Water parameterPAR-000006

Silt density index

An index of the particulate fouling potential of RO feed water, from the rate at which a 0.45 µm filter plugs at constant pressure.

SaveStandardizedScientifically reviewedLevel 3 · Monitoring, measurement & analytics

Also known as: SDI, SDI15

Symbol: SDI15
Unit: dimensionless
Typical range: below 5, preferably below 3 (RO feed)

is measured by

Test method · MET-000002Silt density index testOpen

is monitored in

Process · PRC-000001RO pretreatmentMedia or membrane filtration, cartridge filters, antiscalant dosing and dechlorination ahead of the membranes, to protect them from particles, mineral scale, biofouling and oxidation of the polyamide layer.Open

cites

Standard · STD-000002ASTM D4189 — Silt density index (SDI) of waterOpen

Version history

Version 1 · published 3 Nov 2026