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Silt Density Index (SDI) Measurement & Testing

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What SDI Values Require Prefiltration?

Is SDI the same as Turbidity?

Is there any direct relation between SDI and Turbidity?

What quantity test water is required  to perform an SDI Test?



What is SDI (Silt Density Index?)

Suspended solids and colloidal materials in feed water are one of the biggest problems in reverse osmosis systems. Even though most systems have some pretreatment including 5 micron prefilters, these fine particles are responsible for fouling of reverse osmosis membranes.

In order to have some measure of the degree of this fouling problem, a concept called Silt Density Index is used. Here a 0.45 micron filter is exposed to the feed water under pressure and filtration rates are calculated. A detailed description of the test is available here: Test for Determining Silt Density Index

An SDI of less than 5 is considered acceptable for the reverse osmosis systems. This means that at values of SDI of less than 5, the membranes should foul at a very low rate. Even though the concept works most of the time, there are exceptions when a lower SDI (less than 3) is desirable due to the nature of the suspended solids in that feed water.


Once a SDI is calculated, how is that value used to determine RO prefiltration?

  • SDI  of <5:  No prefiltration is necessary.
  • SDI of 5-10:  A media (sand-type) filter is required.
  • SDI of >10: A 2-stage media filtration is necessary - possibly with the aid of coagulants or settling tanks.

Is SDI the same as Turbidity?

SDI is a measurement of the fouling potential of suspended solids. Turbidity is a  measurement of the amount of suspended solids. They are not the same and there is no direct correlation between the two. In practical terms, however, the membranes show very little fouling when the feed water has a turbidity of less than 1 NTU.  Correspondingly the membranes show very low fouling at a feed SDI of less than 5.

Is there any direct relation between SDI and turbidity? will proper flocculation bring down SDI to desired levels? which method you would recommend to bring down SDI?


No. However, from membrane fouling standpoint, an SDI of less than 5, is equivalent to a Turbidity of less than 1 NTU.   

In the SDI test, are the t2, t3 (etc) tests made by passing water through the membrane for 5, 10 and 15 minutes each -- in other words, do you keep the water running through the whole test. This implies that we need large quantities of the test water. True?

True. You need fairly large quantity of water. That is why the SDI test is generally done at the source of water.

What causes an SDI value to be slightly higher than 5 following the pretreatment system? What can be done to lower the SDI and be able to maintain it at the lower rate?

A high SDI can be caused by a fouling membrane. You can reduce the SDI by injecting a coagulant that is compatible with the membranes, before the media filter. You may also inject a dispersant to keep particles from fouling the membranes.

Is it possible to obtain a SDI value if the pressure is only 4.5 psi? And are there any calculations for extrapolation of the results?

No. You will need a pump to pressurize this water to the test conditions.

Does the function of water temperature affect your SDI readings? Is there a specific correlation?

As long as the temperature remains constant during the test [+/-1 deg C] there is no effect. Only if the temperature varies during the test will the data be distorted.

Since the SDI procedure requires a constant 30 psi pressure, is there a correction that can be done to the result if the pressure is not 30 psi (id 20 psi)? What does %P30 stand for?

There is no correction factor or correlation for running the SDI test at pressures other than 30 psi. However, the SDI value at 20 psi, should still be useful. %P30 is a term used for plugging factor at 30 psi. It equals SDI multiplied by the duration of the test [standard 15 minutes].


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Applied Membranes, Inc. 2007