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Silt Density Index (ASTM D4189 SDI) Calculator environmental
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Silt Density Index (ASTM D4189 SDI) Calculator

Calculate ASTM D4189 Silt Density Index (SDI15, SDI10, SDI5) from filtration times, percent plugging, and reverse osmosis membrane colloidal fouling risk.

ASTM D4189 Test Timers & Volume

First 500 mL sample through 0.45µm disk
Time collected after total duration T
ASTM D4189 standard = 30.0 ± 0.5 psi

Test Water Temperature

Temperature must remain constant (± 1°C) throughout test

Silt Density Index & RO Suitability

Calculated Silt Density Index
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SDI-15 Value
Percent Plugging (%P)
-- %
Max valid: 75%
Membrane Fouling Risk
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ASTM D4189 Formula: SDI = 100 × (1 - t0/tT) / T
Time Flow Ratio (tT / t0): --
RO Warranty Compliance: --
Pretreatment Requirement: --
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Frequently Asked Questions

What is the Silt Density Index (SDI) and why is it crucial for RO systems?

Silt Density Index (ASTM D4189) is an empirical measure of the concentration of sub-micron colloidal particles, suspended silt, silica, and organics in water. Unlike turbidity (which measures light scattering), SDI measures the actual rate of physical cake formation and pore clogging across a 0.45 µm membrane disk, accurately predicting spiral-wound RO membrane fouling rates.

What are the manufacturer warranty limits for SDI in reverse osmosis?

All major RO membrane manufacturers (DuPont FilmTec, Hydranautics, Toray) specify a maximum allowable feedwater SDI15 of 5.0, with a strongly recommended design target of SDI15 < 3.0. Operating with feed SDI > 5.0 voids membrane performance warranties.

What is the 75% maximum percent plugging rule in ASTM D4189?

The ASTM test requires that the final flow time (tT) not exceed 4 times the initial flow time (t0), corresponding to a maximum plugging percentage of 75%: %P = (1 - t0/tT) * 100 <= 75%. If %P exceeds 75%, non-linear cake compaction distorts the index, requiring the test to be truncated to a shorter 10-minute (SDI10) or 5-minute (SDI5) duration.