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Yiwen Ng's Individual Contribution Page

Spring 2012 Mid-Semester Contributions

This semester, I am continuing research as part of the Sedimentation Tank Hydraulics Team. I will be conducting experiments and collecting quantitative data to evaluate the design requirements for floc blanket formation and maintenance. The goals for the semester are:

  • Test a hypothesis that describes the conditions required for floc blanket formation. The hypothesis is that the hydrodynamic pressure of the jet must exceed the hydrostatic pressure of the debris flow in order for flocs to be resuspended.
  • Determine the maximum upflow velocity in the sedimentation tank that will reliably give a performance of 2-3 NTU.
  • Evaluate the effect of high jet velocities on floc breakup and effluent performance.
  • Develop an understanding of how floc consolidation works, and determine design guidelines for the floc hopper.

Summer 2011 Contributions

As part of the Sedimentation Tank Hydraulics Team, I improved upon the existing geometry of the sedimentation tank bottom to promote floc blanket formation in the tank. Experiments with different bottom geometries revealed that the existing flat bottom geometry did not allow for adequate resuspension of settling flocs by the incoming jet. Based on visual observations, the best bottom geometry was determined to be a semicircular trench with a 10 cm diameter. This new geometry contributed to the design of modified diffusers and a jet reverser, which were retrofitted in one of the AguaClara plants in Honduras in January 2012.

Spring 2011 Contributions

As part of the Sedimentation Tank Hydraulics Team, I determined that the minimum angle of repose required for floc blanket formation to be about 25 degrees. The minimum angle of repose is the minimum angle of the sloping sides of the sedimentation tank that allow settling flocs to be returned to the jet. Angles smaller than the angle of repose resulted in sludge accumulation, while angles larger than the angle of repose would result in a taller and more expensive sedimentation tank, but also quicker floc return to the jet.

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