Improving mixing in anaerobic digesters with a jet recirculation system

Inefficient mixing is a common problem encountered in anaerobic digesters operated in wastewater treatment plants. This problem could become worse in the future when these plants receive a large volume of sewage with higher solids concentration due to fast growing population and industries in cities. Processing thickened sludge could easily lead to poor mixing in digesters and eventually digester failure. Therefore, it is warranted to investigate the mixing hydrodynamics of anaerobic digesters to achieve a better design of digesters. This study examines the efficiency of mixing due to liquid jets in a model anaerobic digester using transparent Xanthan Gum Keltrol T (XGKT) solution as the model liquid because it has rheological characteristics similar to those of digested sludge. Experiments were conducted using downward orientated jet in 0.15 and 0.4 wt% XGKT (low and high viscous) solutions. An acid-base decolourisation reaction was used in the flow visualisation of the mixing phenomena. Results showed that well-mixed volume in the vessel decreases with increasing liquid viscosity. Complete mixing was achieved in the low viscous solution within a shorter mixing time. In the case of high viscous solution, a well mixed region was found only near the jet outlet. The stagnant regions outside the well-mixed region remained undestroyed even after a long mixing time. These results indicate that homogeneous mixing of sludge with the rheology of low viscous solution can be achieved with liquid jet mixing whereas it cannot be achieved in thickened sludge with the rheology of high viscous solution.