Evaluation of water treatment capacity, nutrient cycling, and biomass production in a marine aquaponic system

The need to expand marine fish production and improve the economic viability and sustainability of recirculating aquaculture systems led to the development of a zero-discharge, marine aquaponic system. In this study, water treatment capacity, nutrient cycling, and biomass production were evaluated in a prototype, commercial-scale marine aquaponic system that included a moving bed bioreactor (MBBR) for nitrification, a sand filter for solids removal and denitrification, and hydroponic plant beds. Red drum (Sciaenops ocellatus), two species of edible halophytes sea purslane (Sesuvium portulacastrum) and saltwort (Batis maritima), and organic solids, were successfully produced over a 9-month period. Extensive analysis of solids, organic matter, and nutrients (nitrogen and phosphorous) in water and plant biomass was used to develop detailed mass balances on the system. Simultaneous operation of the moving bed bioreactor (MBBR) and plant beds resulted in high ammonia removal rates, allowing the system to support a high fish biomass density (38.8 kg/m3). Passive denitrification was the main nitrate removal mechanism, contributing to approximately 59% of aqueous nitrogen removal. Conversion of a sand filter to a side-stream denitrification reactor resulted in removal of 17% of the daily aqueous nitrogen load and prevented nitrate accumulation in the system. In addition to fish and edible halophyte production, 34 kg of organic solids were harvested from the sand filter and provided to a commercial nursery that used the solids as a fertilizer. This study demonstrates that marine aquaponics is an effective way to simultaneously produce marine fish, edible halophytes, and fertilizer. Addition of biological MBBR and a denitrifying sand filter was shown to be beneficial in situations where there are space limitations for plant growth, unexpected plant losses, or to support high densities of fish.