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14 2. MEMBRANE FOULANTS AND SALINE WATER PRETREATMENT
in the source water and does not quantify the content of dissolved organic, inorganic and mi- crobial foulants. The size of particles contained in the source water matters because RO-membrane feed and concentrate spacers, through which the saline source water is distrib- uted inside the membranes, are of limited width (typically 0.7e0.9 mm).
2.2.2.2 Silt Density Index (SDI)
Silt density index (SDI) is a parameter that provides an indication of the particulate fouling potential of the saline source water. If RO system is operated at a constant transmembrane pressure, particulate membrane fouling will result in decline of system productivity (mem- brane flux) over time. SDI gives an indication of the rate of flux decline through a filter of standard size and diameter operated at a constant pressure for a given period of time.
A standard SDI test procedure is described in ASTM Standard D4189-07 (AWWA, 2007) and is based on the measurement of the time in seconds it takes to collect a 500-mL sample through a paper filter of size 0.45 mm and diameter of 45 mm both at the start of the test (t0 1⁄4 0 min) and after the source water has passed through the filter under a driving filtration pressure of 2.1 bars (30 lb/in.2) for a certain period, “n” measured in minutes (standard time for measurement of filtered water SDI, tn 1⁄4 15 min). The two sample durations (t0 and tn) are applied to a formula (Eq. 2.1), and the resulting SDIn value indicates the particulate fouling potential of the tested source water:
SDIn 1⁄4 ð1  ðt0=tnÞÞ=n  100 (2.1)
where n is the total test run time (which for the standard test of filtered water is 15 min) and t0 and tn are the respective times in seconds it takes to collect 500 mL of filtered water at the beginning of the test and after running water through the filter for the duration of the selected test run time.
Fig. 2.1 shows a typical SDI measurement system installed at a desalination facility.
It should be pointed out that while the standard time between the first and second mea- surements for SDI test of filtered (pretreated) source water is 15 min, this test is typically run for 5 or 10 min on untreated source water, depending on the solids’ concentration. Based on this formula, the maximum value of SDI15 is 6.7. This condition would occur if the time to collect 500 mL after 15 min of filtration were infinite.
Typically, source water with an SDI15 lower than 4 is considered to have adequately low RO-membrane particulate fouling potential, and its use in membrane desalination is expected to result in a reasonably slow membrane flux decline over time (10%e15% RO system productivity reduction over a 3e4 month period). SDI15 levels of the RO feed water in a range of 2e4 are indicative of well-performing pretreatment system, and if the pretreatment system produces water with SDI15 less than 3 at all times, this pretreatment system would be consid- ered of excellent particulate removal efficiency.
Source water with an SDI15 lower than 2 typically has very low particulate fouling poten- tial and is not likely to require RO-membrane cleaning more frequently than once every 6 monthsdindustry standard is RO-membrane cleaning once every 3e4 months. Source wa- ter with SDI15 lower than 1 usually results in very slow increase in fouling and associated differential pressure (DP) and would yield RO-membrane cleaning intervals of 12 months or longer.