Flow visualisation of unmixed regions in a model anaerobic agitated digester

Increasing wastewater produced in large cities due to the rapid growth of population and industries is forcing local wastewater treatment plants to process more wastewater than before with existing equipment such as digesters. Increasing throughput without increasing digester capacity could result in poor mixing of sludge in digesters and lead to digester failure. Therefore more studies are required to understand the mixing phenomenon inside digesters especially when thickened sludge is processed. This paper reports an experimental study on the mixing of digested sludge in an unbaffled cylindrical vessel agitated by a 6-bladed 45° pitched turbine pumping in both downward and upward directions. A transparent model liquid (xanthan gum Keltrol T) was used as it exhibits rheological properties similar to those of digested sludge. An acid-base neutralization reaction was used in the flow visualization of the mixing process. The effect of specific impeller power input (P/V) on the mixing pattern and unmixed volume was investigated using 0.15 and 0.4 wt% (low and high viscous) solutions. Both downward and upward pumping impellers produced a cylinder-shaped cavern (well-mixed region) with ringshaped unmixed regions inside in the low viscous solution. With an increase in mixing time, the rings were found to disappear and the cavern size increased. In the case of the high viscous solution, both downward and upward pumping impellers produced just a cavern. The cavern increased in size with an increase in mixing time but never disappeared regardless of (P/V) used.