Silt Density Index - Silt Density Index, often abbreviated to SDI has become the accepted standard for assessing the suitability of membrane processes, particularly reverse osmosis (RO) systems. The SDI value is merely a measure of the decline in filtration rate of a membrane filter under standard test conditions. In effect the test is an indirect "measure" of all suspended solids, bacterial, and colloidal matter in the water to be treated through an attempt to sacrificially plug a microporous cellulose ester membrane filter.

Silt Density Index test equipment

Reverse Osmosis Chemicals International have two precision SDI test instruments that allow the accurate monitoring and analysis of Silt Density Index.

Manual SDI test kit ... manual SDI >>

Automatic SDI test kit ... auto SDI >>

The Silt Density Index (SDI) test is used to determine the fouling potential of water feeding a membrane filtration process such as a reverse osmosis system. This test is defined by its specific procedure (ASTM D-4189). The ASTM procedure should be referenced for a more detailed description of the procedure.

The nature of this test is such that it cannot be run in the laboratory. SDI tests should be run daily on the water entering the RO membranes after the cartridge filters. This frequency can be reduced to weekly once background data proves that less frequent sampling is sufficient. As such, a sample tap should be installed on the RO machine after the cartridge filters.

This test can also be run across vessels such as filters or clarifiers to see if they are doing the job expected of them. SDI tests on the raw supply water should be part of every feasibility study for an RO system and it is good to run one periodically during operation of the system to make sure changes haven't occurred.

It is recommended practice to keep a record of SDI values and filters to observe changes over time.

Technical support and advice

Reverse Osmosis Chemicals International work closely with a diverse range of global organisations, intelligently combining advanced treatment technologies with practical solutions to resolve complex issues. If you have a project you would like to discuss, or you require technical support and assistance; or if you simply have a question about our Silt Density Index analysis equipment please contact one of our specialist advisors using our Technical Support page or call us on +44 (0) 330 223 31 31.