Ideal flow regimes of gas spargers: Towards optimal gas bubble generation for biological wastewater treatment

Gas bubble dispersion is involved within a large number of chemical, biochemical, and other processes. Of particular importance is the dispersion of air bubbles in the activated sludge process in biological wastewater treatment plants. This is an expensive procedure, which is responsible for the largest share of energy bill in the whole WWTP in the range 45% to 75%. The rubber membrane diffusers, which are currently used for aeration in the treatment facilities, offer relatively low standard oxygen transfer efficiency in range of 40-60 %, which is mainly due to generation of large bubble sizes. Among the parameters which influence the system efficiency, bubble size is of great importance, since it directly influences the gas holdup and the bubble residence time. Moreover, bubble size determines the surface area to volume ratio, which affects the volumetric oxygen transfer coefficient and oxygen transfer rate. Bubble formation is mainly studied at openings bigger than 1 mm. Hence, the leverage of sub-millimeter orifices on volume of the final bubble is still not understood. Current endeavor focuses on bubble formation from submerged orifices under constant flow condition. Subsequently a set of experiments is design to capture the bubble generation from small orifices in the range of 20-800 μm diameter. The initial bubble size was measured by means of a high-resolution optical measurement technique. Changes in the mechanism of bubble formation and detachment within the dynamic bubbling regime were monitored and reported over a progressive trend of the volumetric gas flow rate.