METERING OF RHEOLOGICAL PROPERTIES OF SEWAGE SLUDGE TO PREDICT SLUDGE FLOW BEHAVIOUR
Lars Uby, Magnus Fahlgren, Gert Hallgren, & Anna E Lindberg
ITT Flygt, SWEDEN
(free)
Design of sludge pumping and mixing systems requires substantial knowledge of sludge flow behaviour. Important information is contained in the sludge flow curve, which generalises the concept of viscosity. Unless the flow curve is well defined and accurately determined from rheometry, the pump or mixer selection runs the risk of leading to either oversized equipment, with the effect of unnecessarily high capital and operational cost, or undersized equipment, with the effect of – at the least – poor performance and secondary effects like sedimentation in sludge storage tanks or poor heat transfer in a heat exchanger.
A well known, fairly general flow curve model often used for engineering purposes is the three parameter Herschel–Bulkley model. Within ITT Flygt, sludge flow curves according to i.a. this model are obtained by rheological measurements followed by an in-house developed data analysis procedure. This procedure removes inaccuracies that are present in commercially available standard rheometry software because of simplifying assumptions that are invalid for sludge and similar liquids.
For the rheological measurements a concentric cylinder rheometer is used, where the test liquid (sludge) is sheared in the gap between two cylinders. The primary output from the rheometer is the torque required to run the inner cylinder at some selectable speed.
The analysis method devised for the transformation of primary rheometry data to the correct flow curve involves solving the equation of motion for the flow in the rheometer. A simplex method is used to obtain the best Herschel-Bulkley (or other model) parameters to describe the sludge. Measurement data that are outside the range to which the analysis is applicable are also detected and omitted.
By using the described rheometry and analysis method developed by ITT Flygt we have an objective method to determine the flow curve of any sludge. The method is accurate within the limits of the flow curve model (e.g. Herschel-Bulkley) approximation, it is repeatable and leaves little room for user dependent results. All those properties are essential for a system that aims for flow loss predictions based on flow geometry and rheological properties. Examples are given to indicate the nature of the refinement of head loss calculations obtained by adopting the present method.
KEY WORDS Rotational viscometry, non-Newtonian, Herschel-Bulkley, sludge, viscosity parameter, flow prediction, viscous losses, pipe flow.
