Aquaponics and recirculating aquaculture systems are often affected by poor water quality, which can lead to low plant yields and the potential collapse of fish stock. Key to these systems is dissolved oxygen (DO), which, if not present in sufficient concentrations, can result in the accumulation of toxic ammonia and nitrite. Conventional macrobubble (i.e., millimeter-sized bubbles) aeration, usually via porous ceramic diffusers, cannot maintain high DO, leading to poor water quality. Nanobubble (NB) aeration technology, a novel gas delivery method that produces bubbles with size less than 1000 nm, can effectively alleviate these bottlenecks, supersaturate DO concentrations (above 8 mg L⁻¹), and improve overall water quality. Mechanistically, their small size allows for a long retention time due to reduced buoyancy forces, and high interfacial surface area for enhanced oxygen mass transfer to the aqueous phase.
In this study, NB aeration (IDEC, AgriGALF 15, Japan) was applied at an airflow rate of 1.2 L min⁻¹ in a media-filled aquaponic system to grow radish (Raphanus sativus), with effluent supplied by Nile tilapia (Oreochromis niloticus). This system operated in parallel with a control system (ceramic diffuser) under the same airflow rate. Both systems consisted of five components, arranged according to which water flowed: a fish tank, settleable solids removal sump, aeration sump (where NB/control aeration is applied), biofilter, and grow bed filled with clay aggregates.
Results showed that nitrate concentrations were higher in the NB-aerated system, with a final concentration of 74.2 mg-N L⁻¹ and a nitrate accumulation rate of 12.9 mg-N L⁻¹ week⁻¹, compared to 66.0 mg-N L⁻¹ and 10.9 mg-N L⁻¹ week⁻¹, respectively, in the control system (Figure 1A). Moreover, the total and volatile solids were lower across most components (i.e., fish tank, solids removal tank, biofilter, and grow bed) in the NB-aerated system (Figure 1B). In addition, the total fresh yield of radish was 32% higher. These findings suggest that NB aeration enhanced nitrification and sludge mineralization, likely because of improved DO availability (Figure 1C). Overall, the application of this technology holds promise for improving water quality in aquaponic and aquaculture systems.