How SDI calculated?

 Silt Density Index (SDI) is a measure of the level of particulate matter or suspended solids present in a water sample, such as in the feedwater of a reverse osmosis (RO) system. It is determined by filtering a water sample through a membrane with a known pore size and measuring the pressure drop across the membrane over a given time period. The higher the pressure drop, the higher the level of suspended solids in the water.

SDI is an important parameter in RO system design and operation, as high levels of suspended solids can lead to fouling and reduced membrane performance. It is also used as a measure of the effectiveness of pre-treatment processes, such as sedimentation, filtration, and coagulation, in removing suspended solids from the feedwater.

SDI is typically expressed as a unitless number, with values ranging from 0 (no suspended solids) to 5 (very high levels of suspended solids). An SDI value of less than 3 is generally considered acceptable for RO system operation.


The formula for calculating Silt Density Index (SDI) is:

SDI = (Initial pressure drop - Final pressure drop) / Initial pressure drop x 100

where:

  • Initial pressure drop: the pressure drop across the membrane filter at the beginning of the test
  • Final pressure drop: the pressure drop across the membrane filter at the end of the test, after a specified time period (usually 15 minutes)

The unit of pressure used in the calculation will depend on the system of measurement being used (e.g. psi or bar). The resulting SDI value is a unitless quantity

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