Submerged, recirculating jets: Flow regimes in the mixing of municipal slugde simulant

Inadequate mixing of anaerobic digesters in wastewater treatment results in less than optimal biogas production and solids settling, which leads to costly shut-downs and cleaning. This study uses a sludge simulant to investigate the mixing phenomena of a submerged recirculating jet in a model anaerobic digester. Using a flow visualisation technique, two separate regions, namely an active region (Va) and an inactive region (Vi), are recognised. When the ratio of inactive volume and total volume (Vi/VT) is plotted against normalised time scale, Nt, which corresponds to the number of liquid turnovers due to recirculation, it is found to decrease continuously and three distinct flow regimes are identified over the time scale. By manipulating the parameters of fluid rheology and nozzle injection velocity, while holding the geometric factors constant, the three regimes were further investigated and characterised. Regime 1 occurs at low values of Nt, and can be thought of as an initial phase, in which a well-mixed cavern begins to form in the fluid body and spreads towards the tank wall. Regime 2 is the one most heavily affected by rheology, and is characterised by a welldefined cavern that reaches the vessel wall. Regime 3 seems independent of rheology and is characterised by a breakdown of the well-defined cavern as the Va approaches VT.