Experimental study on mixing of activated sludge with or without air in a mixing tank through electrical resistance tomography (ERT) and response surface methodology

Abstract

In such industrial processes as oxidation, hydrogenation, and biological fermentation gas and liquid are contacted and mixed to reach steady condition. The unit operations applied for gas/liquid processes include but are not limited to bubble column, plate column, mechanically agitated vessels, in-line static mixers, jet mixing devices/jet mixers, and surface aerators. Among aforementioned unit operations, aerated mixing vessels are mostly employed for the gas-liquid processes in such industry as biochemical, pharmaceutical, cosmetics, and waste water treatment. While gas-dispersion in Newtonian fluids, especially in low-viscosity systems, has been successfully understood, not enough information may be found in the literature about this process in non-Newtonian fluids. Therefore, in this study electrical resistance tomography (ERT) was utilized to assess the mixing of the activated sludge as shear thinning non-Newtonian fluid in presence of aeration. ERT results revealed that shorter mixing time can be achieved in presence of aeration. The following three central impellers were employed: ASI (a combination of A200 and the Scaba impellers), ARI (a combination of A200 and the Rushton impellers), and Rushton (fully radial impeller). An ERT system with a two-plane assembly equipped with 16 sensors on each plane was employed to assess the impact of the impeller type, impeller speed, and gas flow rate on the mixing of activated sludge in terms of mixing time, specific power consumption, and gas flow number. A statistical-based experimental design with RSM (response surface methodology) was applied to evaluate the individual and interactive effects of the design parameters and operating conditions. Experiments and RSM demonstrated that among all independent variables in this study, impeller speed was the common independent variable which impacts mixing time, specific power consumption and gas flow number significantly.