Determining SDI

Determining SDI


  • Millipore® Filter Pad Holder with a 0.45 micron filter disc.
  • 500 ml. Graduated cylinder
  • Stopwatch
  • Dull Tweezers


  • Unbolt the filter holder, wet it, and place a 0.45 micron filter on the back-up plate using the dull tweezers.
  • Place the "O"-ring properly and then replace the top half of the filter holder and bolt loosely.
  • Connect apparatus to the feedwater line (minimum 30 PSIG) in a vertical, downflow position and open the needle valve a crack.
  • Loosen two adjacent filter-holder bolts, tilt the apparatus and bleed out all trapped air, then retighten bolts and adjust the pressure to 30 PSI while starting the stopwatch.
  • Immediately run the flow into the graduated cylinder and measure the time required to collect 500 ml, maintaining a pressure of 30 PSIG during the run. Mark the time (t1) and keep the watch running.
  • Repeat the previous step immediately after 5, 10, and/or 15 minutes of total elapsed time. Mark the collection times (t5, t10, tF).
  • Close the sample valve and disconnect he apparatus after the 15 minute reading. Remove the disk and place it in a plastic bag for later examination.


  • Calculate the ratios R = t1/t5 , t1/t10 and/or t1/tF.
  • Estimate % P30 at the selected time(s) from the following equation:
    % P30 = [1-R] × 100.
  • Estimate the Silt Density Index, SDI, from the following equation:
    SDI = % P30 ÷ Elapsed Time (minutes)
The 15-minute index will generally be the lowest of the three, and should be used for filter sizing purposes.