1. A method for controlling operation of a fluid disinfecting system by means of UV radiation, wherein the UV radiation is generated by at least one UV lamp comprising a pair of heating cathodes having a discharge voltage (UD), said UV lamp operated by an electronic ballast unit equipped with a control algorithm for adjusting UV power of the UV lamp by pulse-width-modulation to reduce UV power, the method comprising using the control algorithm of the electronic ballast unit to perform the steps of:decreasing a current generated by the electronic ballast unit to a level (Ikmin);
increasing a voltage amplitude (U) generated by the electronic ballast unit above the discharge voltage (UD) until a desired UV power level is reached; and
with increasing the voltage amplitude (U) decreasing a pulse width (PW) generated by the electronic ballast unit, until PWmin is reached,
wherein decreasing the current and increasing the voltage amplitude (U) generates an ineffective current-voltage-ratio in which excess current heats the cathode.