Some demonstrative embodiments of the invention include a system and a method for disinfection of a liquid including monitoring the disinfection process. The system may include a short-path light detector located in proximity to the illumination source to detect the intensity of light emitted from the illumination source at a short distance from the source. The system may include at least one additional long-path light detector located in proximity to the transparent walls of the conduit of liquid to be disinfected that detects light emitted from the illumination source after propagating in the liquid and exiting through the transparent walls.