PURPOSE:To attain to simplify heat insulation and maintenance and to conserve power, by providing an anaerobic fluidized bed to a submerged type underground treatment tank. CONSTITUTION:Waste water is poured in a sedimentation basin 7 from a supply line 6 and supernatant water is sent to a raw water pump tank 8 and further sent to the bottom part of an underground treatment tank 3 by a pump 9. This waste water passes a fluidized bed 5 and rises to reach an upper part 10. Supernatant water in the upper part 10 is flooded to a recirculation pump tank 12 to be discharged as treated water through an outflow line 13. A part of water in the recirculation pump tank 12 is returned to the treatment tank 3 by a pump 14 and mixed in waste water from the sedimentation basin 7 to circulatorily receive treatment by anaerobic bacteria in the fluidized bed 5..By this method, heat insulating property is well and the safe treatment of waste water can be performed.
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