Buyer guide · Updated 23 Oct 2026 · By the Filtsap technical team

SDI test procedure: measuring silt density index for RO feed water

Short answerSDI (silt density index) shows how quickly RO feed water will foul a membrane. Pass the water through a 47 mm, 0.45 µm membrane filter at 30 psi (2.07 bar). Time how long it takes to collect 500 mL at the start (t0) and again after 15 minutes (t15). SDI15 = (1 − t0/t15) × 100 / 15. Most RO membrane makers want SDI below 5, and below 3 for long membrane life.

Why SDI matters

Turbidity tells you how cloudy water looks. SDI tells you how fast fine colloids will plug an RO membrane. Water can be clear to the eye and still have an SDI of 6 or 7, which will foul membranes in weeks.

Equipment

SDI test kit with a 47 mm filter holder
0.45 µm membrane filter discs
Pressure regulator and pressure gauge set to 30 psi (2.07 bar)
500 mL graduated cylinder and a stopwatch

Procedure

Connect the kit to the RO feed line, after the cartridge filter and before the high-pressure pump.
Flush the line for a few minutes. Fit a new 0.45 µm disc, wetted, without trapping air.
Set the pressure to 30 psi and keep it constant throughout.
Open the valve and start the stopwatch. Record the time to collect 500 mL: this is t0.
Leave the water flowing through the filter for 15 minutes.
Collect another 500 mL and record the time: this is t15.
Calculate: SDI15 = (1 − t0 ÷ t15) × 100 ÷ 15.

Example: t0 = 30 seconds, t15 = 50 seconds. SDI15 = (1 − 30/50) × 100 / 15 = 2.7.

Enter your readings in the SDI calculator to get the result instantly.

What the result means

SDI15	Meaning
Below 3	Good RO feed. Normal membrane cleaning interval.
3 to 5	Acceptable for most membranes, but expect more frequent cleaning.
Above 5	Improve pre-treatment before the RO.

If more than about 75 percent of the filter plugs within 15 minutes, the test is invalid; repeat it with a 5 or 10 minute interval and use the matching formula.

How to bring SDI down

Coagulant dosing ahead of a multimedia filter
Ultrafiltration ahead of the RO, which usually gives SDI below 3
Correct micron and regular change of the RO cartridge filter

FAQs

What is a good SDI value for RO?

Below 3 is good. Most membrane makers allow up to 5, but fouling and cleaning frequency rise as SDI goes up.

What is the SDI formula?

SDI15 = (1 − t0/t15) × 100 / 15, where t0 and t15 are the times to collect 500 mL at the start and after 15 minutes at 30 psi.

Where should I take an SDI sample?

On the RO feed after the cartridge filter and before the high-pressure pump.