PROBLEM TO BE SOLVED: To enable to exactly and quickly suppress increase in nitric acid concentration without changing water, when breeding fishes in a closed water area.SOLUTION: There is provided a system including denitrification circulation tank 7 taking in water in a closed water area 2 and reducing the nitric acid contained in the taken-in water to be converted into nitrogen gas, a water flooding means 13 for taking in water in the closed water area 2 into the denitrification circulation tank 7, a circulation means 16 for circulating water in the denitrification circulation tank 7, an extrusion means 20 for returning the water in the denitrification circulation tank 7 to the closed water area 2, a water level detection means 23 set in the denitrification circulation tank 7 to detect the water level in the denitrification circulation tank 7, a heater 24 set in the denitrification circulation tank 7, and an oxidation-reduction potentiometer 25 set in the denitrification circulation tank 7 to measure the oxidation-reduction potential of water in the denitrification circulation tank 7, wherein the inside of the denitrification circulation tank 7 is divided into a denitrification area 10 having a denitrification part 12 and a circulation area 11 for circulating water in the denitrification part 12 to the denitrification area again.COPYRIGHT: (C)2012,JPO&INPIT【課題】閉鎖系水域で魚介類を飼育する場合において、換水すること無く硝酸濃度の上昇を正確、迅速に抑えることを可能にする。【解決手段】閉鎖系水域内2の水を取り込むとともに取り込んだ水に含まれる硝酸を還元して窒素ガスにまで変換する脱窒循環槽7と、閉鎖系水域2内の水を脱窒循環槽7に取り込む水張り手段13と、脱窒循環槽7内の水を循環する循環手段16と、脱窒循環槽7内の水を閉鎖系水域2に戻す押出手段20と、脱窒循環槽7内の水位を検知するために脱窒循環槽7内に配設された水位検知手段23と、脱窒循環槽7内に配設されたヒーター24と、脱窒循環槽7内の水の酸化還元電位を計測するために脱窒循環槽7内に配設された酸化還元電位計25を具備して、脱窒循環槽7はその内部を、脱窒部12を有する脱窒エリア10と、脱窒部10内の水を再び脱窒エリアへ循環するための循環エリア11に分割した。【選択図】図1
