Understanding SDI
Silt Density Index, or SDI, is a practical fouling test for feed water. It estimates how quickly suspended matter blocks a fine membrane filter. Plants use it before reverse osmosis, nanofiltration, and other sensitive membrane systems. A lower value suggests cleaner water. A higher value signals faster filter plugging and greater pretreatment risk.
Why the test matters
The test uses constant pressure, a clean membrane, and a fixed collected volume. Operators time the first collection, then wait for a chosen test period. They time the same volume again at the end. If the final time is much longer, solids, colloids, biofilm fragments, or chemical precipitates are restricting flow. This calculator converts those readings into SDI, plugging percent, flow decline, and a simple risk note.
Reading the result
SDI is usually reported with the test time, such as SDI15. The number is not a direct concentration. It is an index based on flow reduction. Many membrane projects prefer lower values, often below three, but limits can vary by membrane supplier and site design. Always compare the result with the plant specification, pressure stability, filter pore size, and sample handling practice.
Good testing practice
Use a clean 0.45 micron membrane unless your method says otherwise. Keep pressure steady during the full test. Remove air from the housing and tubing before timing. Record temperature, pressure, sample location, and any unusual conditions. Do not touch the filter surface. Replace damaged membranes. If the first collection time is too short or too long, review the setup before trusting the index.
Using the calculator well
Enter the first collection time and final collection time in the same unit. Use the actual elapsed test period in minutes. The default volume is 500 mL, because this is common in SDI work. The calculator also reports flow rates, so operators can see the physical change behind the index. Save CSV or PDF records for logs, audits, troubleshooting, and chemical optimization. Repeat tests after media changes, coagulant adjustments, cartridge replacement, or sudden turbidity events. Trends are often more useful than one isolated reading. A rising SDI warns that pretreatment is losing control before membrane pressure problems become costly. Confirm unusual results with fresh samples and calibrated pressure gauges.