Experimental Study on Rotating Spar Mixer in Energy-Efficient Aeration Systems

Abstract

Abstract. Aeration is a critical and energy-intensive process in wastewater treatment, and its efficiency is strongly influenced by bubble size. In this study, a novel rotating spar mixer is proposed to generate smaller bubbles than conventional fine-bubble diffusers while simultaneously providing controlled mixing. A mathematical model was developed to evaluate the influence of impeller rotational speed on bubble detachment and bubble size, and the model's predictions were experimentally examined. An experimental investigation was conducted to compare the proposed spar mixer with a commercial fine-bubble diffuser in terms of bubble characteristics and distribution behavior. The results demonstrate a nonlinear relationship between rotational speed and absolute velocity at the spar tip. The average bubble diameter produced by the spar mixer decreased from 0.653 mm to 0.458 mm compared to a conventional membrane diffuser, while the Sauter mean diameter decreased from 1.067 mm to 0.757 mm. The induced mixing generated a well-distributed bubble field throughout the tank, enhancing oxygen transfer efficiency. Bubble coalescence was observed at higher rotational speeds, indicating an optimal operating range for maximizing breakup while minimizing aggregation. Overall, this study provides new insights into the coupled roles of mixing and aeration and offers a promising approach to improving energy-efficient oxygen transfer in wastewater treatment systems.