Silt Density Index (SDI) - Reverse Osmosis Systems Print E-mail

Silt Density Index (SDI) - The Silt Density Index or SDI is an important empirical test used to characterize the fouling potential of a reverse osmosis feedwater stream. The SDI test is based on measuring the rate of plugging a 45 micron filter using a constant 30 psig feed pressure for a specified period of time. SDI 15 refers to a silt density index test which was run for 15 minutes.


  • Spiral wound reverse osmosis systems require an SDI < 5
  • Hollow fiber reverse osmosis systems require an SDI < 3

Most deep well waters have an SDI of 3 and most surface waters have SDI’s greater than 6.

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