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 and nanobubbles 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. The review illustrated that oil removal efficiency in microbubble and nanobubble-based gas flotation was affected by various factors including initial oil concentration, pH, temperature, flotation time, and oil droplet size.

This study investigated the preparation of stable conventional heavy crude oil-in-water (O/W) emulsions by mechanical homogenization with the addition of a non-ionic surfactant, Tween-20. A four-factor, five-level central composite design was carried out to investigate the effects of four independent variables, including mixing intensity (4,000–10,000 rpm), mixing duration (5–45 min), water salinity (0–40 g/L), and the concentration of emulsifier (0.1–2.1 wt%) on the emulsion stability. Emulsion stability was determined by quantification of creaming index, turbidity change rate, and average oil droplet size. The results demonstrated that the salinity of 30 g/L, mixing intensity of 8,500 rpm, mixing duration of 35 min, and emulsifier concentration of 1.6 wt% led to the formation of the most stable emulsion.