PURPOSE:To enhance the denitrifying rate, BOD removal rate, and degree of transparency of treated water using less diluting water by returning part of mixture liquid aerated to mix it with urine for an aeration, adding coagulant thereto to settle and separate sludge, returning part of sludge thus separated to the aeration step, and decoloring supernatant by chlorination. CONSTITUTION:After crude night-soil 1 is pertreated by a device 2, it is supplied to an aeration tank 3. Since part of mixture liquid aerated and exhausted is returned through a return path 5 to the supply side of the tank 3 together with diluting water 4, it can largely reduce the volume of the water 4. Part of sludge produced at later step is supplied through a path 6 as return sludge to the tank 3 and is biologically treated to perform higher than 90% of denitrifying rate and 95% of BOD removal rate. The mixture liquid is then supplied to a coagulation unit 7 added with inorganic coagulant 9, neutralizer 10, and high molecular coagulant 12 to thereby foam floc. After solid is separated from liquid in the settling basin 13, supernatant liquid is decolored with chlorine water 15 in a chlorine mixture tank 14.
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