Oxidative cleaning of surface fouling in heated sanitary systems at low ozone concentrations: Unveiling the role of nanobubbles in mass transfer

Nanobubbles enhance gas-liquid mass transfer, particularly in gas-limited or diffusion-restricted systems, owing to their minuscule sizes and high interfacial surface areas. However, their potential to enhance ozone mass transfer in heated aqueous systems for removing abiotic fouling in sanitary applications remains understudied. This study simulated application conditions for a typical sanitary system to evaluate ozone nanobubbles' efficacy for surface cleaning. First, 71×106 nanobubbles/mL were produced by circulating 120L of tap water at 39 °C with 0.5 LPM ozone-Air flow rate (target ozone = 1.5mg/min) for 2h. This resulted in a dissolved ozone concentration of 5.μg/L i.e., 2.7 times higher than the conventional ozone (micro-macrobubble). Ozone nanobubbles exhibited a more rapid increase in oxidation-reduction potential and dissolved oxygen than conventional ozone. Although dissolved ozone was depleted within 1h, 12×106 nanobubbles/mL remained in suspension after 4h. The minimal decline in ORP over this period suggests that the residual nanobubbles continued to release ozone and/or oxygen, sustaining the oxidative environment in the absence of external ozone input. Nanobubbles significantly enhanced fouling removal by up to a 2.4-fold on PVC and acrylic surfaces than conventional ozone solutions within a 30-minute treatment period. SEM, FTIR, and XRF analyses expounded the role of enhanced ozone transfer by nanobubbles in improving surface cleaning, and also revealed potential surface interactions between ozone and polymer substrates. The findings here feature the potential of ozone nanobubbles to enhance cleaning performance in sanitary systems, offering an effective solution for fouling control.