What is an SDI Test and How to Understand it?
The Silt Density Index (SDI) test is an indicator for estimating membrane maintenance intervals for reverse osmosis systems.

In general, Silt Density Index (SDI) is a measure used to estimate the potential for membrane fouling due to high silt content. It is important to note that SDI tests do not measure the amount of dissolved solids (TDS) in the water; TDS is typically measured in milligrams per litre.

On the other hand, the SDI test is a widely accepted method for estimating the rate of colloids and particulate fouling in water purification systems, especially in applications using reverse osmosis membrane elements. In other words, the SDI test is commonly used to predict the impact of suspended solids in the feed water on RO membrane performance.

Note that sources of silt can be organic colloids, iron corrosion, algae, precipitated iron hydroxyl or other suspended particles.

How to Perform an SDI Test

Although the SDI test can be performed manually using mathematical calculations, the ideal way to ensure an efficient and fast test is to use the AutoSDI tool. To perform the test, the AutoSDI package uses a 0.45 micron membrane filter. This filter is used because it is similar to the behaviour of reverse osmosis membranes in its tolerance to clogging by colloidal material. In this way, the presence of these contaminants in the water quickly causes a decrease in flow through the filter and allows the AutoSDI to estimate the membrane fouling potential.

Regardless if you have an AutoSDI tool or not, the procedure is similar, except that with the tool, everything is automated and the calculations are done by the device.

If you do not have the AutoSDI tool, make sure you have a filter disc with 0.45 micron pores and a 47 mm diameter.

Once the filter is installed, apply a constant pressure of 30 psi (2.07 bars). Then, measure the time it takes for 500 ml of water to pass through the membrane (T0). Repeat this test every 5 minutes  for 15 minutes (T5, T10, T15).

Then, calculate the plugging factors PF5, PF10 and PF15 with the formula PFn = (1-Ti/Tn)*100.

Finally, use the lowest PF values to calculate the SDI at each interval with the formula SDI = PFn / Tn. The T15 result is usually be the lowest and should be used to define membrane cleaning intervals or for filter selection.

What is a "Good" SDI Test Result?

Generally, the higher the SDI test result, the higher the expected frequency of cleaning the membranes. Typically, membrane manufacturers recommend an SDI of less than 5. A result below 3 is generally an indicator of water that is not very problematic for the membranes. However, this result does not mean that there is no potential for clogging.

SDI < 1

Cleaning maximum once a year

SDI < 3

Cleaning +/- 3 times per year

SDI [3, 5]

Frequent cleaning: +/- once a month

SDI > 5

Requires additional pretreatment

How to reduce the amount of silt in water?

There are several methods to reduce the SDI of the feed water to a reverse osmosis system. One of them is the addition of a cartridge filter that captures contaminants of a variable size: generally, 1 to 5 microns. If the water used is not too problematic, this simple addition may be sufficient. If not, a combination of technologies such as ultrafiltration, carbon filters or even other pre-treatment technologies may be necessary to reduce the SDI to an acceptable level.

Please note that sometimes an acid feed or cationic deionization may be used to adjust the pH in a water treatment system. If this is the case, it is important to perform the SDI tests both during and without the pH adjustment to compare the impact of the pH adjustment on the particulate and their ability to clog the filter.


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What is Preventive Maintenance?
Generally speaking, preventive maintenance is a practice that aims to ensure the proper functioning of your system while minimizing operational costs.