Plate Settler Spacing

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Overview

This research is focused on a more thorough understanding of the lamellar 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.

A schematic of the system used for measuring the performances of different tube sizes is given in Figure 1. The sequence of events for a typical experiment is as follows:

The concentrated clay (10g/L) is diluted into the turbid water source until it reaches 100 NTU.
The system switches to a floc blanket formation state, adding alum before mixing and flocculation. Prior experimental data indicated that an alum dose of 45 mg/L was optimal for 100 NTU influent.
After the floc blanket forms, the system enters a loading state where tube settler effluent is sent to a reservoir (installed to prevent settling in turbidimeters that happens at around 50 mL/min for suspended clay particles). The reservoir delivers finished water to the turbidimeters at greater than 50 mL/min during a withdrawal state. As a consequence data collection for tube settlers is cyclical. The clarified effluent zone above the floc blanket is sampled so the tube settler effluent can be assessed relative to the tube settler influent.
Figure 1 - Schematic of the experimental design for testing different tube sizes with 10m/day capture velocity.

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 the plate. This causes flocs to roll up the side of the wall and exit with the finished water. The result of this phenomenon may vary from marginal increases in effluent turbidity to dramatic failure with turbidities closer to the system influent, depending on the magnitude of the velocity gradient in the tube.

Experimental Methods & Results

Fall 2010

Clay Experiments for Assessing the Effects of Velocity Gradients on Settlers Performances at Constant Capture Velocity

This semester's research was designed the failure prediction of the velocity gradient model developed over the previous semesters for various sets of tubes diameters at upflow velocities of 1 mm/s, 2 mm/s, and 5 mm/s.

To isolate the effects of the velocity gradients from the effects of capture velocity, we chose set a constant capture velocity of 10 m/day for all tubes and tubes were designed without a constant length to diameter (L/D) ratio to meet these requirements. The L/D ratio is found by taking the length of a plate and dividing by the spacing between plates or in the case of tube settlers, the diameter. The design approach taken by the team was deemed acceptable because the research aims to propose improved plate settler design parameters (capture velocity and velocity gradient) over conventional parameters like L/D.