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Learn what the silt density index measures, why it matters for reverse osmosis membranes, and how pretreatment helps keep it under control.

What Is the Silt Density Index (SDI)?

Reverse osmosis membranes are sensitive to fine particles in the feed water. The silt density index, or SDI, is one of the most important measurements for understanding that risk.

What SDI Measures

SDI indicates how quickly suspended particles and colloids plug a fine 0.45-micron filter under controlled conditions. The test follows the ASTM D4189 standard method and gives a number that reflects the water's fouling potential.

Why It Matters for Reverse Osmosis

Water with a high SDI can deposit particles on the membrane surface. Over time, this reduces water production, increases pressure, and leads to more frequent cleanings. Membrane manufacturers generally recommend an SDI below 3, and most require it to stay below 5.

SDI vs. Turbidity

Turbidity measures how cloudy water looks. SDI measures how the water actually behaves against a filter surface, which makes it a better indicator of membrane fouling.

How to Keep SDI Under Control

When SDI is high, the solution is usually improving pretreatment. Common options include:

• Multimedia filtration

• Coagulation for fine particles

• Cartridge filtration

• Ultrafiltration

Regular SDI testing helps confirm that pretreatment is working, especially when the water source changes.