Abstract Dissolved air flotation (DAF) is generally used for thickening of waste activated sludge (WAS). Mechanical flotation (MF) is used in the metallurgical industry for selectively recovering minerals from ore. In this study the potential of MF to thicken WAS was investigated. The Taguchi experimental design was followed to investigate the effect of airflow rate, residence time, impeller speed, surfactant dosing, sludge dilution and their interactions. Results indicate that MF with surfactant addition shows potential to thicken WAS and should be investigated further on a bigger scale. The surfactant dosing contribution to thickening is significant at a 95% confidence level. The interactions between residence time and impeller speed, residence time and airflow rate and residence time and surfactant dosing were also found to be significant at a 95% confidence level.