In an aquarium and the aquaculture institution, water management of the breeding water that is the growth environment of the aquatic organism is the most important. In most recirculating systems, ammonia (referring to NH3 and NH4 +) removal by nitrification, sludge removal by sedimentation or mechanical filtration, and water exchange are vital forms of water treatment. Long-term operation of closed aquarium and aquaculture systems results in nitrate accumulation．Nitrate causes toxicity problems for invertebrates and affects the immune system of fish. Thus, to prevent accumulation of nitrate and dissolved organic matter, water exchange of 5-10% of the system volume is performed in everyday. However, this water exchange need a high costs. So, reduction of the exchange water is very attractive as reduction of the maintenance cost.
In this study, we are developing new recirculating nitrogen removal system for marine aquarium using an USB reactor for denitrification to avoid the accumulation of nitrogen in aquarium seawater.