We would like to offer the following based on our research and knowledge gathered over the years in the water treatment industry. Firstly we agree with what states below – “it is the bubbles that are too small to be seen that are important.”
Our comments are as follows:
1. Given our experience over the years especially with Dissolved Air Flotation (DAF) we have always seen that the most efficient performance of a DAF occurs when the aeration system is working effectively, producing small bubbles (a cloud like an appearance) that do not disturb the surface of the water.
2. Larger bubbles rise to the surface quickly and do not efficiently assist the water treatment process in a DAF, or any other aeration process.
3. We have seen many examples of installed DAF units, where simply by the addition of a more effective air absorption system – the customer achieves better water treatment results immediately.
4. There is also a research paper that was published in 2014 – “Effect of Bubble Size on Aeration Process” – where bubble size was measured and correlated with the effect the aeration had on reducing water hardness in this case. The conclusion from that research stated – “The results show that bubbles of smaller size with maximized surface area, take more time to reach the surface, increasing the residence time of each bubble in the water, allowing a better oxygen transfer rate.”
5. It is all very well to introduce the air / oxygen into the water, but ultimately it has to be transferred to the water to have an impact on the water characteristics. The transfer of gas into a liquid is a mass transfer process; that is made more effective the greater the surface area available to allow the transfer of the oxygen from the bubble to the water around it.
6. Smaller bubbles have a greater surface area, making them more efficient at oxygen transfer. Also, the smaller bubbles are less buoyant and rise more slowly through the water (Stokes Law), increasing the contact time with the water and allowing more diffusion of oxygen into the water
7. If a single large bubble and eight smaller bubbles have the same total amount of air inside them, the surface area of the eight smaller bubbles will always be greater.
-A bubble with a 5mm diameter has a volume of 65.44 mm3 and a surface area of 78.53 mm2
-A bubble with a 10mm diameter has a volume of 523.53 mm3 and a surface area of 314.12 mm2
-The larger 10mm bubble volume 523.53 mm3 divided by 8 = 65.44 mm3 that are the volume of the smaller 5mm bubbles
-If we look at the total surface area of the eight smaller bubbles = eight multiplied by 78.53 mm2 = 628.24 mm2 – this is twice the surface area of the single larger 10mm bubble
-Doubling the surface area of the smaller bubbles doubles the surface contact that is being made with the water to allow mass transfer of the oxygen to the water and increase the DO level
8. When very small bubbles are being generated white cloudy water can normally be seen, however, these bubbles may not be seen individually or in lower volumes by the naked eye
In conclusion, we believe that the scientist guy mentioned below is incorrect in this instance. That is if the objective of the aeration is to impact the water quality positively by increasing the DO level, then bubble size is very important.
There is a challenge in using the same water over and over in the test tank when conducting repeated dissolved oxygen tests. Namely, once the water in the tank is charged with O2, you need to have this O2 removed from the water so that it does not interfere with the next aeration test. There are several chemicals used by the aeration industry which will drive the O2 out of the water. These chemicals are known as oxygen scavengers. The laboratories with which you will be working will have their preference for the type of chemical used.
In our tests, we use Sodium Sulfite Anhydrous, 98% (Na2SO4) as our O2 scavenger. The goal is to use only enough O2 scavenger to drive the O2 levels downward to just above “O” parts-per-million (ppm or mg/liter) of O2. If you use too much scavenger and drive the O2 level to “0”, the excess chemical will linger in the test tank and then begin to drive out the O2 you will be putting into the water as part of your next aeration test. This will obviously distort the reading and give you false results.
During our tests in this particular portable tank, we tried to measure the smallest size of the O2 molecules using a ruler denominated in millimeters. It was a crude measurement, but we found bubbles clinging to the side of the tank that were considerably smaller than 0.25 mm in diameter. In fact, the sides of the tank were covered with these minuscule bubbles, yet they could not be seen in the water since they were so small. We tell our clients that it is not the bubbles that can be seen breaking the water surface that is critical, but rather the bubbles that are too small to be seen that are important. However, we have had a scientist in here tell us that the bubble size is not that important so We are hoping that OxyTurbine and its other distributors can help all of us determine the truth in all of the conflicting scientific feedback we are receiving!
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