Silt is composed by suspended particulates of all types that accumulate on the membrane surface. Sources of silt are organic colloids, iron corrosion products, precipitated iron hydroxide, algae, and fine particular matter. Silt Density Index testing is a widely accepted method for estimating the rate at which colloidal and particle fouling will occur in water purification systems, especially using reverse osmosis (RO) or Nanofiltration membranes. SDI is a measurement of the fouling potential of suspended solids. It’s not measuring the quantity of particular matter, since the size, shape vary. Turbidity is a measurement of the amount of suspended solids. They are not the same and there is no direct correlation between them. In practical terms however, the membranes show very little fouling when the feed water has a turbidity of < 1 NTU. Correspondingly the membranes show very low fouling at a feed SDI of less than 5. The SDI test is used to predict and then prevent the particulate fouling on the membrane surface. Other names for it are the Kolloid-Index (KI) or the Fouling-Index (FI). The test is defined in ASTM Standard D4189, the American Standard for Testing Material. It measures the time required to filter a fixed volume of water through a standard 0.45µm pore size microfiltration membrane with a constant given pressure of 30 psi (2,07 bar). The difference between the initial time and the time of a second measurement after normally 15 minutes (after silt-built up) represents the SDI value. At All Chemical we can test Silt density Index.
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