Silt Density Index (SDI) Testing
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In the early days of using semi-permeable membranes for water treatment, there was a need to estimate the potential of the raw water to foul the membrane. Finding this was essential to identify the need for pretreatment. With some water, membranes are able to treat tens of thousands of gallons before requiring cleaning, while other water plugged the membranes after a few thousand gallons. Although attempts were made to correlate the fouling propensity of water with turbidity, those attempts were only somewhat successful. This led to the development of the Silt Density Index Test (SDI test), as it better correlates the “silt” or solids in water with membrane fouling tendencies. Often, water with very low turbidity (<0.5 Nephloturbidimetric Units or NTU) can still cause unacceptable membrane fouling. This is due to the presence of many particles with small diameters (commonly <3 microns in diameter) which don’t efficiently reflect light and show up as higher turbidity. Nonetheless, these small particles can rapidly foul spiral wound RO membranes. Sources of these particles can include: • • • • •
Iron corrosion products Organic colloids Precipitated iron hydroxide Algae Fine particulate matter
The SDI test involves forcing water through a small diameter (47 mm), 0.45-micron absolute membrane filter while maintaining a pressure of 30 psi. Depending on the time required to pass 500 mls at the end of 5, 10 and 15 minutes is taken and used to calculate the unit-less SDI value. The silt density index is calculated as follows: • An SDI test less than 3 is usually considered acceptable for membrane operation without needing pretreatment for silt removal • An SDI value of 3-5 may result in frequent cleaning (quarterly to weekly) • An SDI value greater than 5 usually requires upstream filtration capable of reducing submicron particles Silt Density Index must be measured on-site the proposed location for the RO installation. difference in SDI between a QC laboratory in installation site. This can be due to corrosion potable water distribution.
by a professional, from a sample point close to Our experience has shown a sometimes marked a large manufacturing plant and the proposed in old piping that is often used for process or
The WaterProfessionals® have both manual and semi-automated sampling systems to perform on-site silt density index testing in order to accurately specify the pre-treatment equipment necessary to assure economical RO performance.
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