Abstract: The present invention relates to a process for the treatment of arsenic-containing sludge obtained by adjusting arsenic-containing waste water to a PH of 12 or greater by the addition of a calcium compound and subjecting the waste water to solid-liquid separation, the process comprising the steps of adding a calcium compound to the arsenic-containing sludge slurry obtained by the solid-liquid separation, dewatering the resulting sludge, drying the dewatered sludge, and calcining the dried sludge. According to the present invention, arsenic-containing sludge obtained by treating waste water to precipitate arsenic present therein can be calcined to yield a calcined product which involves no risk of arsenic redissolution when it is dumped. Moreover, there is no possibility that arsenic compounds may be volatilized during calcination.

Abstract: The present invention provides an evaporative concentration apparatus for waste water having an evaporating vessel 2 for evaporatively concentrating waste water and a heater 3 for circularly heating an in-vessel liquid in the evaporating vessel 2, in which the whole or part of thermal energy of generated steam 6 discharged from the evaporating vessel 2 is recovered by an absorption heat pump 29, and the thermal energy is supplied to the heater 3. According to the present invention, evaporative concentration can be effected efficiently in the evaporating vessel by effectively utilizing the thermal energy of generated steam, and also the operation of evaporative concentration apparatus can be performed stably by stably securing heating steam quantity.

Abstract: The present invention provides a process for treating arsenic-containing waste water, comprising adding an oxidizing agent, if necessary, to arsenic-containing waste water to oxidize trivalent arsenic in the waste water into pentavalent arsenic; adding a calcium compound to adjust the pH to 12 or higher; separating it into solid and liquid (first solid/liquid separation); calcining the resulting sludge, while adding a ferric salt to the treated solution to adjust the pH to 6-9 after the solid/liquid separation; and separating it into solid and liquid (second solid/liquid separation). According to the present invention, it is possible to remove arsenic from waste water efficiently to yield treated water satisfying various tolerance limits under various regulations, while the arsenic-containing sludge separated from the waste water can become harmless in subsequent continuous steps.