Dissolved Air Floatation (DAF)

We often find sea water and river water polluted with oil, especially in areas frequently passed by oil carriers or near jetties. Oil spills will usually spread following the direction of the wind and ocean waves, forming a thin layer on the sea surface. Depending on the temperature, wind speed, tides and water turbulence, the thin layer of oil will gradually break up into small droplets. Some of the light and volatile oil droplets will evaporate, the heavy oil droplets will settle, and some remain in the seawater in the form of an emulsion (a mixture of two liquids measuring <20 microns).

As we all know, seawater has been widely used as a new source of clean water with SWRO technology. Then can the sea water contaminated with oil still be used for SWRO feed water? Of course, it cannot be used directly, because the SWRO membrane requires that the feed water does not contain oil and grease. So we have to remove the oil content before the seawater enters SWRO.

Then how can we remove the oil content from seawater? There are several methods of removing oil content in seawater, depending on the size of the oil droplet as follows:

• Emulsified oil (Oil droplet 0-20 micron)          : Dissolved air floatation (DAF)
• Dispersed oil (Oil droplet 20 – 150 micron)   : Corrugated Plate Interceptor (CPI)
• Free oil (Oil droplet > 150 micron)                  : API Oil water Separator

In this article we will discuss about Dissolved Air Floatation (DAF). DAF is an equipment in the form of an open basin to remove oil and grease content in seawater by injecting pressurized water that has been mixed with micro air bubbles (microbubbles) so that oil will be trapped in the bubbles and lifted to the surface, and then can be set aside with scrapers. Two factors that must always be controlled because it determines the success of DAF are pressure and air saturation in seawater. Typical oil and grease reduction efficiency using DAF is 90%.

In addition to oil and grease, DAF can simultaneously reduce the TSS and BOD content in the water with an efficiency of 70-85%, especially if assisted with the right injection of chemical coagulant and flocculant. For more details about this DAF, please contact and discuss further with our engineering team. We hope you find this article useful, and see you in the next article.