Page 33 - ro membanes

16 2. MEMBRANE FOULANTS AND SALINE WATER PRETREATMENT
FIGURE 2.2 SDI pads from tests of pretreated seawater of various quality.
on filtration of saline water sample through microfiltration (MF) membrane pad with 0.45-mm pore openings. Therefore, the SDI measurement is mainly indicative of the content of particulates and colloids in the source water that can be retained by such size filters. The fouling mechanisms of permeable MF membranes and semipermeable RO membranes may differ significantly, depending on the types of foulants contained in the source water.
Fouling of MF and UF membranes usually occurs due to a combination of micropore plug- ging and cake formation on the membrane surface. In contrast, RO membranes are typically fouled by the formation of cake of deposits on the membrane surface (without pore plug- ging). Since smaller particles create significantly higher resistance in the filter cake than big particulates, their effect on RO-membrane fouling can be much more pronounced than that of the large-size particles captured by the SDI test.
In order to address the effect of smaller particles on SWRO-membrane fouling, a number of alternative filtration indices have been developed over the past 10 years. Advantages and disadvantages of these modified fouling indices are discussed in the following publications (Khirani et al., 2006; Boerlage, 2007; Mosset et al., 2008).
2.2.2.3 Total Suspended Solids
Total suspended solids (TSS) concentration is a measure of the total weight of solid resid- uals contained in the source water and is customarily presented in milligrams per liter (mg/L). TSS is measured by filtering a known volume of water (typically 1 L) through a pre- weighed glass-fiber filter, by drying the filter with the solids retained on it at 103C, and then by weighing the filter again after drying. The difference between the weight of the dried filter and of the clean filter, divided by the volume of the filtered sample, reflects the total amount of particulate (suspended) solids in the source water.
It should be pointed out that because saline water contains dissolved solids that will crys- tallize and convert into particulate solids when the sample is heated at 103C, often TSS