A controller (100, 120, 130) adapted to regulate at least one de-bacterisation unit of a fluid system (200, 210, 230), the fluid system (200, 210, 230) comprising the de-bacterisation unit, a production unit for producing a heated fluid heated to a predetermined temperature, at least one pipe for transporting the fluid from the production unit to at least one fluid outlet, the at least one fluid outlet and at least one first sensing unit for sensing a first temperature of the fluid at a first location in the fluid system as a function of time, the control unit (100, 120, 130) - including a module (101, 124), the module including an input for receiving the first temperature of the fluid as a function of time, wherein - the module is being adapted for obtaining second temperatures at a plurality of locations throughout the fluid system as a function of time, and wherein, for obtaining the second temperatures, ∘ the module is being programmed for thermohydraulic modelling the fluid system and for determining based thereon and taking into account said first temperature, the second temperatures as a function of time at a plurality of locations throughout the fluid system, and/or ∘ wherein the module comprises an input for receiving the second temperatures as a function of time from a plurality of second sensing units located at a plurality of second locations throughout the fluid system. - the module is being programmed for modelling bacterial growth in the fluid throughout the fluid system as a function of time based on the obtained second temperatures and for predicting, based on the modelling, a bacterial concentration at the at least one fluid outlet over time, and - being adapted for driving the de-bacterisation unit based on the predicted bacterial concentration, wherein the control unit is adapted for determining, based on said predicted bacterial concentration over time obtained by the module (101, 124), moments in time when the bacterial concentration reaches a predetermined value in the fluid system and for, in reply thereto, driving at these moments in time the de-bacterisation unit so as to reduce bacterial concentration in the fluid system (200, 210, 230), characterized in that the module is further being programmed for modelling bacterial growth in a biofilm throughout the fluid system, and for modelling interaction between bacterial growth in the fluid and bacterial growth in the biofilm. - including a module (101, 124), the module including an input for receiving the first temperature of the fluid as a function of time, wherein - the module is being adapted for obtaining second temperatures at a plurality of locations throughout the fluid system as a function of time, and wherein, for obtaining the second temperatures, ∘ the module is being programmed for thermohydraulic modelling the fluid system and for determining based thereon and taking into account said first temperature, the second temperatures as a function of time at a plurality of locations throughout the fluid system, and/or ∘ wherein the module comprises an input for receiving the second temperatures as a function of time from a plurality of second sensing units located at a plurality of second locations throughout the fluid system. ∘ the module is being programmed for thermohydraulic modelling the fluid system and for determining based thereon and taking into account said first temperature, the second temperatures as a function of time at a plurality of locations throughout the fluid system, and/or ∘ wherein the module comprises an input for receiving the second temperatures as a function of time from a plurality of second sensing units located at a plurality of second locations throughout the fluid system. - the module is being programmed for modelling bacterial growth in the fluid throughout the fluid system as a function of time based on the obtained second temperatures and for predicting, based on the modelling, a bacterial concentration at the at least one fluid outlet over time, and - being adapted for driving the de-bacterisation unit based on the predicted bacterial concentration, wherein the control unit is adapted for determining, based on said predicted bacterial concentration over time obtained by the module (101, 124), moments in time when the bacterial concentration reaches a predetermined value in the fluid system and for, in reply thereto, driving at these moments in time the de-bacterisation unit so as to reduce bacterial concentration in the fluid system (200, 210, 230), characterized in that the module is further being programmed for modelling bacterial growth in a biofilm throughout the fluid system, and for modelling interaction between bacterial growth in the fluid and bacterial growth in the biofilm.