Surface Tension
The affect of surface tension has the potential to seriously hinder the effective and accurate dosing of alum in AguaClara plants. Surface tension is caused by the cohesive forces of liquid molecules at the water surface. The concept of surface tension can be visualized by the figure below.
Figure 1: The affect of surface tension
As can be seen in the figure above, a certain head, h, is required to overcome surface tension and form a water droplet. The graph above indicates this relationship between the height required to form the water droplet and the diameter of an orifice subject to the water head. Any value beneath of the line indicates that there is not sufficient head to form a water droplet, and no flow will occur. If the head subject to a set orifice size is above the line, there is adequate water height to cause flow.
The relationship shown in the graph above is represented by the formula:
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$$
\Delta h = {{2\sigma } \over {\rho g
Unknown macro: {d over 2}
}} - 2{d \over 2 \over 3}
$$
where:
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$$\Delta h$$
= the height of water above the orifice
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$$\sigma $$
= Surface tension, force/length
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$$\rho $$
= density of alum, mass/volume
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$$d$$
= Diameter of the orifice, length
The affect of surface tension can manifest itself in errors in the calculated head values in the dosing system. A certain headloss on the lever arm corresponds to a certain flow rate of alum. If surface tension increases the necessary head required to achieve this flow rate, underdosing will result. This concept can be shown in the figure below:
Figure 2: Headloss error caused by surface tension
The error caused by this difference is especially prevalent in lower plant flow situations where the head between the orifice and water level in the constant head tank is small. Another words, at lower concentration scale positions, ex. 5 mg/l, the effects of surface tension will be especially prevalent.
The affect of surface tension has not been integrated into the generation of the dosing scales yet, but several solutions are proposed to minimize the error caused by surface tension.
Solutions
There are three proposed solutions to mitigate the affect of surface tension, they are presented below:
Option 1:
The creation s
Figure 3: Three scale
Option 2:
The addition of a submerged orifice inline between the constant head tank and the
