The present invention provides a method for treating flue gas desulfurization wastewater to prevent scale attachment to the liquid contact surface of an evaporator tube and a heater in a flue gas desulfurization plant. This method comprises the steps of (a) incorporating seed crystals of gypsum into flue gas desulfurization drainage, and (b) concentrating the desulfurization drainage containing seed crystals in a vaporization tube until a constant concentration ratio is obtained. The step of precipitating and (c) releasing the concentrated desulfurization drainage from the evaporation tube and then washing the inner surface of the evaporation tube with washing water are repeated.
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