Dissolved Air Flotation for Solids? How to cope with the problem of white-water systems and pump blockage.
A true DAF generates dissolved air. The process involves dissolving atmospheric air to the greatest saturation possible under pressure and then releasing the dissolved air in the form of microscopic bubbles with the shock developed in a back-pressure valve or valves.
Modern DAFs operate the white-water system at about 95 to 120 PSI when operated in a recycle mode to achieve the greatest practical degree of air saturation. There are numerous ways to create the necessary interface between air and water to do this, but the most popular lately is to inject metered air into the inlet of a multi-stage pump. A saturation tank follows the pump. Older designs saturated the full inlet flow at about 35 PSI with a larger saturation tank and a back-pressure valve.
All “white-water” systems can be trouble due to the solids that are entrained in waste water. They can also be very expensive with tight clearances, unable to tolerate solids passing through the pump.
Dan Keys of Keystec developed a new, solids tolerant white-water DAF pump which can work with unscreened waste streams and is low cost and very easy to repair if it’s needed. As a DAF design engineer, it had long been one of Dan’s goals to solve this old problem. The development is now complete and the pump is enjoying great success in every place where it has been applied. Its ability to handle solids has made it possible to return to full flow inlet aeration and split flow inlet aeration, thus increasing the DAF throughput capacity by freeing the throughput capacity used by recycle flow.
To summarize, proper white-water is achieved by aggressive air saturation which involves maximum air to water interface under pressure and some residence time in the pressurized state to complete the saturation, followed by a high-shock back-pressure valve or valves to force the sudden release of the saturated air.
This represents a revolutionary advance in applications of Dissolved Air Flotation technology.
There are also “entrained air” systems on flotation devices. Be aware that while this approach can work many cases, the bubbles produces by spinning discs and blowing air through porous filters have different surface charges and are often too large for good flotation. The ideal environment is produced by 30 to 50 micron bubbles combined with good chemistry that attracts the bubbles to the flocs formed by the chemistry. If the bubbles are able to coalesce while entraped in the floc, the rise rate can increase and allow more aggressive separation. So, chemical choices are also key to good performance.
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