Water tools

Silt density index from a plugging test

The silt density index is the standard fouling screen for membrane feed water: a 0.45 µm filter at 207 kPa, two timed 500 mL samples, one number.

Time to collect the first 500 mLmin

Time to collect the final 500 mLmin

Total test durationmin15 min is the common standard; use 5 or 10 min if the filter plugs before then.

Membrane

Silt density index

5.33

Above the limit: pretreatment required (media or membrane filtration, coagulation).

Plugging

80.0%

Limit for this membrane

Plugging above 75%: repeat with a shorter test duration.

How it works

From the two timed 500 mL samples and the elapsed test time in minutes:

S D I = \frac{100 (1 - \frac{t_{i}}{t_{f}})}{t}

(1)

where

$S D I$: silt density index, % per minute
$t_{i}$: time to filter the first 500 mL, min
$t_{f}$: time to filter the final 500 mL, min
$t$: elapsed test time, min

Test conditions: 0.45 µm filter, 47 mm diameter, 207 kPa constant pressure, dead-end flow. Recommended maxima: nanofiltration 0 to 3, hollow-fibre RO 0 to 2, spiral-wound RO 0 to 3; manufacturers accept 4 to 5 at most. SDI is a static two-point measure in dead-end mode and misses colloids below 0.45 µm, so it predicts cross-flow fouling only loosely.

Silt density index from a plugging test

How it works

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