Studies on unmixed regions in a mechanically agitated anaerobic digester

High volume of wastewater is produced in cities with high population density and rapid growing industries. This has been a big concern to local wastewater treatment plants as they have to process more wastewater than before with existing equipment such as digesters. As the digester throughput increases, the sludge gets thickened. Consequently, mixing of sludge becomes poor leading to digester failure. Therefore, more studies on digester mixing are required to understand the mixing phenomenon in the equipment. Such knowledge will help in improving the mixing efficiency of anaerobic digesters. This paper reports a study on the mixing of digested sludge in a mechanically agitated vessel. Instead of using real sludge, a transparent model liquid (Xanthan Gum Keltrol T), which exhibits rheological properties similar to those of digested sludge, was used in the experiments. Experiments were conducted in an unbaffled cylindrical vessel equipped with a six-bladed Rushton turbine. An acid-base neutralization reaction with fluorescent dye as the tracer was used in flow visualization of the mixing process. The effects of specific impeller power input on the mixing pattern and unmixed volume were investigated. Results showed that 0.15 wt% of Xanthan Gum solution has rheological behavior similar to that of digested sludge with 2.23 wt% dry solids. Isolated mixing regions (IMRs), which are unmixed regions, were found to appear above and below the rotating impeller in the beginning and disappear gradually with increasing mixing time. However, the rate of destruction of IMRs was found to be different for different specific power inputs.