Performance evaluation of mixing systems in a model anaerobic digester

Performance of three mixing systems (mechanical mixing, pumped liquid recirculation and gas mixing) was investigated in a laboratory scale model anaerobic digester. A transparent model fluid, xanthan gum Keltrol T (XGKT) solution was used to represent digested sludge due to their similarity in rheological characteristics. Mixing performance in 0.15 and 0.4 wt% XGKT solutions was investigated using a flow visualisation technique involving a decolourisation method with the assistance of a fluorescent dye tracer. A constant specific power input was used for all the three mixing systems. The changes in inactive mixing volume were determined as a function of mixing time for each system. The final inactive volumes in the three mixing systems were compared to determine their mixing efficiencies. Complete mixing with less than 5% inactive volume was achieved in the 0.15 wt% solution in all the three systems. However, a significantly large inactive volume remained undestroyed in the 0.4 wt% solution. The mechanical mixing had the best performance in the 0.15 wt% solution whereas the gas sparging system showed the best mixing efficiency in the 0.4 wt% solution.