PROBLEM TO BE SOLVED: To effectively and appropriately treat arsenic-containing muddy water even if a large quantity of the arsenic-containing muddy water to be treated is generated in a series of whole process from generation of shield muddy water to production of dehydrated cake.SOLUTION: A system of treatment for rendering arsenic-contaminated soil harmless comprises: an iron powder adding device 40 adding iron powder to pH-adjusted muddy water containing fine-grain fractions while neutralizing the pH of the muddy water to cause the iron powder to adsorb arsenic; a gravity separating device 50 using a cyclone to separate, by gravity separation, the muddy water into muddy water containing no iron powder and muddy water containing the iron powder having absorbed arsenic; a drum-type magnetic separation device 60 recovering, using permanent magnet, the iron powder from the muddy water containing the iron powder having adsorbed arsenic; a filter-type electric magnet separating device 70 recovering the iron powder that the drum-type magnetic separation device 60 cannot recover, from the muddy water containing the iron powder; and a dehydration processing device 80 performing dehydration processing of the muddy water from which the iron powder has been separated.SELECTED DRAWING: Figure 1
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