Current Team Research Focus - Spring '11
Velocity Gradients
Past research has emphasised the importance of flow regime characteristics on the performance of tube settlers. Perhaps one of the most critical characteristics of the flow inside the plate settlers are velocity gradients - the means by which the colloidal particles experience a drag force acting on the edge of the particle closest to the center of the tube. When velocity gradients become large, particles at the bottom wall of the tube experience an upward drag force that is greater than gravity force. This force dynamics creates and overturning torque and causes flocs to roll up the side of the wall. Consequently, they leave the platte settler with the effluent. 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. Current research focuses on using a bench-scale system to validate the theoretical velocity gradient model developed last spring.
A schematic of the system used for measuring the performances of different tube sizes is given in the figure on the right. 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.
