Silt Density Index (SDI)
Computes the silt density index of ASTM D4189, the test that measures the fouling potential of the feed water of a reverse osmosis membrane: the sample is filtered through a 0.45 µm membrane at 207 kPa, the time to collect 500 mL is measured at the start and again at the end of the test, and the index is the percentage of flow loss divided by the duration. The expression is SDI = (1 − initial time ÷ final time) × 100 ÷ duration, and the number tells how much the filter plugged per minute of test: membrane makers usually require under 5 for a spiral wound element and under 3 for extended warranty, and water above that forces stronger coagulation or filtration upstream. The test is only valid if the flow loss stays below 75%; if the filter plugs before that, run it again with a shorter duration. The duration was adopted as an input instead of fixing 15 minutes, the standard value, precisely because poor water demands repeating at 10 or 5 minutes — results from different durations are not comparable, which is why the value is recorded as SDI₁₅, SDI₁₀ or SDI₅. Enter the initial time, the final time and the test duration.
Result
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SDI test: will this feed water foul the membrane?
Membrane warranties normally demand an SDI below 5, and whoever runs the reverse osmosis unit performs the test at the inlet with a filter holder, a stopwatch and a graduated cylinder. The tedious part lies not in the filtering but in turning two recorded times into a comparable number — comparable with what, given that the run may have lasted 15, 10 or 5 minutes. Low turbidity settles nothing either: silica and iron colloids slip past a turbidimeter unseen and plug the membrane all the same.
The ASTM D4189 arithmetic is SDI = (1 − t₁/t₂) × 100 ÷ T. Here t₁ marks the time in seconds for the first 500 mL to pass a 0.45 µm membrane under 207 kPa, t₂ marks the time for another 500 mL at the end of the run, and T is the elapsed duration in minutes between the two collections. The fraction captures how much flow the filter lost, and dividing by the duration turns that into plugging per minute. Makers of spiral wound elements normally ask for SDI₁₅ under 5, and under 3 for extended warranty; anything above 5 calls for stronger coagulation, filtration or ultrafiltration upstream. With 20 s, 32 s and 15 min the index reads 2.50 %/min at 37.5% flow loss.
The test holds only while flow loss stays under 75%, and that check falls on you, never on the calculator. With t₁ at 20 s, any t₂ above 80 s voids the measurement, and the fix is a shorter run. One consequence trips people up: a valid 15-minute SDI can never exceed 5.0, because 75 divided by 15 equals 5; at 10 minutes the ceiling becomes 7.5, at 5 minutes it becomes 15. That is why results from different durations never compare, and why the value gets recorded as SDI₁₅, SDI₁₀ or SDI₅. The usual unit slip is the duration in seconds: 900 in place of 15 gets rejected, since the field accepts at most 60 minutes. Temperature and pressure have to hold steady, or viscosity shifts and t₂ lies.
Frequently asked questions
How does 2.50 %/min come out of the values on screen?
My filter plugged before the 15 minutes were up. Is the test lost?
Does a low SDI guarantee the membrane will stay clean?
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.