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 dirt particles from dirty water as efficiently as possible, allowing for successful chlorination of the outgoing water. Currently, the 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 make things simpler for a lab, we use tube settlers of various diameters, which emulate the effects of the lamella plates. A majority of flocs that enter these channels settle out due to the force of gravity, significantly reducing effluent turbidity to increase the effectiveness of chlorination. It is inevitable that controlling the conditions of the influent water at the various plants in Honduras is nearly impossible , so there is a keen interest in developing a settling system that is robust --- that is, finding both a geometry and flow rate that can handle a wide range of influent turbidities and disturbances in alum dosage.

Plate settler spacing is an important parameter for AguaClara because the lamella are 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 plant costs, since lower lamella also means a smaller facility.

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 underdosed and overdosed conditions to measure performance through effluent turbidity from the tube settler. Other non-ideal conditions that should be investigated are organic matter in the effluent, varying influent turbidities, and other changes in the chemistry of the water.

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.

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.

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

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.