New Challenges
Air entrapment in vertical rapid mix tube below micro mix orifice
We have strong evidence that air is being trapped in the vertical section of the rapid mix tube below the micro mix orifice. The head loss in the rapid mix tube increases causing the entrance tank to overflow even though the flow rate is less than the design flow rate.
We hypothesize that the air slow accumulates below the vena contracta because the velocity in the vena contracta is too high for air bubbles to travel upward. Below the vena contracta where the velocities are lower the water velocity is too low to carry the bubbles down and into the flocculator. In addition, the pressure in the middle of the vena contracta can be reduced significantly. At Agalteca the dosing tube was hanging down into the vena contracta. The low pressure in the vena contracta may have caused air to flow through the dosing tube into the vena contracta.
To solve these problems we need to make several changes.
- The dosing tube should not be in the vena contracta. The dosing tube should be centered at the very top of the rapid mix tube.
- We need a low velocity escape route for air that is caught below the vena contracta. We are proposing that a 1/2" PVC pipe be inserted inside the rapid mix pipe against the inside wall of the pipe and down to a hole in the micro mix orifice plate. This 1/2" pipe will extend up through the maximum water surface in the entrance tank. It will provide a stagnant water path for air bubbles to travel up and out of the rapid mix pipe.
- It may be better to have the orifice plate attached to a pipe coupling rather that have the orifice cut into a pipe cap that is at the very top of the pipe going to the flocculator. The plate would be submerged further below the water surface and the dosing tube would not be so close to the orifice. That would provide some opportunity for some mixing before reaching the micromixing orifice.
Powdered Poly Aluminum Chloride 
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This topic has come up several times and could be a significant contribution. Agua Para el Pueblo has mentioned that they would like to have access to a portable water treatment plant. The challenge is to create a design using lightweight materials that can easily be transported to a location where safe water is needed.
- From CEE 4540 in 2008 we have an emergency water treatment plant proposal.
- From CEE 4540 in 2009 we have 2 proposals
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