NOVELTY - A water treatment plant (10) comprises biological treatment tank which biologically treats organic wastewater by microorganisms, solid-liquid separation tank (11) which separates organic wastewater processed by biological treatment tank into solid and liquid, dehydration mechanism which dehydrates sludge obtained by solid-liquid separation in solid-liquid separation tank, reactor (14) filled with biological treatment adjuvant which dominates specific microorganism group of microorganisms, and sludge refining tank (12) which is provided with aeration mechanism (12a) for aerating sludge.    USE - A water treatment plant.    ADVANTAGE - Volume of excess sludge can be reduced, and the need for complicated operation control can be eliminated.    DETAILED DESCRIPTION - A water treatment plant comprises biological treatment tank which biologically treats organic wastewater by microorganisms, solid-liquid separation tank which separates organic wastewater processed by biological treatment tank into solid and liquid, dehydration mechanism which dehydrates sludge obtained by solid-liquid separation in solid-liquid separation tank, reactor filled with biological treatment adjuvant which dominates specific microorganism group of microorganisms, and sludge refining tank which is provided with aeration mechanism for aerating sludge and refines sludge. An INDEPENDENT CLAIM is included for biological treatment method of organic wastewater which involves performing biological treatment process of processing organic wastewater by contacting sludge and biological treatment adjuvant, separating sludge into solid and liquid, aerating sludge and reducing volume of sludge, and dehydrating sludge.    DESCRIPTION OF DRAWING(S) - The drawing shows an explanatory diagram of the water treatment plant. (Drawing includes non-English language text).    Water treatment plant (10)    Solid-liquid separation tank (11)    Sludge refining tank (12)    Aeration mechanism (12a)    Reactor (14)