Designing a Waste Treatment Sludge Mixing System:
Mixers for sludge storage tanks are used to suspend the solids prior to thickening or de-watering. The mixers are also used to blend-in conditioning agents. Occasionally, some sludge storage require sparging in air, which is beyond the scope of this discussion.
Defining the process characteristics to define a mixing requirement for a sludge storage chamber or tank can be difficult to define. Sources of sludge include raw septage, waist activated and thickened sludge, as well as sludge from the primary and secondary clarifier. The proportions from each source are generally variable, which result in an infinite number of consistencies. Therefore, conservative sizing must be considered for a worst case condition.
Viscosity studies of various sludge's indicate that they are special pseudo-plastic shear thinning non-Newtonian slurries. This sizing and mixer selection procedure is based upon general assumptions about sludge characteristics. Viscometer readings that determining shear rate vs. viscosity, which are typically used for flow evaluation (tank turnovers), should be somewhat tempered as sludge has very unusual visco-elastic characteristics. In short, thickened sludge does not act like general comparable pseudo-plastic fluids.