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The benefits of CFD as applied to the design of new wastewater treatment plants (WWTP), or the expansion of existing ones, have been featured in previous posts. These posts dealt primarily with wastewater that had only small amounts of dissolved solids so the fluid exhibited Newtonian viscosity properties. However, CFD can be especially useful in the analysis of sludge transport in which the high solids content of the sludge leads to non-Newtonian fluid viscosity behavior. Characterizing the fluid rheology accurately can be challenging, so CFD can be a very useful tool to predict the system behavior for a range of rheological properties to ensure successful operation of the pumping system.

One application was for the sludge piping that was feeding a digester in which the piping was extended to accommodate additional equipment and capacity. The concern was whether the overall pressure loss of the new piping system would exceed the head capacity of an existing pump. Furthermore, other changes in the WWTP resulted in some uncertainty of the sludge solids content, so although the sludge could be represented as a Herschel-Bulkley fluid, the rheology could cover a fairly wide range of parameters.

A detailed model of the piping system was created that consisted of several hundred feet of piping with numerous elbows and T-junctions. Then, simulations for a range of rheological parameters were conducted to determine the required pump head for various conditions. Based on these simulations, it was determined that the existing pump would still be able to perform satisfactorily for the proposed piping and range of possible solids content.

It is noteworthy that other methods of predicting pressure losses in piping systems with non-Newtonian fluids can be challenging, as they typically rely on 1D loss coefficients for pipe and pipe fittings with uniform inlet flow. CFD simulations, on the other hand, provide relatively accurate predictions not only in straight pipe sections, but also through fittings such as elbows and T-junctions and additionally account for interaction between closely coupled fittings and through pipe and fittings with non-uniform flow profiles.