A method for monitoring and controlling the chemistry of a ZLD (Zero Liquid Discharge) system in a power plant, comprising a treatment plant for desulfurizing flue gas coming from said power plant, a unit for treatment wastes coming from said flue gas treatment plant, a first fraction of the liquid effluent coming from said unit being sent to an evaporation plant and a second fraction being recycled to a storage tank providing for the liquid make-up to said flue gas treatment plant, said evaporation plant comprising a softening unit and a crystallization - evaporation unit, said method being characterized in that It comprises the following steps: - periodically sampling liquid streams circulating in predetermined, critical sections (SC1 - SC4) of said system, comprising the outlet section from the waste treatment unit of the flue gas treatment plant, the outlet section from the softening unit, the feed section to the crystallization/evaporation unit, the make-up section from said storage tank to said flue gas treatment plant; to determine chemical - physical parameters relating to the calcium sulphate and calcium carbonate concentrations at said sections; - calculating saturation ratios relevant to calcium sulphate and calcium carbonate for each of said critical sections (SC1 - SC4); - Identifying the critical sections which are subjected to the precipitation of calcium sulphate or calcium carbonate, in which the liquid streams passing through them have calculated saturation ratios for calcium sulphate and/or calcium carbonate higher than a prefixed attention threshold; - carrying out actions comprising variations of reactants/additive dosage to said crystallization/evaporation plant and/or to said waste treatment unit or variation of the ratio between the flow rates of said fractions of the liquid effluent coming from said unit, said variations being such that the saturation ratios for calcium sulphate and calcium carbonate is kept lower than, or equal to 1 during time, - periodically sampling liquid streams circulating in predetermined, critical sections (SC1 - SC4) of said system, comprising the outlet section from the waste treatment unit of the flue gas treatment plant, the outlet section from the softening unit, the feed section to the crystallization/evaporation unit, the make-up section from said storage tank to said flue gas treatment plant; to determine chemical - physical parameters relating to the calcium sulphate and calcium carbonate concentrations at said sections; - calculating saturation ratios relevant to calcium sulphate and calcium carbonate for each of said critical sections (SC1 - SC4); - Identifying the critical sections which are subjected to the precipitation of calcium sulphate or calcium carbonate, in which the liquid streams passing through them have calculated saturation ratios for calcium sulphate and/or calcium carbonate higher than a prefixed attention threshold; - carrying out actions comprising variations of reactants/additive dosage to said crystallization/evaporation plant and/or to said waste treatment unit or variation of the ratio between the flow rates of said fractions of the liquid effluent coming from said unit, said variations being such that the saturation ratios for calcium sulphate and calcium carbonate is kept lower than, or equal to 1 during time,