Slit Density Index

Measurements of Silt Density Index (SDI) are carried out for relatively clean water supplies (water from waterworks) in order to determine the contents of colloids, thereby indicating how fast an NF or RO membrane will be clogged (fouling).

As colloidal materials vary considerably in form and character the measurements are only normative. The method is, however, considered the most applicable for predictions of the risks of fouling RO and NF membranes.

SDI MEASUREMENT:

Step 1: Measure the time required to filter a fixed volume of water through a standard 0.45μm pore size microfiltration membrane at a constant pressure of 30 psi (2.07 bar). Record this as Ti, or T initial.

Step 2: Take additional time measurements, normally after 5, 10 and 15 minutes (after silt build up)

Step 3: Calculate the Plugging Factor after 5, 10 and 15 minutes as follows:

PF5 = (1-Ti /T5)*100
PF10 = (1-Ti /T10)*100
PF15 = (1-Ti /T15)*100

Step 4: The SDI value is then determined at each interval as SDI = PF/T.

The lowest SDI value, which is most often at 15 minutes, is the value reported. Example: Your time measurements indicate a Ti of 1 minute and T15 of 4 minutes. Plugging factor is then calculated as PF= 1-(1/4)*100 = 75 The SDI is then calculated as SDI = 75/15 = 5 This means there has been a 4X decay of flow which equates to about 75 % plugged on the 0.45 micron disk. SDI above 5 means the plugging percentage is even higher. On waters with high SDI, it’s often useful to measure the SDI at 5 and 10 minute intervals. The resulting values, SDI-5 and SDI-10 can provide a better indication of the rate at which the membrane is plugging.

A high SDI can be caused by a fouling membrane. You can reduce the SDI by injecting a coagulant that is compatible with the membranes, before the media filter. You may also inject a dispersant to keep particles from fouling the membranes.