The survival rate under the congested
experimental conditions was 91%. The foam separation unit maintained oxygen saturation in the
rearing water at about 80%. Furthermore, fine colloidal substances were absorbed on the stable foam
formed from eel mucus and were removed from the rearing water by foam separation. Ammonia
oxidation and the removal of suspended solids were accomplished rapidly and simultaneously in the
nitrification unit. The ammonia concentration and turbidity were kept at less than 1.2 mg of N per litre
and 2.5 units, respectively.