The water treatment system includes: a wet desulfurization device that removes sulfur oxide present in boiler flue gas; a dewatering device that separates gypsum from desulfurization waste water containing gypsum slurry; a reaction tank to which separated water from the dewatering device is introduced, and which immobilizes heavy metals present in the separated water by inputting a chelating agent; a solid-liquid separation unit that performs solid-liquid separation with respect to heavy metal sludge present in the separated water; a mixing unit that mixes the separated water obtained with effluent generated in a plant facility; a desalination device that removes salt content from the mixed water mixed by the mixing unit; and a spray drying device that includes a spray unit, which sprays concentrated water in which the salt content has been concentrated by the desalination device, and that performs spray drying using a part of the boiler flue gas.