Response surface optimisation of nanobubble/microbubble-assisted gas flotation for separating emulsified heavy crude oil from saline wastewater

Oily wastewater generated during marine oil spill response operations can adversely affect the environment, and its emulsified oil droplets are difficult to remove using conventional methods. This study investigated the performance of nanobubble/microbubble (NB/MB)-assisted gas flotation for oil–water separation of saline wastewater. Experiments were conducted in a flotation column (internal diameter: 5.2 cm; height: 100 cm) connected to an NB/MB generator. Response surface methodology (RSM) was used to evaluate the individual and interactive effects of initial oil concentration, flotation time, and wastewater temperature on oil–water separation efficiency. Analysis of variance confirmed that the regression model was highly significant (p < 0.0001, R2 = 0.992), with initial oil concentration showing the strongest effect (F = 395.98). The model predicted a maximum separation efficiency of 98.6% at an initial oil concentration of 524.5 mg L−1, a flotation time of 28.6 min, and a wastewater temperature of 21.2 °C. Validation experiments conducted under these conditions achieved an average oil–water separation efficiency of 97.3%, whereas gravity separation removed only 6.7% of the oil over the same treatment period. The process also achieved an oil–water separation efficiency of 98.2% for the weathered CHCO emulsion. These findings demonstrate the potential of NB/MB-assisted gas flotation for fast and efficient oil–water separation without the addition of flotation chemicals or demulsifiers.