Plate Settler Spacing

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Introduction

This research is focused on a more thorough understanding of the sedimentation process so that AguaClara plants may remove flocculated particles from their effluent streams as efficiently as possible. Currently the plants use lamella, which are a network of stacked, sloped plates with narrow channels between them. As water flows through these channels gravity pulls out coagulated dirt particles, resulting in a clean effluent. In the lab the Plate Settler Spacing Team uses tube settlers to simulate the effects of lamella, where different tube diameters represent different spacing between the plates. The properties of these two technologies are analogous and our results from the lab are directly applicable to plate settlers.

Since we are unable to control the turbidity level of the influent water entering the AguaClara plants, there is a significant interest in developing a robust system with high performance over a wide range of influent conditions. Nephelometric Turbidity Units, or NTU, are a measure of how concentrated a dirty solution is based upon how much that solution scatters light. In the lab we aim to produce clean water with less than 1 NTU turbidity---and though this surpasses the WHO's 5 NTU demarcation, it does not meet the EPA's 0.2-0.3 NTU requirement, which is difficult to achieve without a filtration step.
We strive to optimize the lamella design in order to achieve effluent water with 1 NTU or less turbidity, even under water chemistry fluctuations and variations in alum dosage. 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.

Current Team Research Focus: Velocity Gradients

Past research has illuminated the importance of flow regime characteristics on the performance of tube settlers. Specifically, when velocity gradients established in the tube become too large, flocs at the bottom wall of the tube experience an upward force greater than the gravity pulling them down and out of the effluent stream. This causes flocs to roll up the side of the wall and exit with the clean water, dramatically affecting the final turbidity. Since flocs are fractal particles whose effective diameters are sensitive to shear stress, natural organic material in the water, and influent turbidity, determining their response to the velocity gradient is a complex problem that goes beyond a force balance.

Plate Settler Spacing Goals