This paper investigates the effect of controlled periodic illumination by visible LEDs on the performances of a flat-plate photocatalytic reactor for wastewater treatment. Different LED dimming techniques are investigated and compared, including the classical Pulse Width Modulation (PWM) technique and a novel proposed Variable Peak PWM technique. A modulation of dimming duty-cycle is adopted as well, which allows to control the light irradiation in a new way with respect to previously used methods. Experimental results highlight the improvement in the photocatalytic degradation process obtained by using the proposed modulation techniques.