NOVELTY - A wastewater treatment system has a wastewater treatment apparatus, a sludge reduction tank having an alkali treatment tank (15) and an ozone treatment tank (16), a fermentation tank (17) and a crystallization tank (19). An alkali-treating waste sludge is transferred from the wastewater treatment apparatus to alkali treatment tank, and ozone-treated. A liquid solubilized in alkali treatment tank is separated through a membrane (18), and pure solubilized liquid is transferred to crystallization tank. The crystallization tank produces a crystallized material.    USE - System is used for treating wastewater (claimed).    ADVANTAGE - The system decreases the amount of sludge and recovers organic compound efficiently and economically.    DETAILED DESCRIPTION - A wastewater treatment system has a wastewater treatment apparatus, a sludge reduction tank having an alkali treatment tank and an ozone treatment tank, a fermentation tank and a crystallization tank. An alkali-treating waste sludge is transferred from the wastewater treatment apparatus to the alkali treatment tank. The alkali-treated waste sludge is subjected to ozone-treatment in the ozone treatment tank to increase solubilization ratio of the sludge and reduce the volume of the sludge. A liquid solubilized in the alkali treatment tank is separated through a membrane submerged into the alkali treatment tank, and pure solubilized liquid is transferred to the crystallization tank. A crystallized material is produced in the crystallization tank from a solubilized liquid transferred from the alkali treatment tank. The crystallized material is separated and supernatant from the crystallization tank is recovered. The supernatant recovered from the crystallization tank is recycled to the wastewater treatment apparatus and used as a carbon source.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of the system for treating wastewater. (Drawing includes non-English language text)    Alkali treatment tank (15)    Ozone treatment tank (16)    Fermentation tank (17)    Membrane (18)    Crystallization tank (19)