Submerged, recirculating jets: nozzle geometry and its effect on active volume creation in the mixing of municipal sludge simulant

Anaerobic digestion (AD) of sludge is an integral step in municipal wastewater treatment. The complex rheology of the feed to digester often leads to poor mixing and the formation of deadvolume. This work attempts to address the poor mixing issue. Submerged, recirculating jets, which are frequently used in anaerobic digesters for the purpose of agitation, are the focus of this study. In particular this study systematically explores the effects of the orientations of the injected jets on the evolution of dead-volume within the experimental reactor when the point of injection of the jet is maintained constant. A well characterised viscoelastic fluid that is known to mimic the rheological behaviour of municipal waste-water sludge is used, and the decay of the dead-volume is observed as a function of different injection velocities, and solution rheology. By measuring the decline of the deadvolume (Vi) as a percentage of total volume (VT) over a time-scale, normalised by the hydraulic retention time, several distinct mixing regimes were identified that allow a rational comparison of the efficiencies of the configurations used. The study suggests that when the jet is pointed upwards towards the air-liquid interface the overall rate of decay of Vi, increases compared to the case when the jet is pointed to the base of the tank. Also, the results suggest that when the jet is pointed upwards the influence of the rheological nature of the fluid on the decay of the dead-volume is reduced. A mechanism that can explain this observation is also discussed.