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.

Oily wastewater is generated from various sources within the petrochemical industry, including extraction, refining and processing, storage, and transportation. Over the years, large volumes of oily wastewater from this industry have made their way into the environment, negatively affecting the environment, human health, and the economy. The raw waters from the petrochemical industry can differ significantly and have complex features, making them difficult to treat. Membrane bioreactors (MBR) are a promising treatment option for complex wastewater; it is a combined physical and biological treatment. The biological component of the MBR is one of the main contributing factors to its success. It is important to know how to control the parameters within the bioreactor to promote the biodegradation of hydrocarbons to improve the treatment efficiency of the MBR. There have been many reviews on the effects of the biological factors of membrane fouling; however, none have discussed the biodegradation process in an MBR and its impact on effluent quality. This review paper investigates the hydrocarbon biodegradation process in an aerobic MBR system by gathering and analyzing the recent academic literature to determine how oily wastewater characteristics and operational parameters affect this process.