Accelerating the spread of the active mixing region in a sludge simulant using submerged jets

In anaerobic digesters operated in a municipal wastewater plant, submerged recirculating jet streams are often used for the purpose of agitating sludge. However, due to the complex rheology of the sludge, large portions of the digester can remain stagnant and the stagnant volume changes slowly with time. The rate of change varies with time which is determined by the rheological nature of the sludge. In our previous flow visualisation study involving a downward facing jet recirculating a transparent, non-Newtonian, sludge simulant in a model digester, we have shown that the rate of decrease of inactive volume occurs in three distinct flow regimes. In practice, some of these flow regimes can persist for an undesirably long period of time and delay the acquisition of a fully “active” state within the vessel. Our experiments in the present work carried out using the same sludge simulant reveal that the time-span of the flow regimes can be remarkably shortened by a simple change in the geometry of the recirculating jet. The findings reported here open up the possibility of improving the extent of mixing of highly viscous, non-Newtonian feeds within shorter time frame and lowering specific power input in systems operated with recirculating submerged jets.