A method and apparatus for controlling the mixing efficiency of the mixer, the method comprising the steps of: injecting a chemical into a process fluid flowing in a pipe, the chemicals in a mixer operating at a first operating speed a and a step to be mixed with the process fluid. The method includes the steps of measuring the efficiency of mixing the chemical with the process fluid within the pipe downstream of the mixer, comparing the measured efficiency of mixing with a predetermined range of efficiency of mixing, the range of a predetermined mixing efficiency so as to adjust the mixing efficiency against further comprising a step of controlling the operation rate of the mixer. Mixing efficiency, it is preferred to be measured by using a set of electrodes disposed on the periphery of the pipe, mixing efficiency is preferably obtained by the use of electrical impedance tomography.