the method comprising - receiving first measurement data which are indicative for at least one input process variable (V1_M, V2_M), wherein the at least one input process variable (V1_M, V2_M) is a process variable of the filtration process measurable during the filtration process with help of a measurement unit (11, 31), - determining output data by executing a filtration process model and using the first measurement data, characterized in that - executing the filtration process model comprises solving at least one physical equation describing the interaction between solutes of the first solution, the physical equation takes into account the mass balance, the electroneutrality condition and thermodynamic equilibrium of the filtration process, - at least one parameter of the filtration process model is dependent on the at least one input process variable (V1_M, V2_M) derived from the first measurement data, - the output data is indicative for at least one modeled process variable (V3_P, V4_P) of the filtration process, wherein the at least one modeled process variable (V3_P) extractable from the output data is the concentration of a solute of the first solution and/or of a permeate of the filtration process. - receiving first measurement data which are indicative for at least one input process variable (V1_M, V2_M), wherein the at least one input process variable (V1_M, V2_M) is a process variable of the filtration process measurable during the filtration process with help of a measurement unit (11, 31), - determining output data by executing a filtration process model and using the first measurement data, characterized in that - executing the filtration process model comprises solving at least one physical equation describing the interaction between solutes of the first solution, the physical equation takes into account the mass balance, the electroneutrality condition and thermodynamic equilibrium of the filtration process, - at least one parameter of the filtration process model is dependent on the at least one input process variable (V1_M, V2_M) derived from the first measurement data, - the output data is indicative for at least one modeled process variable (V3_P, V4_P) of the filtration process, wherein the at least one modeled process variable (V3_P) extractable from the output data is the concentration of a solute of the first solution and/or of a permeate of the filtration process. - executing the filtration process model comprises solving at least one physical equation describing the interaction between solutes of the first solution, the physical equation takes into account the mass balance, the electroneutrality condition and thermodynamic equilibrium of the filtration process, - at least one parameter of the filtration process model is dependent on the at least one input process variable (V1_M, V2_M) derived from the first measurement data, - the output data is indicative for at least one modeled process variable (V3_P, V4_P) of the filtration process, wherein the at least one modeled process variable (V3_P) extractable from the output data is the concentration of a solute of the first solution and/or of a permeate of the filtration process.