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.

Partners in Dissolved Air Flotation Applications

Dissolved Air Flotation – Applications and Limitations in Wastewater Treatment

Dissolved Air Flotation is widely used in Wastewater Treatment as well as in less traditional applications such as:

•   drinking water clarification and color removal
•   Algae removal
•   By-product recovery  (i.e. metals, etc.)
•   Dissimilar solids separation

Within Wastewater Treatment, DAF application can be divided into
•   Primary Treatment
•   Secondary Treatment
•   Tertiary Treatment

In all cases, DAF (dissolved air flotation) is used to separate fine solids (any constituents that are not dissolved) by flotation, assisted by the attachment of microscopic bubbles.  Often, solids that would normally settle are forced to the surface for removal through the attachment of those fine bubbles.

DAF is used in a wide range of situations where the suspended material  can be as little as 20 mg/l (as in algae or color removal) to tens of thousands of mg/l as in rendering or some petro-chemical oil/water separation situations.  Each case must be evaluated for size, flow, and chemical requirements.

To further complicate the issue, the general term, DAF, no longer covers all the approaches to separating solids with microscopic bubbles.  Other similar technologies that are frequently referred to as DAF are:

•   CAF (cavitation air flotation)
-Bubbles produced by a spinning disk without pressure
•   BAF (bubble air flotation)
-Bubbles produced by blowing air through a micro-filter into the water
•   SAF (saponification air flotation)
-Bubbles produced with the aid of a surfactant for more aggressive attachment
•   EAF (entrained air flotation)
-Bubbles produced with relatively low pressure and high shear, entrained in the water

Dissolved Air Flotation

DAF Pumps

DAF Design

Dissolved Air Flotation – DAF • Algae Removal Effective?

Dissolved Air Flotation (DAF) has proven very effective for algae removal.  (Full Algae TSS Removal article)

DAF evaluation was prompted by a state imposed limit of 45 mg/l TSS to discharge into a river. The bulk of the residual TSS in the effluent was daphnia “water fleas” that was visible to the eye.

Contact P-TEC at 800-450-0398 for more information on effective use of DAF Systems.

DAF evaluation to solve problems with existing sand filters being plugged with algae floc.