Understand the fouling potential of your feed water before it reaches your membranes. Silt Density Index (SDI) helps evaluate the feed water quality for you to select the right pretreatment before reverse osmosis and nanofiltration treatment.

We help you explore SDI testing, interpretation guidelines and pretreatment solutions.

How the SDI test works

The SDI test compares the time required to collect a specific water volume before and after filtration. The increase in filtration time indicates the amount of particulate and colloidal matter present in the water. The standard test duration is 15 minutes.

Calculate your SDI value

  • ti = the time it takes to tap 100 ml.
  • tf = the time it takes to tap 100 ml after having filtered for 15 minutes.
  • T = the total test time (always 15 minutes or less)

SDI measuring unit

The SDI measuring unit provides a simple and reliable way to evaluate feed water quality before RO and NF treatment. Designed with high-quality components, it helps ensure accurate and consistent SDI measurement.

Key components:

  1. Inlet valve
  2. Pressure adjustment valve
  3. Filter fixture base

 

Contact us for further information and request of data sheet for the SDI measuring unit. 

How to measure SDI with the SDI measuring unit

Learn how to carry out the SDI test and calculate the result accurately. The video walks you through the equipment, test procedure and interpretation of the measurement.

Reference sheet for calculated SDI

Use the reference chart to quickly evaluate your SDI result and gain insight into the fouling potential of your feed water.

For reliable results, at least two measurements must have similar SDI values (+/- 0,5). 

  • SDI < 3: Low fouling tendency
  • SDI 3 to 5: Moderate fouling risk. Acceptable for many applications, but careful monitoring is required.
  • SDI > 5: High fouling risk. The water requires additional pre-treatment before used as feed water for RO or NF.

If your water quality is not suitable for RO or NF treatment

Water with an SDI above 5 is generally considered unsuitable for direct RO or NF treatment due to the increased risk of membrane fouling. However, appropriate pre-treatment can often reduce fouling potential and enable reliable production of demineralised water. Explore our water treatment solutions for proper pretreatment.