Test for determining Silt Density Index

Equipment

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

Procedure

• 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, t_F).
• 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.
  

Calculation

• Calculate the ratios R = t₁/t₅ , t₁/t₁₀ and/or t₁/t_F.
• 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.