To surely prevent the re-elution of arsenic at a time when arsenic-containing sludge is subjected to disposal treatment by adding a calcium compd. to arsenic- containing waste water to adjust the waste water to a specific pH value and subjecting this waste water to solid-liquid separation treatment to obtain arsenic-containing sludge and adding slaked lime to the sludge before calcinating the sludge.
When slaked lime 12 is added to arsenic-containing waste water 11 flowing in a first reaction tank 1 to adjust the pH of the waste water to 12 or more, hydroxide flocs are formed. Next, this reaction soln. is subjected to solid-liquid separation within a flocculation and sedimentation tank 2 to form flocculated sedimented sludge 17 and a part of this sludge 17 is returned to the first reaction tank 1 as return sludge 17a and the remainder is stored in a sludge storage tank 5. When the sludge in the storage tank 5 becomes a definite amt., the sludge is dehydrated by a dehydrator 6 to be dried by a dryer 7 and, after slaked lime 9 is added to the dried sludge, the lime added sludge is calcined in a calcinator 8. The treated soln. of the sedimentation tank 2 is introduced into a second reaction tank 3 and an iron salt and an acid are added to the treated soln. to convert residual arsenic to iron arsenate which is, in turn, included in ferric hydroxide flocs formed at the same time to be co-precipitated.
TAO KOZO
SHIMIZU HIROSHI
YOKOSE MAMORU
