sedimentation tank hydraulics

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Plate Settler Spacing, Fall 2010

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Tanya Cabrito, Jae Lim, Cosme Somogyi

Abstract:

The goal of the Plate Settler Spacing Team (PSS) is to study the lamellar sedimentation process in plate settlers and the efficiency of the removal of flocculated particles and establish improved guidelines for plate settler spacing. The traditional guidelines for plate settlers state that the spacing between plates cannot be less than 5 cm and little to no justification can be found for this. The team's results show that performance in accordance with the US drinking water standard of 0.3 NTU can be achieved with spacings smaller than 5 cm. Also, all but one of the experiments meet the World Health Organization standard of 5 NTU. For AguaClara plants, having smaller spacings between plate settlers allow the sedimentation tank to be shallower and therefore cheaper.  Smaller spacings also allow for increased head loss across the plate settlers. This would help even out the distribution of flow in AC plants and allow the plate settler system to function at its design capture velocity of 0.12 mm/s throughout. The team had finished velocity gradient experiments with a clay aluminum hydroxide system; however, a recently discovered mistake in documentation caused the team to reassess the data collected this semester and more tube trials must be run. Due to this error, the team changed the capture velocity for the velocity gradient from 0.12 mm/s to 0.10 mm/s.  The major tasks completed by the Fall 2010 PSS team are catching a documentation error that happened at the end of Spring 2010, studying the effects of high velocity gradients and floc rollup on plate settler performance, developing a macro that significantly facilitates data analysis and a plate settler dynamics model that may better shed light into processes governing plate settler performance.

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Sedimentation Tank Hydraulics, Spring 2011

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Yiwen Ng, Anna Lee, Tiffany Tsang

Abstract

Previously our team worked on designing a scaled down model of sedimentation tank in order to study floc blanket formation in 3D models. However, we decided that it would be more effective to continue the study with the existing 2D sedimentation tank. Using this tank our first objective was to determine the minimum angle of repose. We have hypothesized that for an insert angle below 60 degree that a floc blanket would not form. The slope of the insert would not be sufficient for the flocs to be transported to the jet, thus the flocs would accumulate on the incline. However, even when we decreased our angle of repose to 30 degrees, we were successful in forming the floc blanket.

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Plate Settler Spacing, Spring 2011

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The Effect of Floc Roll-up on Clay-Aluminum Hydroxide Flocs

Matt Hurst, Monroe Weber-Shirk*, Tanya Cabrito, Cosme Somogyi, Michael Adelman, Zachary Romero, Richard Pampuro, Rachel Phillipson, Sarah Long, Colette Kopon, Ying Zhang, Ashleigh Sujin Choi, Adela Kuzmiakova, Jae Lim, Alexander Duncan, Christine Catudal, Elizabeth Tutunjian, Ling Cheung, Kelly Kress, Tiara Marshall, and Leonard W. Lion

Abstract:

Inclined plate and tube settlers are commonly 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. The greatest concern with small plate spacing is the danger of settling sludge being swept out with the finished water. This research presents the basis of this failure mechanism as high velocity gradients present at small tube settler diameters and small plate settler spacings.

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Sedimentation Tank Hydraulics, Summer 2011

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Elana Liskovich, Mahina Wang, Jill Freeman, Yiwen Ng

Abstract:

A floc blanket is a dense fluidized blanket of flocs that helps to reduce effluent turbidity in the sedimentation tank by trapping other flocs. The geometry of the sedimentation tank is crucial in determining the extent of floc resuspension by the jet and hence floc blanket formation. To improve tank bottom geometry, nine experiments were conducted, each testing a different tank bottom geometry. The experiments were run in a 1/2 inch wide tank to model a thin slice of the full scale sedimentation tank. The geometry that resulted in the least sludge accumulation and therefore best floc resuspension was two 60 degree inserts leading to a semicircular trench 10 cm in diameter. We also provided initial designs and calculations for a floc weir to maintain the height of the floc blanket. A preliminary experiment was also conducted to evaluate the feasibility of our initial floc weir design.

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Sedimentation Tank Hydraulics, Fall 2011

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Jill Freeman, Mahina Wang, Saied Khan, Matthew Hurst

Abstract:

A floc blanket is a dense, fluidized bed of particles that forms in the sedimentation tank and helps to reduce effluent turbidity by trapping small flocs and reduces clean water waste through less frequent draining of the sedimentation tank. Floc resuspension is necessary for floc blanket formation so that flocs are recirculated through the tank instead of settling on the tank bottom as sludge. Research was conducted to examine mechanisms for floc resuspension. Parameters important for floc resuspension include energy of the jet stream on its upward flow path, position of the jet as it interacts with solids, and hydrodynamic pressure of the jet compared to hydrostatic pressure of the returning solids. Several geometries were tested with red dye and fully built floc blankets to observe the jet path and velocity profile around the bottom geometry. Best results are achieved through geometries that preserve jet momentum, especially through splitting the jet flow, and geometries that maintain a high jet velocity when contacting solids. Later, quantitative measurements were taken to determine floc blanket performance for various bottom geometries.

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Sedimentation Tank Hydraulics, Spring 2012

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Jill Freeman, Mahina Wang, Matthew Hurst, Saied Khan, Yiwen Ng

Abstract:

A floc blanket is a dense, fluidized bed of particles that forms in the sedimentation tank. It helps to reduce effluent turbidity by trapping small flocs and reduces clean water waste through less frequent draining of the sedimentation tank. Floc resuspension is necessary for floc blanket formation so that flocs are recirculated through the tank instead of settling on the tank bottom as sludge. Research was conducted to examine the effectiveness of the retrofitted Marcala sedimentation tank. At high influent turbidities, a steady floc blanket was obtained, but performance was slightly compromised when the influent turbidity was lowered to simulate Marcala conditions during the dry season. A floc blanket visibly formed with an influent turbidity of 5 NTU after about 1 week but “seeding” the tank with coagulated flocs will minimize floc blanket formation time. Images were also acquired for hindered sedimentation velocities of 0.6 mm/s, 1.2 mm/s, and 1.6 mm/s and analyzed with a floc-water interface program using a region of interest to better understand hydraulic processes within a floc blanket. Complete settling curves from this data confirmed wall effects significantly affect settling velocity. A floc hopper proved to be effective at controlling the height of the floc blanket when the accumulated flocs were drained at an adequately high flow rate. A lower alum dose of about 39 mg/L for an influent turbidity of 100NTU resulted in a less sticky sludge that could be more easily drained from the hopper.

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Sedimentation Tank Hydraulics, Summer 2012

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Danielle Feng, Jill Freeman, Cari Gandy

Abstract:

In the sedimentation tank of an AguaClara water treatment plant, water flows through the inlet manifold with vertical diffusers that channel the water into the bottom of the tank as a line source. As water exits the vertical diffusers, a semi circular half pipe jet reverser directs the water upward to resuspend flocs to form a floc blanket, or a dense, fluidized bed of particles, in the sedimentation tank below the plate settlers. A floc blanket increases the particle removal efficiency of the sedimentation tank by capturing smaller flocs that would otherwise escape through lamellar sedimentation. A floc blanket also leads to less clean water waste because without a floc blanket, sludge builds up at the bottom of the tank and will require constant draining. While current plant designs use a 0.5” radius jet reverser and centered jet placement, other jet reverser sizes and jet placements were explored to increase floc resuspension and floc blanket stability. A 1.5” radius reverser with asymmetric jet placement was found to be the optimal design for floc resuspension. Floc blanket stability in relation to coagulent dose was also explored, and optimal alum doses for several influent turbidities were determined.

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Sedimentation Tank Hydraulics, Fall 2012

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Frances Ciolino, Hongyi Guo, Ethan Yen

Abstract:

The sedimentation tank hydraulics team this semester focused on optimizing the floc hopper. Our main goal was to learn more about the floc hopper geometry and reasons for floc blanket failure. We started the semester by looking at the vertical sedimentation velocity. This velocity is controlled by the flow into the tank and affects how fast the particles settle. If this velocity is too high or too low, the sedimentation tank will not form a proper floc blanket.

We also looked into how the plan view area of the floc hopper would effect the floc blanket formation and performance. Changing the size of the floc hopper effects how much of the area allows for up-flow of the water and how much captures the flocs. In our current experiments we are trying different sizes at different wasting rates. We are looking for a size and rate that keeps the plant running efficiently, meaning the least amount of water wasted while keeping the water leaving the plant clean.

One of the ideas we are working on is continuous wasting of the flocs. The idea behind this is to allow for constant removal of flocs instead of collecting the flocs and then manually opening a valve to let them leave the tank. When the wasting rate is optimal, the flocs will be allowed to compact before they are removed so that the least amount of water is lost in the process. Our last experiment looked at finding the proper wasting rate where the rate of particles flowing into the floc hopper is the same as the rate at which the particles are being removed.

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