Why sludge rheology matters: Reducing OPEX and improving mixing efficiency with cutting-edge CFD

When it comes to mixing tank optimisation, addressing the challenges posed by sludge is high on the agenda. Operating with thick sludge or failing to design for its presence can lead to significant increases in energy expenditure as the system attempts to mix effectively. At the same time, energy inefficiency and costs can rise if the system is oversized or inadequately designed to handle sludge. Let’s take a look at sludge and some important considerations for mixing systems.

What is sludge?

Sludge is a byproduct of wastewater that does not ‘flow’ like water and has non-Newtonian properties. This means that its viscosity depends on the forces that are applied to it, rather than being constant like Newtonian fluids (water, for example). Therefore, to understand how sludge behaves within a mixing system, rheology – the study of flow and deformation of matter – must be used.

Impact of sludge on sustainability, CAPEX, and OPEX

The primary issue with sludge in a mixing system is that it can dramatically increase the power needed to effectively mix the volume. The thicker the sludge is, the higher yield stress it exhibits, meaning more power is required to mix it – an unoptimised system means increased costs and energy-associated emissions. However, there is also a trap in oversizing the system.

All too often, oversized mixers are selected for sludge applications, just to be on the “safe side”. In most circumstances, the resulting mixer is too large for the application, resulting in unnecessary capital expenditure. In turn, this leads to higher operational expenditure, as the system consumes more energy than a mixer that is the “right” size. 

Rheology in design: avoid common mistakes

Mixing system design is never as straightforward as it seems. Complex or unforeseen fluid behaviours and the impact of sludge rheology can significantly reduce system efficiency and efficacy. When applied correctly, rheology allows for intelligent system design for real-world applications. 

For example, the cooling of mixer motors is usually done by the flow of liquid through the system. Sludge can impact the effectiveness of this cooling and is, therefore, an important factor in determining the number of mixers and their positioning – this is often overlooked. Equally, different sludge types, thickening methods, and chemical additions will impact the sludge’s flow, even though they might all have the same level of total suspended solids content. It is vital to take all of this into consideration in system design.

CFD as a tool to understand sludge impact

Here at Xylem, we have developed complex CFD modelling solutions for mixing tanks. For customers looking to design a new mixing tank or to investigate issues with a current system, our engineers use a combination of real-world laboratory testing and CFD to identify and resolve issues for an optimised result.

Our lab testing uses scale copies of the real-world mixing system. This allows us to map the whole tank volume and investigate how bubbles, solids, and liquids behave in the tank. Our engineers also conduct rheology tests to measure sludge properties – valuable data for mixer sizing and validated input for our CFD models. Through the combination of scale testing, CFD, and extensive experience in tank design, Xylem can develop the solution.

Predict and prevent

By understanding sludge rheology and modelling mixers using a non-Newtonian approach, Xylem can help customers mitigate the risks posed by sludge. Evaluating sedimentation, crust formation, and homogeneity, engineers work to understand each specific system and the challenges it faces. Whether they are developing a new design or looking for a technical solution to an existing system, they know how to predict and prevent the common problems caused by sludge, while increasing system efficiency across the board.