Application of Air Nanobubbles in Floating Raft Aquaponics

Abstract

There were three main objectives in this study. (1) Air nanobubbles were generated using the pressure dissolution method (AgriGaLF i15, IDEC, Japan), then physicochemical properties were investigated to determine their stability; (2) The oxygen mass transfer of the nanobubble generator AgriGaLF was evaluated; and (3) The nanobubble generator was deployed and assessed in field-scale, floating-raft aquaponics. Results show that air nanobubbles with a 30 minute generation time are stable and exhibit negative surface charges under varying pHs. Average zeta potentials dropped from -5.96 to -27.6 between pH 6.0 and 8.5, respectively. Size (100 nm on average) and concentration (108 bubbles/mL) did not significantly change. This indicates that negatively charged nanobubbles can exist in slightly acidic aquaponic solutions (pH 6–7). Generation of air nanobubbles with a 30 minute generation time in distilled water improved oxygen mass transfer by 4.5-fold and could supersaturate the aqueous phase by 120%. Three field-scale trials from Fall 2021 to Summer 2021 improved butterhead lettuce yields between 25% and 48%. Moreover, total organic carbon was lower, and nitrate concentrations higher on average in nanobubble-aerated aquaponics than in the conventionally-aerated system during all 50-day trials. In addition, nanobubble-aerated aquaponics had no significant effect on nitrite and ammonia production. This suggests that the effective oxygen mass transfer or the electrostatic interactions of air nanobubble induces higher mineralization of fish waste (shown by a higher positive accumulation of nitrate) and higher deliverability of nutrients to the plants. The implications of these findings can help further advance nanobubble technology for other environmental and agricultural systems.