PROBLEM TO BE SOLVED: To provide a method and system for coagulation-sedimentation treatment of a slurry containing dissolved heavy metals capable of separating and removing the heavy metals dissolved in water from the slurry.SOLUTION: The system 1 for the coagulation-sedimentation treatment of the slurry containing the dissolved heavy metal includes a first reaction tank 2 for forming a first coagulated flock for sedimentation by adding an inorganic coagulant and a polymeric coagulant to the heavy-metal-containing slurry S1 which contains suspended particles and the dissolved heavy metals, a first sedimentation tank 3 for separating and settling treatment of the first coagulated flock, a second reaction tank 4 for forming a minute flock by adding an immobilizing agent for immobilizing the dissolved heavy metal to a supernatant liquid S2 after the separating and settling treatment, a third reaction tank 5 for forming a second coagulated flock for sedimentation in a supernatant liquid S3 by adding the inorganic coagulant and the polymeric coagulant to the supernatant liquid S3, and a second sedimentation tank 6 for separating and settling treatment of the second coagulated flock in the supernatant liquid S3.
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