A modelling approach to explore the optimum bubble size for micro-nanobubble aeration

Area of Science:

• Fluid dynamics
• Chemical engineering
• Environmental science

Background:

• Conventional bubble aeration in water/wastewater treatment suffers from low gas utilization and high energy consumption.
• Micro-nanobubbles (MNB) offer a potential solution due to their high surface area and efficient gas-liquid mass transfer.

Purpose of the Study:

• To develop and validate a dynamic model for micro-nanobubble behavior in water.
• To determine optimal MNB sizes for complete gas dissolution at various water depths.
• To provide guidance for upgrading bubble aeration systems.

Main Methods:

• A dynamic model integrating multiple forces (buoyancy, gravity, drag, Basset, virtual mass) and mass transfer was developed.
• The model describes MNB rising velocity and radius variation.
• Model predictions for microbubbles were validated against experimental data (R² > 0.85).

Main Results:

• MNB of optimal size can shrink and burst below the water surface, achieving 100% gas dissolution.
• Bubble shrinkage is influenced by initial size and water depth; greater depth promotes shrinkage.
• Optimal air (42-194 μm) and oxygen (127-470 μm) MNB sizes were calculated for 100% gas transfer at depths of 0.5-10 m.

Conclusions:

• The study provides insights into MNB dynamics and offers guidance for optimizing aeration systems.
• Complete gas dissolution and potential free radical generation are key benefits of MNB.
• Further validation is needed for nanobubble (<1 μm) models due to limited empirical data.