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Original Press Release

What is SDI?

Press release date: May 26, 2017

What is silt density index (SDI)?

Silt density index (SDI) estimates the quantity of suspended solids and colloids inside of water. SDI is measured following the ASTM D19.08 Standard Test Method for Silt Density Index (SDI) of Water, using a 0.45 micron membrane. SDI provides information about the fouling potential of water treatment equipment, including membrane filtration systems, and therefore is commonly used in their design and choice.

Is SDI a reliable fouling propensity parameter? How is it used?

SDI is a simple and helpful tool extensively used in pilot or large-scale treatment plants as a standard test to verify the fouling potential of RO and NF membranes. However, using SDI to estimate the fouling intensity of water treatment equipment has some limitations. Even though SDI is measured using a dead-end filtration unit, hydrodynamic conditions in dead-end filtration units are not representative of the hydrodynamic conditions of NF/RO membranes. Hydrodynamic conditions in membranes filtration systems directly affect the fouling rate. SDI does not provide information about natural organic matter fouling, which can play a significant role in membrane fouling. It is also important to run SDI testing at representative temperatures of the ones present in the treatment systems because the SDI may vary as a function of water temperature.

Are there any SDI test kits available?

There are a number of commercially available SDI test kits. At the moment, Sterlitech offers two kinds:

GE Auto SDI kits

Simple SDI kits

These kits are available in 115 V, and 220 V versions. For more information, please visit or contact us at

Sterlitech Corp. 22027 70th Ave. S.

Kent, WA 98032-1911

Phone: 253-437-0844


Sterlitech Corporation

Sales Department