Water Online

November 2017

Water Innovations gives Water and Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.

Issue link: https://wateronline.epubxp.com/i/896704

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Page 22 of 29

By Sami Sarrouh M ixing sludge is more complex than mixing other fluids because of its non-Newtonian characteristics. Some manufacturers provide mixing systems for sludge in holding tanks and digesters with varying degrees of success. In order to properly design such systems, it is important to understand the physical characteristics of the fluid and its response to applied forces. Sludge behaves differently at different solids content. To simplify the discussion, we shall categorize sludge solids content, or total solids (TS), from a fluid dynamic aspect according to the following: • TS ≤ 2 percent — Low content where sludge behavior is not much different from water. • 2 < TS < 4 percent — Average content where sludge behaves as a non- Newtonian fluid, but not drastically different from water. • 4 ≤ TS < 8 percent — High content where the solids start to behave as pseudo-plastic fluid. Here the ratio of volatile solids (VS) content to TS becomes a major factor. • 8 ≤ TS < 12 percent — Sludge is pretty much a semisolid and not achievable without some form of thickening. • TS ≥ 12 percent — Sludge is dewatered and handled as a solid. TS < 4 percent sludge is not highly viscous and may be handled with ease by most commercial systems. Eight ≤ TS < 12 percent is already thickened sludge and is not typically mixed, as a fluid, in water or wastewater applications. This article will discuss 4 ≤ TS < 8 percent solids sludge mixing in holding or fermentation tanks. Micromixing of such sludge using high-velocity agitation requires a considerable amount of energy due to the high-viscosity sludge, especially in large tanks. Higher sludge concentration slows down settling speed due to particle interactions. Therefore, proper design attempts to achieve macromixing of the sludge by redistributing the sludge three-dimensionally using density currents to further homogenize the volume. Some sludge may include sand particles. Sand is abrasive and can cause much faster equipment wear, but the solids content required to change flow characteristics is higher than treatment sludge and dependent on particle size. Literature shows that sand slurry exhibits shear-thinning, non-Newtonian behavior for all solids concentrations at low shear rates. However, at higher shear rates, high concentration sand solids suspensions transition from a shear thinning to a shear thickening. Such high shear rates may be present in a pump, for example; therefore, sludge "behavior" may vary considerably with sand content. Accordingly, mixing system design has to accommodate variable conditions and assist in removing potential blockages caused by unforeseen conditions. Mixing efficiency is determined by the behavior of the fluid interfaces, and knowledge of the dynamics of the interfaces is crucial. The volume enclosed by the outer interfaces between recirculated or influent and ambient sludge, rather than the interfacial surface area between the different sludge, is what determines mixing efficiency. Large-scale dynamics of the outer interface provide the dominant contributions to the mixing efficiency. The mixer design presented in this article utilizes a combination of dispersive and distributive mixing. The latter, also known as macromixing, relies on swirl created by directed flow that causes laminar thinning of the interfaces, thereby increasing volumetric combination of the sludge. A repeated cutting-and-folding action of the mixture also increases the distribution of different sludge components. The effectiveness of a mixer in distributive mixing is a function of how the influent jets interact with the ambient sludge 20 wateronline.com n Water Innovations Computational fluid dynamics (CFD) modeling helps wastewater operators derive a formula for highly effective and cost-efficient sludge mixing. Effective Sludge Mixing Through Distributive Mixing Principles Figure 1. From top to bottom, Design Layouts 1, 2, and 3

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