New Setup for Spring 2008

The tube flocculator team has made significant improvements to the experimental setup, including improving the settling column dynamics and the actual tube flocculator. Apart from redoing our experiments using new turbidimeter, we are also planning to make use of PIV to get better understanding on how flocs are forming and behaving.

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Apparatus Changes

Concerns

The Tube Floc research team has known for a while now that sedimentation was occurring within the flocculator and that such premature sedimentation was bad. The switch from a horizontally coiled flocculator to a vertically coiled one was made partly to try to overcome this issue. Throughout the Fall 2007 semester, the Lab Floc team performed numerous experiments with the flocculator apparatus despite not having completely eliminated sedimentation. It seemed, at the time, that the sedimentation accrued per run was minimal and that it would not contribute a significant amount of error into our measurements. Further investigation by the Lab Floc team this semester, however, has given new light into the significance of this premature sedimentation as well as a new problem of fluid circulation inside the settling column immediate after the termination of flow through the apparatus. These issues are significant not only because they are unexpected and are not optimal experimental conditions, but they also put into question the validity of the data obtained last semester and the method by which we have been analyzing and describing these past results. Below are more detailed discussions of the different aspects of these problems.

Issues with premature sedimentation

We have hypothesized that because our experimental apparatus is currently limited to generating Poiseuille flows, there are no transverse velocities with respect to the streamlines of the flow strong enough to keep large flocs in suspension once they are large enough to settle out. Therefore, large flocs are settling out on the bottom walls of the tubes where the streamline velocity is at a minimum and where there are negligible velocities normal to the wall to scour the flocs back towards the center of the pipe. There are several reasons why this is unwanted:

1. If flocs are settling out, they are not making it out of the flocculator and we loose information about the performance of our flocculator. Since we are interested in how shear/strain affects floc size and the final clarity of the water, we need the flocs to stay in suspension so we can measure how well they were formed. With significant premature sedimentation, the measurements of final settling effluent turbidity clearly underestimates flocculation performance.
2. Not only does premature settling underestimate flocculation, but it also makes determining how parameters like flow rate (and other measurements of shear and strain) have affected flocculation. For instance, it is nearly impossible to determine for certain whether the steeper settling curves at higher flow rates was a result of larger flocs being formed or whether the higher flow rate just kept more flocs in suspension and thus was able to settle out faster.
3. The efficiency of the flocculator is compromised when big flocs settle out because the primary mechanism by which fine particles are removed from solution is adsorption onto large flocs. The poor final turbidity observed in our experiments may be due to too many large flocs settling out of the flow before they reach the end of the flocculator.

Options for fixing premature sedimentation

A new observation: While sedimentation may happen during the loading state, much of the settling in the flocculator is permitted to happen during the sedimentation state, when the flow is shut off to allow sedimentation in the settling column. Since there are flocs present in the flocculator during this settle state (which usually as a 10 minute duration,) those flocs will settle out into the bottom walls of the flocculator. Additionally, since the flow during the clean flocculator phase is not turbulent, the majority of settled flocs do not get resuspended into the flow and washed out of the flocculator. As these sediments continue to build up over each experimental run, it is difficult to determine if the bulk of the sedimentation occurs during loading or is simply residual flocs that have settled there during the settle state and which the flow was unable to clean out. The Lab Floc team will make a few apparatus adjustments to see if we can eliminate the amount of sediment present in the flocculator by redirecting the flow around the setting chamber during the settle state and commence the flushing-out of the flocculator at the same time as settling inside the column occurs. This may elucidate when the majority of the flocs are settling.

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