What is SDI?

The Silt Density Index (SDI) test is a means of quantifying the amount of particulate contamination in a water source. The test was specifically developed as means of predicting the rate of colloidal and particulate fouling of Reverse Osmosis (RO) membranes.

Description of the silt density index test

The silt density index test described in ASTM 4189-07 is performed using a .45 micron, 47mm diameter filter. The water to be tested is supplied to the filter at a constant pressure of 30 psi. The test involves measuring the time it takes to collect a 500 ml sample through the filter at the start of the test and comparing it with the time it takes to collect a 500ml sample after water has flowed through the filter (at 30psi) for 15 minutes. The sample times are applied to the formula below to obtain the SDI15 value.


The resulting value, SDI-15, indicates the plugging of the membrane in percent-per-minute. Accordingly, the maximum SDI-15 value is 6.7. (100÷15=6.7) An SDI15 value greater than 5 is generally considered too high to be meaningful (75% plugging).

On waters with high SDI, it’s often useful to measure the SDI at 5 and 10 minute intervals. The resulting values, SDI-5 and SDI-10 can provide a better indication of the rate at which the membrane is plugging. For example, if you obtain an SDI5 of 15, there’s no point in taking the time it takes for an SDI15 test since you’ve already achieved 75% plugging (15 x 5=75). In this case, the water being tested is simply too high in colloidal or particulate matter to obtain a meaningful result.

Limitations of the manual Silt Density Index test

The Silt Density Index test as defined in the ASTM standard is a valuable tool for assessing and monitoring the potential for fouling in water supplies for an RO systems. However, the test is not performed as often as it should be because the manual test is tedious and time-consuming. The value of the SDI test can also be compromised by inaccuracies due to operator error making it difficult to compare readings made at different times or by different technicians. Tips for improving the accuracy of the Silt Density Index Test can be found on our SDI FAQ page.

The simpleSDI Advantage

simpleSDI overcomes the limitations of the silt density index test and makes SDI measurement a much more valuable tool. simpleSDI helps to eliminate membrane fouling by providing accurate, timely data on the fouling potential of RO feed water.