Plate Settler Spacing

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Introduction

This research is focused on a deeper understanding of the sedimentation process so that plants can remove flocs from clean water as efficiently as possible, allowing for successful chlorination of the effluent water. Currently, AguaClara plants use lamella in their clarification systems, which are a network of sloped, stacked plates that create narrow channels through which clarified water can flow.

To emulate lamellar sedimentation, we use tube settlers of various diameters, which emulate the effects of the lamellar plates.

A majority of flocs that enter these inclined channels settle out due to the force of gravity, significantly reducing effluent turbidity to increase the effectiveness of chlorination. There is no control of influent water parameters going to the plant, so there is a keen interest in developing a settling system that is robust -after floc blanket clarification that will produce less than 1 NTU water.

Ultimately, we hope to optimize the lamella design in order to achieve effluent water with a turbidity of 1 NTU or less -- even under water chemistry fluctuations and alum dose variation. At present the plate spacing, capture velocity, and velocity gradient formation are three of our key design limiting constraints.

Robustness of our plate design is defined as the ability of the plate settlers to produce 1 NTU water over a variety of non-ideal conditions. One set of non-ideal conditions was building a floc blanket with underdoses and overdoses of alum to measure performance through effluent turbidity from the tube settler. Other non-ideal conditions that should be investigated are organic matter in the influent, varying influent turbidities, and other changes, such as pH, in the chemistry of the water. Please put this in another section

Plate settler spacing is an important factor in determining the height of the plant clarifiers. Theoretically, if we could find a way to maximize their performance at the lower-limit of spacing and height it would be possible to decrease height of the sedimentation tank and lower plant costs. Please put this in another section

Velocity Gradient Research

Past research has brought to light the importance of velocity gradients within the tube settlers. Flocs at the bottom wall of the tube that experience an upward velocity pull greater than the force of gravity pulling them down will roll up the wall and exit with the effluent water. Flocs are fractal particles whose effective diameters are sensitive to shear stress, organic material in the water, and influent turbidity, so determining their response to the velocity gradient may require more than just a simple force balance. Furthermore, the maximum velocity gradient achieved at steady state will fluctuate with a plant's conditions and choices for flow rate through their lamella.

Subteam Semester Goals and Future Challenges
Weekly Subteam Progress.
Research Plan

Experimental Methods and Results

Summer 2009

Experiments Varying Alum Concentration

This section contains the experiments in which the alum dose was varied in order to test how well the selected geometry (d = 15.1mm) performed when subjected to non-ideal conditions

Experiments with the Velocity Gradient

The team has conducted physical modeling of the system to investigate the phenomenon of floc roll-up.

Filter Foam

Due to a discrepancy in head loss, flow through each of the plate settlers in plants in Honduras is not equal. A geotextile foam was placed on top of the plate settler to create head loss and equalize the flow.

Spring 2009

Experiments with Flow Rates and Inner Tube Diameters

This section contains two related experiments. The first experiment varies flow rates and the inner diameter of the tube settler. From these results, the ideal L/d ratio is identified and held constant for a certain flow rate and inner diameter by varying the tube length.