3. Water & Environmental

What filter paper is used to measure silt density index (SDI)?

Silt Density Index (SDI)  is a test used to determine the levels of colloidal and particulate suspended solids in the feedwater to reverse osmosis (RO) systems. This in turn will enable you to estimate the fouling potential. This measurement is of importance as it can be used to predict the lifetime of the RO membrane. This can help save time and money.

It is measured following the test standard: ASTM D4189 – 07 (2014) standard test method for Silt Density Index (SDI). Put simply, this test uses a membrane filter to measure the flow rate decline of the feed water. The more the flow declines, the higher the level of suspended solids present.

1192508: SDI Manual Test Kit – Sufficient for 100 tests

This method gives some particular specification on the type of filter to be used for this test:

  • Membrane/filter type: white hydrophilic, mixed cellulose nitrate (50–75 %) and cellulose acetate (MCE)
  • Mean Pore Size: 0.45 μm
  • Diameter: 47 mm
  • Thickness: 115–180 μm
  • Pure Water Flow Time: 25–50 seconds per 500 ml.
  • Pressure: 91.4–94.7 kPa (13.3–13.8 psi).
  • Bubble Point: 179–248 kPa (26.0–36.0 psi)

Lovibond produces a handy SDI complete kit for manual measurement which includes filter paper of the above specification. This kit includes everything you need to perform 100 tests. Each test can be performed in just 15 minutes. Replacement membranes are available here.


If you have any questions about measuring SDI or the type of filter paper and equipment used, please contact us using the form below. Alternatively, you can call us on 01954 233 130 and email on support@camlab.co.uk.

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