Liquid jet recirculation in a model digester: flow characteristics

Wastewater treatment is becoming an increasingly important issue to municipal water suppliers the world over. Anaerobic digestion is a preferred method of treatment given that it can produce energy above and beyond the needs of the wastewater treatment plant. With an increasing population density, the necessity to treat digester feeds with increased sludge concentration becomes apparent. With increasing sludge concentration, however, comes increasingly complex fluid rheology. This study aims to investigate the mixing characteristics of digested sludge in the reduction of inactive volume in anaerobic digesters. Experiments were carried out in a 0.19 m diameter vessel, mixed using liquid jet recirculation. A transparent simulant fluid (xanthan gum Keltrol T solution) at varying concentrations was used to mimic digested sludge. The active volume generated during mixing was determined using a flow visualisation technique involving an acid-base reaction and a fluorescent dye tracer. The decrease in inactive volume formation was monitored over a normalised time scale and the prevailing trends were investigated. The evolution of active volume was found to be influenced by vessel geometry, specific power input, and fluid properties. It was found that the rheological properties of the agitated fluid play a major part in the formation of the active volume. An increase in fluid apparent viscosity leads to an increase in specific power input required to achieve an equivalent active volume.