Current Team Research Focus - Spring '11
Effect of a Floc-Rollup Phenomenon on Flocs
IntroductionTraditionally, inclined plate and tube settlers are used to create compact sedimentation tanks. Conventional design guidelines are based on obtaining a desired sedimentation design capture velocity. Theoretically, this capture velocity can still be achieved while greatly reducing conventional plate spacing or tube diameter. Yet, the greatest concern with small plate spacing is the danger of settling sludge being swept out with the finished water – the phenomenon known as the floc-rollup. It is the purpose here to estimate the effect of the floc-rollup inside the plate settlers. Particle Capture by a LamellaThe experimental testing was performed on tube settlers with the design capture velocity of tube settlers in which the ends of the tube are perpendicular to the axis of the tube. Theoretically, it is possible to reduce L by decreasing the diameter of the tube settlers, D. After a floc settles on the lower surface of a plate or a tube it continues to experience an upward drag caused by the fluid flow. The velocity at the centerline of the floc increases if the spacing between plates or the diameter of the tube is decreased while maintaining a constant average fluid velocity. Gravitational force will cause flocs to roll or slide down the incline while the fluid drag will tend to cause the floc to roll or slide up the incline. When the fluid drag and the gravitational forces balance, the floc remains stationary. This balance point is approximated by determining the point at which the velocity caused by fluid drag at the centerline of the floc is the same as the opposing component of its terminal velocity along the slope (as seen in Figure 1).
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By using the velocity gradient at the wall, we then obtain the expressions both for velocity at the center of a floc resting on the wall and for the diameter of a floc. Finally, we solve for the terminal velocity. The floc terminal sedimentation velocity represents the slowest settling floc that can slide down an incline. Flocs with a terminal velocity lower than will be carried out the top of the tube (i.e., “roll up”) even if they settle on the tube wall. Thus, this terminal velocity represents an additional constraint on the capture velocity for tube settlers. Unlike the design sedimentation capture velocity, which is exclusively a property of the geometry and flow characteristics of the sedimentation tank, the capture velocity needed to prevent flocs from rolling up and out of the tube (referred to here as the “roll up capture velocity”) is a property of the floc as well as the sedimentation tank geometry and flow characteristics. This complexity is a result of the interaction between the size of the floc and the linear velocity gradient.
Additionally, we estimate the floc roll up capture velocity as a function of tube diameter, D, for the case of three different upflow velocities. However, for a given upflow velocity, a decrease in tube diameter results necessitates in a higher particle settling velocity for particle capture to occur.
Experiments and Results Constant 0.10 mm/s design capture velocity experiments have been conducted using tube settlers of two different inner diameters, 6.35 mm and 9.53 mm respectively, for a range of V-alpha. The figure below shows the measured pC*s as a function of V-alpha. When V¬α increases, the performance of the system decreases, while the performance of the floc blanket remains relatively consistent with the increasing V¬α for an average value of pC* of 1.12. The graph shows a first order decreasing trend of the system performance.
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ConclusionsAll the above results are given for tubes. V-rollup is dependent on the velocity gradient, which is ¾ less for plates than for tubes. All equations with the linearized velocity gradient at the wall are therefore changed by a factor of ¾ for plates. Thus for a given V_↑, for plates will be of a smaller magnitude than the V-rollup for tubes. Thus plates are slightly less vulnerable to floc roll up than are tubes given the same diameter and spacing.
The floc roll up model delineates a failure mechanism that prevents flocs from sliding along an inclined surface in the countercurrent direction. This failure is caused by fluid drag resulting from velocity gradients at the plate or tube wall that oppose gravity forces. Velocity gradients increase as the plate settler spacing or tube diameter is decreased and as the upflow velocity is increased. We expect that high velocity gradients will cause flocs to “roll up” an inclined surface and act to increase effluent turbidity. If the tube settler diameter or plate settler spacing is too small, high velocity gradients can cause roll-up of flocs that would otherwise be captured. Evaluated this phenomenonm utilizing a combined tube-settler floc blanket system to characterize the removal effectiveness for colloidal particles at different flow conditions, but at a constant design capture velocity of 0.1 mm s-1. Experimental data suggests that plate spacing as small as 1 cm for an upflow velocity of 1 mm/s can be implemented without causing performance deterioration.
Tube settler performance deteriorated when the the floc roll up capture velocity was larger than the sedimentation design capture velocity Plate Settler Spacing research focuses on developing a more thorough understanding and optimizing the lamellar sedimentation process of AguaClara plants. Currently the plants use lamella, which are a network of stacked, sloped plates with narrow channels between them. These are used to provide more surface area for particles to settle out, thereby significantly decreasing the sedimentation tank plan area. As water flows up through these channels, coagulated dirt particles are caught by the plates and fall down into the sedimentation tank. In the lab the Plate Settler Spacing Team (PSS) uses tube settlers to simulate the effects of lamella, where different tube diameters represent different spacing between the plates. The performance of these two technologies are comparable after adjusting for geometric differences, and results from bench-scale experiments can be applied to plate settlers. The team is focusing on a failure mechanism called floc roll-up, where high velocity gradients near the wall (present in small diameter tubes or at close plate spacings) overcome the floc particles' settling velocity causing flocs that would otherwise be captured to roll up into the effluent. Velocity Gradient theory (detailed in the PSS Fall 2010 Velocity Gradients Experiments) dictates that performance deterioration due to floc roll-up will be more significant for tube settlers than for plate settlers, given that the tube diameter equals the plate spacing. This is due to the geometric differences between tubes and plates, so using tube settlers for the bench-scale system represents the worst case scenario for failure. Since we are unable to control the turbidity level of the influent water entering the AguaClara plants, there is a significant interest quantifying plate settler performance over a wide range of field conditions. Nephelometric Turbidity Units (NTU) is a measure of solution's turbidity based upon how much that solution scatters light. On the laboratory scale, the team has produced finished water that meets the US drinking water standard of 0.3 NTU. Laboratory conditions, however, are an idealization of field conditions and may not completely be representative of field performance. The team's research thus far has used an influent solution of pure clay; however, the existence of natural organic matter in rivers and streams may result in worse plate settler performance. The PSS team's objective is to optimize the lamella design in order to achieve 1 NTU finished water or less, even under water chemistry fluctuations. If filtration technology proves feasible in the field, this would also ease the loading on the downstream filter. Some of the fundamental parameters which control the design of our experiments are plate spacing, capture velocity, and the formation of velocity gradients between the plates.
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