NOVELTY - Treated water is solid-liquid separated by adding a coagulant. The separated upper water is subjected to biological-waste treatment. The separated solid sludge is introduced to an acid fermenter (6) and an organic acid is generated by souring. The fermented sludge of fermenter is dehydrated. The dehydrated filtrate containing an organic acid is supplied to the biological-treatment tank (2).    USE - For treating water.    ADVANTAGE - Since the decomposition-resistance solid sludge amount flowed into the biological-treatment tank is reduced, the average residence time of the waste water in the biological-treatment tank is reduced. The activated-sludge microorganisms utilize decomposition-resistance solid sludge solublility biological oxygen demand as a nutriment. The amount of solid sludge ejected out of the water treating unit is reduced. Since hydrogen donors such as methanol supplied to the system is suppressed, cost is reduced by utilizing the organic acid as the source of organic carbon or hydrogen donor in the biological waste treatment.    DETAILED DESCRIPTION - Treated water is solid-liquid separated in a sedimentation basin (1) by adding a coagulant to the basin. The separated upper water is introduced to a biological-treatment tank (2) to perform biological-waste treatment. The separated solid sludge is introduced to an acid fermenter (6) and an organic acid is generated by souring. The fermented sludge of acid fermenter is dehydrated with a dehydrator (7). The dehydrated filtrate containing an organic acid is supplied to the biological-treatment tank as a source of organic carbon or a hydrogen donor. An INDEPENDENT CLAIM is included for water treating unit using the souring, which has a sedimentation basin for solid-liquid separating the treated water flowed in the system, biological-treatment tank for performing biological waste treatment of the separated upper water of sedimentation basin, acid fermenter for performing souring of the sedimentation solid sludge of the sedimentation basin, dehydrator for dehydrating the fermented sludge of the acid fermenter, and dehydration filtrate supply system (8) for supplying the dehydrated filtrate to the biological-treatment tank.    DESCRIPTION OF DRAWING(S) - The figure shows the block diagram of the water treating unit. (Drawing includes non-English language text).    Sedimentation basin (1)    Biological-treatment tank (2)    Acid fermenter (6)    Dehydrator (7)    Dehydration filtrate supply system (8)