A standardized test that measures the particulate and colloidal fouling potential of feed water for reverse osmosis and nanofiltration systems, defined by ASTM D4189.
The Silt Density Index, or SDI, measures the particulate and colloidal fouling potential of feed water entering a reverse osmosis or nanofiltration system. The procedure is straightforward and defined in ASTM D4189. Water is filtered through a 0.45-micron membrane filter at a constant differential pressure of 30 psi (207 kPa). The time required to collect a fixed volume of filtrate is recorded at the start of the test and again after fifteen minutes. The result is calculated as the percentage reduction in filtration rate over that period, normalized to the test duration. A lower value means cleaner feed water.
Operational benchmarks for SDI are well established: an SDI below 3 is good for most RO membranes, and an SDI below 5 is generally accepted by membrane manufacturers as the warranty limit. Values above 5 indicate that additional pre-treatment is required before water can be passed safely to the membrane array. High-purity applications, including ultrapure water production for semiconductor and pharmaceutical manufacturing, typically require SDI values below 1.
The test has known limitations. It captures particulate and colloidal fouling only and does not measure biological, organic, or scale-related fouling potentials. For waters where colloidal fouling at low particulate loads is the principal concern, the Modified Fouling Index (MFI) offers higher resolution. SDI remains the industry standard pre-screening test for RO feed water quality because most membrane manufacturers reference it directly in their warranty conditions.
