Dissolved Air Floatation (DAF)

It is not uncommon to find seawater and river water polluted with oil, especially in areas frequented by oil transport ships or near jetties. Oil spills usually spread with the wind and waves, forming a thin layer on the sea surface. Depending on temperature, wind speed, tides and water turbulence, the thin layer of oil will eventually break up into small droplets. Some light and volatile oil droplets evaporate, some flattened oil droplets precipitate, and some remain in the seawater in the form of an emulsion (a mixture of two liquids with a size of 20 microns).

As we know, seawater is now widely used as a new source of clean water with SWRO technology. Then can the oil-contaminated seawater 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 must remove the oil content before the seawater enters SWRO.

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

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

Our article this time will discuss about Dissolved Air Floatation (DAF). DAF is a device in the form of an open basin to remove oil and grease content in water by injecting pressurized water mixed with microbubbles into seawater so that oil will be trapped in the bubbles and lifted to the surface, and then can be removed with a scraper. Two factors that must always be controlled as they determine the success of DAF are pressure and air saturation in the seawater. Typical efficiency of oil and grease reduction using DAF is 90%.

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