NOVELTY - A flocculent is added to a treated water and the water is solid-liquid separated. The sewage sludge (SS) separated is guided to an acid fermenter (6) and pH of the sludge is set to 4.5-6.5. An organic acid is produced by souring and maintaining the pH to 5-6. The fermentation sludge is dehydrated and the dehydrated filtrate containing organic acid is used as a source of organic carbon or a hydrogen donor.    USE - For treating waste water.    ADVANTAGE - The discharge amount of sewage sludge ejected out of the system is reduced. The cost of waste water treatment is reduced by utilizing organic acid as a source of organic carbon or hydrogen donor. The sludge load amount for processing inside the system is increased by increase in the sludge concentration. Transfer to methanation is suppressed by maintaining the pH to 5-6. Since the organic acid is collected, the phosphoruses re-melt from the coagulate-and-sedimented sludge floc, and phosphorus flowing to the biological-treatment tank are prevented.    DETAILED DESCRIPTION - A treated water is solid-liquid separated in a sedimentation basin by adding a flocculent. The separated water is guided to a biological-treatment tank (2) and biological waste treatment is performed. The sewage sludge separated is guided to an acid fermenter and the pH of the sludge in the tank is set to 4.5-6.5. An organic acid is produced by souring and maintaining the pH to 5-6. The fermentation sludge of the acid fermenter is dehydrated with a dehydrator (13). The dehydrated filtrate containing 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 a water treating unit. The unit comprises a sedimentation basin, a biological treatment tank, an acid fermenter, a dehydration filtrate supply system (14), an acid supply system (9), an alkali supply system (11), a pH sensor (7) and a control apparatus (8). The sedimentation basin separates the treated water flowed in the system. The acid supply system supplies acid to the acid fermenter and the alkali supply system supplies alkali to the acid fermenter. The dehydration filtrate supply system supplies the dehydration filtrate containing organic acid separated with a dehydrator to the biological-treatment tank. The pH sensor (7) measures the pH of the sludge in the tank of the acid fermenter. The control apparatus controls the amount of supply of acid supply system or alkali supply system based on the measured pH value. The control apparatus maintains the pH of the sludge to a preset value.    DESCRIPTION OF DRAWING(S) - The figure shows the block diagram showing the water treating unit. (Drawing includes non-English language text).    Biological-treatment tank (2)    Acid fermenter (6)    pH sensor (7)    Control apparatus (8)    Acid supply system (9)    Alkali supply system (11)    Dehydrator (13)    Dehydration filtrate supply system (14)