What is Silt Density Index SDI Measurement
Silt Density Index SDI roughly measures the fouling potential of a Reverse Osmosis System feed water. In other words, it is supposed to tell us if a feed water is likely to foul a Reverse Osmosis System or not. For the SDI measurement, the filter used is a cellulose acetate membrane filter with pores of around 0.45 micron. The filters are generally 47mm in diameter.
Silt Density Index SDI Clean Dirty/Plugged Filter Example
These SDI membrane filters will plug with particulate matter when a sample is filtered through them. If the pressure is kept constant, the amount of flow through a filter will decrease as the pores become plugged.
Silt Density Index SDI Filter Measurement
SDI measurements may be taken on any stream. The one that the membrane manufacturers are concerned about, however, is the SDI of the feed water as it enters the elements. This is generally taken at the suction of the high pressure pump, after all of the feed water pretreatment has been accomplished.
Silt Density Index SDI Measurement Method in Reverse Osmosis
Generally, three-eighth inch (9.5 mm) tubing is used to pipe the feed water stream to the filter housing. A shutoff valve is required to isolate the filter when not in use. A pressure regulator is required to maintain the pressure at 30 (2.1) psi(bar) throughout the test. A pressure gauge is required to monitor the pressure. A filter housing is required to hold the 0.45 micron filter.
Silt Density Index SDI Measurement Method Reverse Osmosis System
A 500 ml graduated cylinder is placed under the filter to collect the filtered water (Figure 14.12). The
time required to filter 500 ml is measured at the beginning and at the end of a 15 minute period
during which the feed water is being filtered by the 0.45 micron filter.
Silt Density Index SDI Measurement Reverse Osmosis
If there were no particles in the feed water, the filter would not plug at all, so the amount of time required to pass 500 ml through the filter should be the same at the end of 15 minutes as it was in the beginning. If particulate matter plugs the pores of the filter during the 15 minute test, however it will take a longer period of time to pass 500 ml through the filter at the end of the test than it did in the beginning.
SDI is a relative comparison of the amount of time required to pass 500 ml through a 0.45 micron filter before and after a given time period. The time period for RO membranes, required by the membrane manufacturers, is 15 minutes. The actual calculation of SDI is the following:
SDI =Â 100x(1-t1/t2) / T
Where:
t1 = Time required to pass 500 ml in the beginning, in seconds.
t2 = Time required to pass 500 ml at the end, in seconds.
T = Time between the beginning of t1 and beginning of t2, in minutes.
SDI Measurement Procedure Summary
A quick summary of the procedure is given.
STEP 1:Â Â Flush the SDI apparatus without the filter disc installed in the filter housing. This is required in order to flush out any particles which have become lodged in the lines since the last measurement.
STEP 2:Â Â Install a 0.45 micron filter in the housing. Bleed out any air then tighten the housing nuts. Place the discharge line of the filter into a 500 mL graduated cylinder.
STEP 3:Â Â Open the isolation valve and simultaneously start a stopwatch to begin timing.
STEP 4:Â Â Measure the time required (t1) to pass 500 mL. The pressure to the filter MUST remain at 30 (2.1) psi(bar) the entire time.
STEP 5:Â Â Allow feed water to continue to flow at 30 (2.1) psi(bar) of pressure through the filter for a total (from the beginning of t1) of 15 minutes.
STEP 6:Â Â Right at the end of 15 minutes, again measure the amount of time required to pass 500 ml.
STEP 7:Â Â Perform the SDI calculation.
