Different combinations of operational parameters including water level, paddle rotational speed and influent flow rate were applied to investigate their impacts on mixing characteristics, residence time distribution and gas transfer rate in a pilot-scale high rate algal pond. In closed condition, the paddle rotational speed had a positive correlation with the Bodenstein number (Bo), water velocity and oxygen volumetric mass transfer coefficient (kLaO2) while increasing water level generated a negative impact on these parameters, although the impact of water level on water linear velocity was small. The amplification effect of water level and paddle rotational speed on the sensitivity of Bo and kLaO2 should be noticed. Moreover, paddle rotational speed had more impact on kLaO2 than on Bo. The study in open condition indicated that effective volume fraction had a positive correlation with inlet flow rate and negative correlation with paddle rotation, while the opposite was observed in the case of Peclet number. The impact of water level variation on these parameters was unclear. Both water level and paddle rotational speed had negative impacts on the short-circuiting index, while no correlation was observed when varying inlet flow rate. In this study, the optimal operational conditions included low water level (0.1 m) and medium paddle rotational speed (11.6 rpm).

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