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SDI – Silt Density Index, is a measure of presence of suspended solids (SS) in feed water. Silt Density Index (SDI) testing quantifies the amount of particulate contamination in a water source. SDI is widely accepted for estimating the rate at which colloidal and particulate fouling will occur in water purification systems – especially in applications using reverse osmosis (RO) membranes. Water sources often change their water quality and this test often needs to be done weekly or monthly. The SILTON SDI Analyser automatically calculates a relative value for the amount of suspended matter in feedwater streams. Measured values reflect the rate at which a 0.45-micron membrane filter will plug with particulate material when feedwater is flowing through it. The ASTM chose the 0.45 micron filter because it is more likely to clog from colloidal matter than from hard particles such as sand or scale. SDI testing is commonly used as an “early alert” to ensure that particulates in feedwater do not plug the micropores in RO membranes. Measurement and control of SDI is important in membrane based water treatment systems, since excess SS could clog the membrane surface. SDI values range from 0 to 6.6, with 6.6 as maximum. All RO/ Nano membrane manufacturers specify a limit of SDI < 5 for smooth operation of their membranes. Our Auto Online SDI Analyser is used to measure the SDI value of any given water stream inside a pipe with the least possible human effort.

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SDI is calculated based upon the decay in flow rate during a 15 minute period across a new 0.45 micron filter installed in the built-in holding fixture. Flow rate measurement accuracy is ensured by automatically maintaining a constant 30 psi (2.07 bar) in the feed stream to the filter.Flow rates are measured during the collection of 500 mL samples at the start of the test and after 5, 10, and 15 minutes. SDI units (percent decay per minute) represent the degree of plugging that may occur due to the feedwater stream particulate and colloidal levels. The 15 minute SDI (SDI 15) is defined by ASTM D 4189-95 as the interval required for accurate and standardized testing. The 5 and 10 minute SDI values are only estimates of the 15 minute value.In addition to calculating SDI 15, the measured decay in flow rate is also converted from an SDI value to plugging factor (PF) value, a number between 1 and 100%. 100 percent means the filter is completely plugged. This provides an understandable point of reference for the potential of your RO membranes to plug — the higher the percentage, the faster your membranes may become fouled with particulates.