Dissolved-Air Flotation

by Chris Paris for Reef Aquarium Information Depot

You've probably had the experience of filling a glass full of water from the tap, and finding tiny, almost microscopic bubbles in the water. Dissolved-air flotation uses the same principle in order to introduce tiny bubbles into water. These bubbles form because the water inside the pipes, which is at high pressure, had somehow dissolved enough air that the water becomes supersaturated with air when the pressure drops before the water falls into your glass. As a result, the excess air precipitates out in the form of tiny bubbles. These bubbles are much smaller than we produce by other means of dispersing air in water.

The flotation process was developed in the mining and coal processing industries as a way of separating suspended solids from a medium such as water. The flotation process has found uses in other fields, such as wastewater treatment. The process introduces fine air bubbles into the mixture, so that the air bubbles attach to the particles, and lift them to the surface. The process is similar to the foam fractionation that we use in reef aquariums.

Dissolved-air flotation uses a particular way of introducing the air bubbles into the flotation tank. A dissolved-air flotation machine dissolves air into the water to be treated by passing the water through a pressurizing pump, introducing air, and holding the air-water mixture at high pressure long enough for the water to become saturated with air at the high pressure. Typical pressures are 20-75 psig. After saturating the water with air at high pressure, the water passes through a pressure-relief nozzle, after which air precipitates as tiny bubbles.

This process for creating air bubbles has two advantages over other processes. Dissolved-air flotation typically produces bubbles in the 40-70 micron range, whereas in normal foam fractionation, a bubble of 500 microns is considered small. The smaller bubbles have much more surface area for their volume than do the larger bubbles. A particular volume of air has 10 times the surface area when distributed as 50 micron bubbles as it does when distributed as 500 micron bubbles. Looking at this fact another way, you need 10 times the air flow with 500 micron bubbles as you need with 50 micron bubbles in order to achieve the same air-water interfacial area.

The second, and probably more important, advantage of producing bubbles by precipitation is that the process provides a more positive attachment between air bubbles and the particles or globules that you want to remove. Particles and globules in the water act as nucleation sites for the precipitation process; the precipitating air seeks out these sites to begin bubble formation. This is better than relying on chance encounters between waste particles and large bubbles introduced by some other means.

I'm curious about using dissolved-air flotation in reef aquaria, as an alternative method of foam fractionation. I'm in a learning phase right now, and don't have much more to say about the topic than what you just read. However, I'm accumulating a bibliography of books and papers on the topic. You may find the bibliography interesting.

Dissolved-Air Flotation Bibliography

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Chris Paris
Last modified: Thu Nov 26 21:42:08 EST