Why Silt Density Index, SDI Testing Matter

Protect Your Reverse Osmosis System with Confidence

Most feed water sources contain suspended solids—ranging from bacteria and iron to clay, treatment chemicals, and other microscopic particles. These contaminants can lead to membrane fouling, causing reduced water output, higher operating pressures, and compromised filtration performance.

That’s where the Silt Density Index (SDI) comes in. This quick and reliable field test helps you assess the potential for membrane fouling and predict how often your system may need cleaning. By measuring how fast a 0.45-micron filter clogs under constant pressure, SDI gives you a clear, quantifiable insight into the particulate load in your water supply.

Stay ahead of fouling issues. Optimize your system’s efficiency. Trust SDI testing for smarter water treatment decisions.

The Silt Density index is calculated as follows:
SDIT = (1 – ti / tf) *100
                          T
T = Elapsed time (min) for the test (5, 10, 15 min)
ti = Initial time (sec) to flow 500 ml
tf = Time (sec) to fill 500 ml after T minutes

From SDI values a plugging factor (PF) can be calculated and it is expressed as a number between 1 and 100%. For example, 100% plugging factor indicates that the filter is completely plugged. This factor provides an indication of the potential for fouling in RO membranes. The higher the percentage, the faster membranes may become fouled with particulates.

Plugging factor (PF) is calculated as follows:
PF = [(tf - ti ) / tf] *100
 

While the Silt Density Index (SDI) is a trusted tool for assessing membrane fouling potential, it’s important to understand its limitations:

1. Different Flow Dynamics: SDI testing uses direct filtration, capturing particles larger than 0.45 microns. In contrast, reverse osmosis systems use cross-flow filtration, which helps sweep away many of these particles.
2. Pore Size Differences: The test filter has a pore size of 0.45 microns, whereas RO membranes are much finer—less than 0.002 microns—making them sensitive to much smaller contaminants.
3. No Concentration Factor: SDI doesn’t account for concentration effects within RO systems, where fouling often results from precipitation rather than just suspended solids.

Despite these factors, SDI remains a widely accepted industry standard. It offers a quick, practical snapshot of water quality and helps predict the likelihood of membrane fouling—making it a valuable tool for system design, operation, and maintenance.

Use SDI as your first line of defense in protecting your RO investment.

 

Customer Benefits of SDI Testing

1. Predict Membrane Fouling Risks

Gain valuable insight into the potential for reverse osmosis (RO) membrane fouling caused by colloidal and particulate matter—before it impacts system performance.

2. Optimize Pretreatment Efficiency
Evaluate the effectiveness of your pretreatment processes, such as clarifiers and multimedia filters, ensuring your RO system receives the cleanest possible feed water.

3. Identify Fouling Agents
Analyze filter disc deposits to pinpoint the specific contaminants in your feed water—helping you take targeted action to prevent future fouling.

4. Enable Proactive Maintenance

Regular SDI testing—daily or as needed—provides critical diagnostic insights that help fine-tune pretreatment processes and extend membrane lifespan.

Look for us if you need the appratus and membrane filter to do the Silt Density Index test for your RO system!