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Dissolved Air Flotation/Dissolved Gas Flotation (DAF/DGF) is a separation technique whereby the adherence of micro gas (air) bubbles to suspended pollutants in a carrier liquid is used to enhance separation efficiency. DAF/DGF is applied when the density of the suspended pollutants is close to that of the carrier liquid so that gravity separation becomes impractical if not impossible.

Micro bubble generation
Micro bubbles can be generated by dissolving gas (air) in pressurized water and by subsequently de-pressurizing this water. In the DAF/DGF units, designed by Pielkenrood Engineering, this process is effected in a recirculation system. Part of the treated water is branched off from the DAF/DGF unit outlet and is recycled to the feed via a multistage lateral channel centrifugal pump. Gas is  added to this recycle stream at the inlet flange of the pump. This type of pump is capable of pressurizing relatively large amounts of gas together with water, causing the added gas to dissolve. Subsequently in a vent vessel, unwanted free gas is continuously bled off. From the vent vessel two different streams of gas saturated water are introduced into the DAF/DGF unit feed line.

DAF/DGF unit recirculation system
1. Feed DAF/DGF unit
2. Discharge DAF/DGF unit
3. Air/gas supply
4. Multi stage lateral channel pump
5. Free gas vent vessel
6. Free gas bleed-off
7. Supersaturators
8. Micro bubble generators


The first stream passes through a number of pipes of a small diameter and serves to supersaturate the feed without generating micro bubbles. The second stream also passes through a number of pipes of a small diameter, each of which is fitted with a micro bubble generator. In the micro bubble generators a sharp pressure drop causes the  dissolved gas to be released in the form of micro bubbles.

Feed supersaturation
Super saturation of the feed is essential in order to maintain a ”cloud” of micro bubbles of the desired size. Micro bubbles tend to re-dissolve and to implode in non supersaturated water due to their internal pressure caused by surface tension. This internal pressure is considerably higher than the hydrostatic pressure of the surrounding liquid.The smaller the bubble, the higher its internal pressure. In a non supersaturated feed, the smallest bubbles are the first lost by dissolving/imploding. The micro bubbles generated will adhere to the suspended pollutants in the feed, causing them to rise rapidly to the water surface in the DAF/DGF unit. The success of this process actually depends on the size of the micro bubbles. The smaller the bubbles, the more efficient the process.

Pielkenrood Engineering DAF/DGF units are designed to result in efficient flotation of suspended, and even of slowly setting, pollutants. The shape of the unit together with the location of the feed and discharge connections are important factors.
The average feed density, after supersaturation and after the introduction of micro bubbles, is considerably lower than the density of the treated water leaving the unit. The low density feed entering the unit quickly spreads in a horizontal direction over the surface water layer.
Along its flow path through the unit the water is depleted of micro bubbles and so its density gradually increases. This phenomena causes a stable layering or stratification effect to occur.

A logical consequence is that the feed must be introduced into the unit at a high level, while the treated water should be discharged at a low level. In this way short circuiting and turbulence are prevented and a limited residence time is achieved. Also detachment of weakly adhering micro bubbles and pollutants is greatly reduced and no micro bubbles will penetrate the lower water levels. Therefore installation of plate packs in a properly designed DAF/DGF unit is not necessary.
Depending on the nature and quantity of the feed, Pielkenrood Engineering also uses absorption columns for the dissolving of gas or air. The efficiency of all Pielkenrood Engineering DAF/DGF systems guarantees  a minimum recirculation flow rate.

 

DAF/DGF systems are used in for example:

    oil refineries
    petro chemical plants
    food industry
    oil/gas extraction

Pielkenrood Engineering DAF/DGF systems offer the following advantages over competing designs:

    Many companies use standardised systems. Pielkenrood Engineering designs and builds unique and tailor made units for each specific situation.
    We have a pool of experienced engineers available. For each project, we form a special project team, thus combining expertise with cost efficiency and flexibility.
    Our decades of experience and extensive know how enable us to analyse each and every waste water problem and to find the best possible way of treatment.

 

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