Enter the initial time required to collect a 500 ml sample and the final time required to collect a 500ml sample to determine the SDI.

SDI Formula

The following formula is used to calculate a silt density index. 

SDI = [(1- (ti/tf) ) * 100] / T
  • Where SDI is the silt density index (%/min)
  • ti is the initial time it takes to collect a 500 ml sample
  • tf is the final time it takes to collect a 500 ml sample after time T has passed
  • T is the total time between tests. This is typically 15 minutes.

To calculate the silt density index, subtract the value of the initial time over the final time from 1, multiply the result by 100, then finally divide by the time between tests.

SDI Definition

What is SDI?

SDI, short for silt density index, is a measure of the fouling capacity of water in reverse osmosis systems.

The SDI measures the rate at which a filter is plugged when subjected to constant water pressure. In other words, as water flows through the filter, how fast does the filter clog with solids.

This test typically uses a .45 micrometer filter and the pressure is set at 30 psi.

Example Problem

How to calculate Silt Density Index?

First, determine the initial time it takes to fill a 500 mL sample of water as it passes through the filter. For this example, it takes 5 minutes to fill the sample.

Next, wait 15 minutes and perform the test again. For this example, the final time takes 10 minutes to fill up the 500 mL sample.

Finally, calculate the SDI using the formula above:

SDI = [(1- (ti/tf) ) * 100] / T

SDI = [(1- (5/10) ) * 100] / 15

SDI = 3.333 % / min