There is described an on-line device for controlling a fluid treatment process configured to inactivate a microorganism in a flow of fluid using ultraviolet radiation and a chemical disinfectant. The device includes: a memory for receiving a calculated database of dose response for the ultraviolet radiation and for the chemical disinfectant for a fluid treatment parameter; means to obtain input data about the fluid treatment parameter from the process; means to compare the input data with calculated database; and means to adjust one or more of the amount ultraviolet radiation and the chemical disinfectant added to the flow fluid in response to a difference between the input data and calculated database. There is also described a process for controlling a fluid treatment process configured to inactivate a microorganism in a flow of fluid using ultraviolet radiation and a chemical disinfectant.