The most important and difficult part of DAF (dissolved air flotation) systems is microbubble generation. Almost all microbubble generation is done by pressure tank. However, it is very difficult to adjust/keep the optimal pressure in the pressure tank, so there are sites where microbubbles are not generated at all.
In addition, microbubbles generated by the pressure tank method inevitably use a large amount of flocculants because the size of the microbubbles is large and the number of microbubbles is small, meaning the ability to separate SS (suspended solids) from water is low. Many water treatment companies have DAF systems but most of them use crude methods. It could be said that there are many sites where the DAF system is not working sufficiently.

According to ‘Henry’s law’ the amount of air dissolved in water is in proportion to the amount of pressure, so five times the amount of air will dissolve in water when the pressure is increased five times more than the atmospheric pressure.
The pressure tank method relies on this law to generate microbubbles.
The theory is to dissolve air in water at several times the atmospheric pressure in the pressure tank, and then release it under atmospheric pressure. This way the saturation point decreases and dissolved air cannot stay in a dissolved state, becoming fine bubbles (microbubbles).

However, in reality, air does not dissolve in the pressure tank according to Henry’s law. The reason is that even when compressed air is blown into the pressurized tank, most of the air will transform into coarse bubbles in an instant. Then most of the air will leave the water quickly in the pressure tank, and large amounts of undissolved air will be generated. The undissolved air is released from the ‘air vent valve’ at the top of the pressure tank or the outlet of the pressure tank. (Please see the illustration on the right.)

The pressure tank method has two fundamental flaws. The first one is that not all of the blown air (raw material) converts to microbubbles.
A lot of the blown air is “discarded” as coarse bubbles. Even if the amount of blown air is increased, the amount of microbubbles will not increase because it is only increasing coarse bubbles. The second fundamental flaw is that it is very difficult to find the optimum operating conditions because of the difficulties of controlling the tank pressure which changes every day. When there is a change, however small, it can affect the ratio of dissolved/undissolved air. Hence microbubbles can not be generated consistently.

The advantage of OHR flotation separation is that a pressure tank and daily adjustment are completely unnecessary. It requires only the initial operation adjustment. After that it can be consistently operated over a long period. The configuration of the equipment is very simple, with one OHR MIXER and one pump, and it can generate very large amounts of microbubbled water up to 15,000L/min (900m3/h).
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Company X, the biggest water treatment engineering company in Japan, analysed and collected data from three companies; company K, company N and OHR. Microbubbled water generated by the equipment of these three companies was collected in 1L measuring cylinders respectively to observe how long it took for all microbubbles to disappear. See the results table on the right. You can see that the OHR method has a much longer ‘staying time’ than other companies’ products.
In flotation separation, the finer the microbubble the better because they are more likely to attach to the finer SS (suspended solids) and raise them to the water surface. Depending on the manufacturer/type of microbubble generator, there is a big difference in the ability to separate SS.