In one of its aspects, the invention relates to a process to optimize the dose of a treatment agent for the treatment of a fluid comprising a contaminant. In this first aspect the process comprises the steps of: (a) calculating the dose of the treatment agent based on the relationship between concentration of the treatment agent at one or more points and residence time distribution of the treatment system, and (b) contacting the fluid with the treatment agent in the concentration required to meet the dose calculated in step (a). In another one of its aspects, the invention relates to a process to optimize the dose of a treatment agent for reduction of a contaminant in a fluid.