PSS Fall 2010 Velocity Gradients Experiment

Experimental Design

The PSS team's current experimental setup is to test the failure prediction of the velocity gradient model for various sets of tubes diameters at different up flow velocities (described in Table1).

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.

Table 1 - Values of the tube sizes for the three different upflow velocities.

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.