I've received many inquiries about bubble size and rise rates in DAF.  Here are some thoughts on the subject for your use.  Comments and further observations are always welcome.  It is my hope that this will help some of those on the forum that have questions about the subject.

BUBBLE SIZE vs RISE RATE FACTORS

The rise rate is a little complicated.  The rate is directly proportional to the size of the bubble.  The smaller the bubble, the slower the rise rate, not necessarily a good thing in a DAF.  The bubbles need to be small enough to be attracted to the surface of the floc formed by the chemicals.  Larger, faster rising bubbles will not be attracted as well.  So, we need smaller bubbles, but a faster rise rate to make the DAF work.

The trick is to get as many bubbles as possible to attach to the floc, giving a cumulative buoyancy.  Even better, if we can entrap the small bubbles inside the floc and get them to coalesce where they are trapped, the rise rate can be improved even more.

Large bubbles rise more rapidly, but won’t cling to the surface of the floc.  However, the surface tension of the larger bubbles will cause them to attract super-fine solids to their own surface, bringing those solids to the surface, a phenomenon used in the mining industry in their “frothers”.

Other factors that have an effect on rise rates of a given bubble size are things like the viscosity of the fluid, oscillation of the bubble as it rises, presence of surfactants in the water and temperature.  The rise rate of larger bubbles can be altered due to the tendency of the bubble shape to change while rising, while smaller bubbles tend to keep their spherical shape and rise a little faster relative to their size.  In other words, while the rate is directly proportional to the size of the bubble, it is not necessarily a flat curve when you plot size vs. rise rate.

BUBBLE SIZE FACTORS

In DAF, two types of bubbles are generated in virtually all designs except the old pure saturation tank methods.  They are bubbles resulting from the release of dissolved air and entrained bubbles resulting from the shearing action of the “whitewater” generator.  The quantity of bubbles generated by actually dissolving air in water and then precipitating them by shock/shear and sudden pressure reduction is governed by Henry’s law.  All bubbles generated in an amount greater than that which defined/restricted by Henry’s Law are entrained air.

The size of bubbles generated by precipitation from dissolved air are generally believed to be between 30 and 50 microns.  Entrained air can be bubbles of just about any size, depending on how much shear is provided.

Various DAF pump manufacturers claim “10% air injection by volume”.  Most pumps used in modern DAFs can operate under these conditions, but only the amount of air defined by Henry’s Law can actually be dissolved for the purpose of generating 30 to 50 micron bubbles of the correct surface tension and charge to be effective in typical DAF application.  The additional air generated by entrained air shearing may or may not be useful for attaching to solids in the DAF process.

CONCLUSION

It is always good to understand the factors involved when claims are made about rise rate, bubble size and air injection percentages.  The DAF PUMP manufacturer wants you to believe theirs can “handle” more air than anyone else’s pump and the DAF manufacturer may want to lead you to believe that their equipment has “better” air, etc.  Bottom line, the factors that matter are hydraulic design of the DAF for capacity efficiency, reliable micro-bubble generation, preferably from precipitation of dissolved air rather than entrained air and well managed chemical programs designed to achieve complete coagulation and flocculation of the proper size and charge to attract as much air as possible to their surface or internally to assist in the most rapid rise possible within the flotation area of the DAF.

Dan Keys

You are very kind.  If I don't get too old too fast, I may write a practical guide to DAF.  With the proceeds, I could make an occasional visit to the local coffee shop

One more time! I've saved this for future use.

grrun