Harn R/O Systems Blog

How to Complete a Silt Density Index (SDI) Test and Why it's Important

Posted by Julie Nemeth-Harn on Nov 16, 2020 9:45:00 AM

Silt Density Index Test Harn ROIt is important to check on the level of semi-colloidal suspended solids in the feedwater to RO systems. The level can be estimated (not measured absolutely) by the Silt Density Index or SDI test which measures the blocking rate of a 0.45 micron filter pad under controlled pressure conditions.

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Silt Density Index Test

This test was originally developed by DuPont and there are several variations using different test times of 5, 10 & 15 minutes. The method used for testing feedwater quality into an RO/NF system is always the 15 minute test – which is the most sensitive.

The SDI test gives a calculated number in the range 0 – 6, where 0 is excellent and 6 denotes very high fouling potential.

Most membrane manufacturers require the feed SDI to be below 3.0 to indicate control of colloidal and particulate fouling. SDI values above 3.0 indicate periodic cleaning will be required and values above 5.0 indicate rapid fouling and very frequent cleaning.

The water plant is equipped with a well flush valve on the raw water line upstream of where it enters the process building. The flush valve allows the raw water to be flushed to waste at start-up for an operator determined period of time in order to reduce the SDI reading to below 3.0 before the water is sent to the R/O trains. An SDI sample point was installed on the raw water line upstream of the plant inlet valve so that SDI tests can be performed during the well flush. Taking these SDI tests during the well flush will allow the operators to determine the length of time needed for the well flush and will also allow for reducing the well flush time if the SDI’s improve over time. We do not recommend reducing the well flush time to less than 30 minutes as it does take a period of time for the water to reach the R/O plant once the wells start running.

An SDI test should be performed once a day on the raw water entering the R/O plant and the results should be recorded in the operator’s daily log.

If you would like to buy an SDI kit, please don’t hesitate to contact Harn R/O Systems, Inc for more information.


Topics: water quality, ro calculations, silt density index