Measuring active volume using electrical resistance tomography in a gas-sparged model anaerobic digester

Inadequate mixing in anaerobic digesters fitted with gas sparging systems is caused by many factors, and leads to dead zones where sludge remains stagnant. The present study explores a range of gas sparging configurations that can be implemented to maximize active volume, and validates electrical resistance tomography (ERT) as an effective measurement tool for analysing mixing conditions without the need for visual access to the liquid volume. Air was used as the gas phase, and xanthan gum Keltrol-T (XGKT) solutions at concentrations of 0.15 and 0.4 wt% were selected as transparent simulant fluids for their rheological similarity to digested sludge. Gas flow rate, sparger nozzle orientation (upward-facing vs. downward-facing), and nozzle height were varied, and mixing performance was assessed using flow visualisation experiments. Results were then replicated with ERT for comparison. It was found that the 0.15 wt% XGKT solution achieved almost complete mixing for all configurations, while the 0.4 wt% XGKT solution developed stable, unmixed regions. Gas flow rate made little difference to the final mixed volume suggesting lower power input does not sacrifice steady-state active volume in the reactor. Positioning the nozzle closer to the bottom of the vessel and sparging gas downward both reduced inactive volume. ERT measurements matched flow visualisation results closely, and were able to capture details that flow visualisation ignores. It has been shown that there is great potential for implementing ERT as a method for researching flow behaviours in complex opaque materials, especially the formation and progression of active volume.