The Importance of Silt Density Index in Seawater RO Applications

Understand the meaning behind your high levels of SDI

What is the silt density index ?​

The SDI (Silt Density Index) is a well-known parameter used in seawater reverse osmosis (SWRO) applications to objectively measure the compatibility of the incoming water with the reverse osmosis membranes. It is a general measure of particulate levels present in the water, but even more importantly, it is an excellent measure of the levels of particles which are most responsible for membrane fouling.

The test method (ASTM D4189 – 07) includes passing water through a 0.45-µm membrane filter at a constant applied gauge pressure of 207 kPa (30 psi). The plugging rate of the filter is then measured, and the SDI is calculated.


The Silt Density Index is Useful in Determining… ​

The SDI can serve as an indication of the quantity of particulate matter in your feed water. The SDI is useful in determining the effectiveness of various processes, such as filtration or clarification, used to remove particulate matter. The SDI may vary as a function of water temperature; as a result, values obtained at different temperatures may not necessarily be comparable. The required level of SDI will vary with the membrane filter manufacturer. Thus, it is important to note that SDI values obtained with filters from different membrane manufacturers are not comparable.

Recent developments have also demonstrated that the equipment of measure, such as the filtration support, can also contribute to some difference in the final SDI value. Nevertheless, the measure of SDI is also useful from an operating point of view. Water experts utilize measures of SDI on a day-to-day basis to ensure the pretreatment filtration solution is delivering high quality, and compatible, water to the reverse osmosis system. Learn about our pretreatment solutions that have been validated to reduce levels of SDI with a variable quality of seawater intake.

Plugging Time Comparison - MMF vs CMF Technology ​

The graph below highlights the results of a recent seawater reverse osmosis (SWRO) pretreatment whitepaper study, measuring the levels of SDI at a SWRO plant in Egypt. Various plugging observations were made from two different types of pretreatment systems (traditional multimedia and cross-flow microsand filtration systems) to better understand the relationship between high levels of SDI and plugging time.


Silt Density Index Objective for Reverse Osmosis Membranes

​The SDI objective of most RO plants is to have an inlet water SDI lower than three. Traditional granular filtration and multimedia filters do not always meet this objective. Often, the RO plants have poor water quality coming from their pretreatment system, yielding an SDI below five. Unfortunately, such high levels of SDI can increase the potential for membrane fouling. Therefore, poor pretreatment in SWRO applications can result in routine cleanings of the RO membranes, eventually leading to early membrane replacement.