The SDI test is calculated from the rate at which a 0.45 µm filter clogs when water passes through it for 15 minutes at a constant pressure of 2.07 bar. A high SDI indicates a heavy colloidal and suspended solids load and the need for pretreatment such as a multimedia filter, UF or coagulation. Membrane manufacturers do not give a warranty for water with SDI > 5.
Technical glossary· Water quality and measurement
SDI (Silt Density Index)
Short definitionA fouling index that measures the tendency of water to clog a membrane; for RO feed, SDI₁₅ should be < 5, preferably < 3.
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