If a closed recirculating aquaculture system that need not discharge the effluents is developed, a large amount of the pollutant load on water bodies can be reduced. In this study, rearing experiment of eel were reared in a closed recirculating system. The system consisted of a fish tank, a foam separation unit, nitrification and denitrification units, pH and temperature control units, and a circulation pump. The foam separation unit has an air inhalation-type aerator and supplies numerous bubbles into the rearing water. Fish were fed a commercial diet for 3 months.
The survival rate of eel was 100% during the experiment under the congested condition. The foam separation unit kept oxygen saturation in the rearing water at about 90%. Furthermore, colloidal matters were absorbed on stable foam formed from eel mucus and were removed from the rearing water by foam separation. Ammonia oxidation and the removal of suspend matters were accomplished rapidly at the same time with the nitrification unit, and ammonia concentration and turbidity were kept at less than 0.21mg-N·l^-1 and 0.981 TU, respectively. When the denitrification process was operated, nitrate accumulated in the rearing water was reduced to a concentration less than 7mg-N·l^-1. Based on these results, an intensive aquaculture of freshwater fish such as eel can be achieved by a closed recirculating system.