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2.2 PARTICULATE FOULANTS 15
FIGURE 2.1 Typical SDI measurement device used in full-scale desalination plants.
In order to maintain their performance warranties, membrane manufacturers usually require that the SDI15 of the source water fed to the RO membranes to be less than 5 at all times and less than 4 at least 95% of the time. If the source water’s SDI15 is higher than 5, this typically means that this water has a very high content of particulate foulants and, therefore, it is not directly suitable for desalination, because it would cause accelerated fouling of the RO membranes. For source seawater with SDI15 value higher than 5, it is often useful to complete the SDI test at shorter (10, 5 or 2.5 min) intervals, which will usually provide more meaningful results (Mosset et al., 2008).
Color and appearance of the SDI pads is also indicative of the content of various foulants in the source water and the effectiveness of the plant pretreatment system. Fig. 2.2 shows filtration pads from SDI15 tests along with the SDI15 values of pretreated seawater. The SDI test pads referenced as “D2” are from iron-salt coagulated seawater pretreated via two-stage upflow sand media filters, while these referenced as “Zenon” are from noncoagu- lated UF filter effluent.
The top two SDI filter pads (5.3 SDI and 5.2 SDI) on Fig. 2.2 from left to right have a red discoloration caused by overdose of ferric coagulant. These SDI tests indicate that the pre- treated influent is not suitable for SWRO treatment due to high particulate content in the source seawater. The other SDI pads are indicative of pretreated filter effluent of relatively low particulate-fouling potential.
It should be pointed out that although widely used in practice today, the SDI15 test has limited ability to measure the particulate fouling potential of saline water. The test is based