A research team in Japan tested a two-stage sponge-based reactor to keep nitrogen low in a marine recirculating aquaculture system. The system pairs an anaerobic down-flow hanging sponge reactor with an aerobic one, removing ammonia and nitrate while reoxygenating the water. Over 116 days with Japanese flounder, the setup kept ammonia and nitrite near detection limits once the aerobic sponge volume reached 40 liters and the carbon-to-nitrogen ratio was set to 6. No routine water exchange was needed. The design uses gravity-driven flow and natural ventilation, so it avoids forced aeration. The sponge media also resisted clogging over the trial. However, the study did not measure energy use, capital costs, or greenhouse gas emissions. The authors note that the system was tested sequentially, not in a replicated design, so results should be interpreted with caution. If validated at commercial scale, this approach could reduce freshwater use and discharge in fish farming.
