(click to enlarge)
^Plate Settlers at Cuatro Communidades AguaClara Plant (Jan 2011).
\\\\ 
(click to enlarge)
^Plate Settlers module being inserted in a sedimentation tank after maintenance at Marcala AguaClara Plant (Jan 2011).
Plate Settler Spacing
The Plate Settler Spacing Team focuses on finding optimal spacing for the plate settlers (see pictures) in the sedimentation tank. Optimal spacing will lead to smaller sedimentation tanks and hence to minimize construction costs.
The team is currently exploring a failure mechanism termed "floc roll-up". It is hypothesized plate settler performance is decreased with high velocity gradients in the plate settlers. For a given capture velocity (the slowest settling particle a plate settler can effectively capture based on geometric parameters and floc properties) these velocity gradients overcome the settling velocity of flocs, which results in flocs rolling up into the effluent instead of settling down to the bottom of the sedimentation tank. With smaller spacing between plates, the velocity becomes greater near the bottom plate and thus increases the likelihood of floc roll-up.
The team predicts floc roll-up by using a dimensionless parameter called the "Pi-ratio". The Pi-ratio compares the terminal velocity of the slowest settling floc to the velocity experiences at the floc's outer diameter. When the Pi-ratio is less than one, floc roll-up should occur. And when this parameter is greater than one, the convention considerations of capture velocity dictate the particle size range that should be captured.
To explore these velocity gradients, the team is designing and running experiments at a constant capture velocity.
Future Work
The team will perform experiments on the velocity gradient theory on different setups to test for the consistency of the theory.