A selective method of dechlorination treatment of circulating water containing disinfectant chlorine in the form of ClO.sup.-,HClO, ClO.sub.2 or chloramine, where an ascorbic acid aqueous solution is injected from a storage reservoir into the water flow, the mass flow rate of the injected ascorbic acid solution being regulated with respect to the mass flow rate of the chlorine in the circulating water to be treated, in such a way that the ratio (R) of the ascorbic acid and chlorine mass flow rates (R=D1/ D2) is between 2.5 and 4, preferably less than or equal to 3. The method can be used for treatment of water that feeds into a biological detector containing a disinfectant chlorine content that is incompatible with the survival of the biological species used in the detector. The disinfectant chlorine level in the circulating water after mixing is maintained at a value lower than or equal to 0.6 mg/l, and preferably lower than or equal to the detectable limit.