Why DAF is used in Mega Seawater Desalination Pretreatment

Starting from early 21st century, DAF technology has been used in Mega Seawater Desalination Pretreatment to remove suspended solid and algae. A Harmful Algae Blooms (HABs) happened in 2008 around Middle East Area, which forced most of SWRO (Seawater
Reverse Osmosis) plants to shutdown. Since then, DAF is the standard pretreatment process in the newly developed Mega SWRO plant in this region.
Why is DAF used? The main function of DAF as a pretreatment system is the separation of algae and plankton from seawater to lower the SDI value of the water. If algae and plankton are not removed from seawater, the decayed body of them can release, which can be dissolved in the water and pass through the secondary pretreatment system. As a result, they finally adhere on the surface of the RO membrane, causing the blooms of bacteria on the RO membrane (bio-fouling) and lowering the membrane’s performance. This leads to a
dramatically shortening of the lifespan of the membrane.
In summer time, rather than the notorious HABs, there is the other natural phenomenon of coral spawning (releasing egg or sperm from coral) in coastal area, which usually occur over several days to just over a week after full moon. Whenever this occurs, the turbidity of
raw seawater increases dramatically to SDI15 values up to 6 or even not be able to be measured. This is barely removed by a dual media filter.
Algae and plankton occur in the filtered water of dual media filter tank regardless of the season. Even the filtered water was clear and clean visually in winter time, SDI15 was surged up to 6 and over, refer to the green dots on the filtration membrane of automatic SDI instrument.
In both above mentioned cases, DAF is the only efficient treatment process to remove these organics and assist maintaining the RO membrane performance and RO membrane lifespan.
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