PURPOSE:To considerably reduce the nitrogen in waste water by providing a nitrifying tank which biologically nitrifies the water used for eel raising, a denitrifying tank which denitrifies the same with anaerobic microbes, and a suspended solids filtering tank. CONSTITUTION:Eel raising pond water enters a nitrifying tank 20 through an inflow water port 22, and circulates the inside of an aerating part 32 and plastic packing material part 25, after which it is introduced through the drop port 28 of the nitrifying tank into a simple sedimentation part 31, where suspended solids are solid- liquid separated and are sent through an outflow water port 30 into a denitrifying tank 40. The treated water flowed through an inflow water port 41 is subjected to reduction of nitrate nitrogen by the anaerobic denitrifying microbes stuck on plastic packing material 43, thence it enters a filter tank 60 through an inflow water port 61. When it passes the filter media 62 in the tank 60, it is removed of micro suspended solids, and is made into outflow water 65. Since the filter media 62 cause clogging upon long-term operation, discharge water is discharged from a backwashing water discharge pipe 68 according to need.
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