Microbubble and nanobubble-based gas flotation for oily wastewater treatment: a review

Gas flotation for oily wastewater treatment is based on the attachment of gas bubbles to oil droplets to produce lighter aggregates that rise to the wastewater surface. It is a feasible, promising, and effective method for oily wastewater treatment due to its high separation efficiency with no secondary contamination, cost-effectiveness, and simple operation. This review focuses on separating oil from emulsions by gas flotation using microbubbles (MBs) and nanobubbles (NBs), which offer the advantages of small bubble size, large specific surface area, and slow rising velocity. The properties of different types of gas bubbles and their generation methods were discussed. Different gas flotation system designs and operational parameters were summarized for dissolved gas flotation, induced gas flotation, and electrolytic flotation (EF). The review illustrated that oil removal efficiency in MB- and NB-based gas flotation was affected by various factors including initial oil concentration, pH, temperature, flotation time, and oil droplet size.