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the sludge by distributing the flow in a three-dimensional manner and making use of density currents. Some nozzles push the heavy solids towards the bottom suction where it is combined with lower- density stream lines from the surface. Other nozzles redistribute the sludge to higher regions of the tank. The mixing system Design Layouts 1 and 2 maximize the effect of density currents that direct the heavier solids towards the suction pipe at the bottom of the cone. The suction inlet is oriented upwards to allow streamlines of lower-density solids originating near the water surface to reach the bottom where it joins the higher-density solids sliding along the conical surface. High-density solids flow by gravity along the sloped bottom towards the recirculation suction pipe aided by momentum imparted by the upper loop nozzles. The nozzles are distributed along the upper cone's perimeter and angled parallel to the conical bottom surface to create boundary layer attachment at the surface of the cone. The nozzles on this upper loop are angled from radial direction. The lower pipe loop is located at about a third of the tank radius. This is the location where all the upper loop's jet paths will intersect. The lower loop has nozzles that are sized to provide the same total flow as the upper loop, although it has fewer nozzles. The lower loop's nozzles are located such that six nozzles are angled up and offset a few degrees from radial, while four nozzles are angled down and offset from radial to continue the solids motion towards the suction pipe. Modeling Results And Conclusions The findings show that the mixing system results in a tank with little potential for major sand/solids accumulation. However, it shows that out of the three design layouts modeled, Design Layout 2 with lower recirculating flow rates provides the best mixing with the lowest operational risk and the best ability to recover from unforeseen conditions. Contour plots of velocity magnitude indicate zones of effective fluid agitation as well as zones of stagnation. Nevertheless, in thick sludge all velocities in the tank are very low due to very high apparent viscosity of the sludge that quickly dissipates the jets' momentum. The velocity distribution in the tank is presented as a chart in Figure 2, which shows the percentage volume of tank where the fluid velocity is less than a defined threshold (1e-6, 1e-5, and 1e-4 m/s in this case). For Case 1 (Design Layout 1 at 3000 gpm), it can be seen that the fluid velocity is less than 1e-4 m/s in nearly 50 percent of the tank volume. The comparison between Cases 1 and 4 versus Cases 3 and 5 shows that Design Layout 2 achieves a better mixing than Design Layout 1, as the volume where the velocity is below 1e-4 m/s is smaller for Design Layout 2 (Cases 4 and 5), although the flow rates for Cases 4 and 5 were two-thirds those of Cases 1 and 3. Another method used to compare design efficacy involves calculating sludge residence time at various regions within the tank. In a direct comparison between Design Layouts 1 and 2 from the CFD analysis, the potential for sand accumulation in Design Layout 2 is decreased due to lower and better distribution of residence time, although the recirculation flow rate and associated energy is reduced by two-thirds. Thus, a simple reorientation of the nozzles had a significant impact. The better flow distribution resulted in a better residence time distribution — or, in other words, less short-circuiting at various tank regions. In addition, Design Layout 2 performance is such that increasing sludge solids content from 3.5 percent to 6 percent required an increase from 2,000 to 4,000 gpm to maintain relatively similar performance. This indicates that using the 3,000/6,000 gpm flow rates with Design Layout 2 provides the resiliency to help recover from unforeseen operating conditions. The mixing system design philosophy described in this article has been used successfully in both an alum sludge holding tank application and a wastewater sludge holding tank. The results indicate that proper sludge distributive mixing can reduce recirculation pumping costs by approximately 50 percent while achieving better performance. n 22 wateronline.com n Water Innovations BIOSOLIDS About The Author Sami Sarrouh, PE, is vice president and senior technical environmen- tal engineer at T&M; Associates. He has over 28 years' experience in applied research, design, and management, working on about 200 different projects in the water and wastewater industry. Figure 3. Residence time distribution for Design Layouts 1, 2, and 3