NOVELTY - Treated water is solid-liquid separated and the separated water is subjected to biological waste treatment. The separated surplus sludge is guided to an acid fermenter (6) and an organic acid is generated by souring. The fermentation sludge is refined and dehydrated. The dehydration filtrate containing an organic acid is supplied to the treatment tank as a source of organic carbon or hydrogen donor.    USE - For treating water using souring.    ADVANTAGE - The surplus sludge is removed from treated water in the first stage of the biological treatment tank. The average residence time of the activated sludge in the biological treatment tank is shortened. The discharge amount of surplus sludge ejected out of the system is reduced. The hydrogen donor supply, such as methanol, is suppressed by utilizing an organic acid as the source of organic carbon or hydrogen donor. The dehydration process is enhanced and moisture content of dehydration cake is reduced. The recovery amount of organic acid is improved.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a water treating unit having a sedimentation basin (1), a biological treatment tank (2), an acid fermenter, a sludge refining tank (7), a dehydrator (14), a dehydration filtrate supply system (15), a coagulant supply system, an acid supply system (10), an alkali supply system (12), a pH sensor (8) and a control apparatus (9). The sedimentation basin solid-liquid separates the treated water flown into the system. The biological treatment tank carries out biological waste treatment of the separated water. The sludge refining tank refines the fermentation sludge of the acid fermenter. The dehydrator dehydrates the fermentation sludge of the sludge refining tank. The filtrate supply system supplies the dehydration filtrate containing the organic acid to the biological treatment tank. The coagulant supply system supplies a coagulant to the sludge refining tank. The acid supply system supplies an acid to the sludge refining tank. The alkali supply system supplies an alkali to the sludge refining tank. The pH sensor measures the pH of the sludge in the sludge refining tank. The control apparatus controls the amount of supply of the acid or alkali supply system based on the measured value of the unit, and maintains pH of the sludge in the sludge refining tank to a preset value.    DESCRIPTION OF DRAWING(S) - The figure shows the block diagram of the water treating unit using souring. (Drawing includes non-English language text).    Sedimentation basin (1)    Biological treatment tank (2)    Acid fermenter (6)    Sludge refining tank (7)    Control apparatus (9)    Acid supply system (10)    Alkali supply system (12)    Dehydrator (14)    Dehydration filtrate supply system (15)