Design Recommendations for Mixing

Design Recommendations for Mixing44 [ 2 ] APPLICATIONS AND RULES INTERMITTENT MIXING OPERATION IN MEDIUM/LARGE PUMPING STATIONS Prior to delivery to a pumping station, raw wastewater may be inefficiently screened or not screened at all. This means that fine and coarse material can accumulate inside the pump sump, forming a thick crust which makes pumping restart operations very diffi- cult. To avoid the build-up of such layers of coarse material, it is necessary to peri- odically mix the pump sump content to make efficient pumping possible. Submersible mixers are therefore in- stalled inside the pump sump and will operate in intermittent duty. Operation can be during filling of the pump sump or when a minimum level in the pump sump is reached, and before pumping operations restart in order to homoge- nise as much as possible of the mix- ture of wastewater and solids. More than one mixing unit can be installed depending on the pump sump feeding conditions and pump sump volume. See figures 27, 28 and 29. Jets from submersible mixers should be directed to avoid mechanical issues with the pumps developing because of the mixing thrust. Fig. 27 Fig. 28 Fig. 29Page 2