SDI (Silt Density Index) — Definition & Engineering Reference | ForeverPure Glossary

SDI (Silt Density Index) (SDI)

Q: What SDI is acceptable for SWRO?

Membrane manufacturers require SDI₁₅ < 5, prefer < 4, and well-designed pretreatment delivers < 3.

Q: How is SDI different from turbidity?

Turbidity measures light scattering by suspended particles in real-time; SDI is a time-based plugging test, more directly predictive of membrane fouling.

SDI is a standardized 15-minute test that measures the colloidal fouling potential of RO feed water. An SDI₁₅ <3 is required for SWRO; <5 for BWRO. SDI is the gold-standard pretreatment specification.

How It Works

A water sample is passed through a 0.45-µm filter at 30 psi. The time to filter the first 500 mL (t₁) and the time to filter 500 mL after 15 minutes of continued flow (t₁₅) yield SDI = 100 × (1 − t₁/t₁₅) ÷ 15.

Why It Matters

Feed SDI directly predicts particulate-fouling rate on the lead element. RO membrane warranties typically void above SDI₁₅ of 5.

Frequently Asked Questions

What SDI is acceptable for SWRO?

Membrane manufacturers require SDI₁₅ < 5, prefer < 4, and well-designed pretreatment delivers < 3.

How is SDI different from turbidity?

Turbidity measures light scattering by suspended particles in real-time; SDI is a time-based plugging test, more directly predictive of membrane fouling.

Can I run RO with SDI > 5?

Yes briefly, but fouling rate accelerates dramatically and membrane warranties become void.

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