Silt Density Index

The silt density index is a measure for the fouling capacity of water in reverse osmosis systems.
SDI values below 5 result in low fouling rates.
The Silt Density Index (SDI) test is used to determine the fouling potential of waterfeeding a membrane filtration process such as a reverse osmosis (RO) system. This test isdefined by its specific procedure (ASTM D-4189) (Updated in July 2007). The ASTM procedure should bereferenced for a more detailed description of the procedure.

The nature of this test is such that it cannot be run in the laboratory. The SDI test shouldbe run daily on the water entering the RO membranes after the cartridge filters. Thisfrequency can be reduced to weekly once background data proves that less frequentsampling is sufficient. As such, a sample tap should be installed on the RO machine afterthe cartridge filters.

This test can also be run across vessels such as filters or clarifiers to see if they are doingthe job expected of them. SDI tests on the raw supply water should be part of everyfeasibility study for an RO system and it is good to run one periodically during operationof the system to make sure changes haven't occurred. It is recommended practice to keep a record of SDI values and filters to observe changesover time.

EQUIPMENT Required
1 Ball Valve (non-metallic preferred)
1 Pressure Regulator (must be accurate over the flow range of 100-2000 ml/min at 30 psi (2.07bar)
1 Pressure Gauge (with 30 psi as mid-range, 1% accuracy at 30psi)
1 47mm Filter Disk Holder
47mm, 0.45 micrometre mixed nitrocellulose membrane filters
1 500ml Graduated Cylinder
Fittings as needed to connect the components.Stopwatch
Thermometer
“Dull” Lab Tweezers
Note: All wetted components including sample lines should be stainless steel or ofplastic construction.

SDI TEST PROCEDURE1. Connect the test kit less filter paper for pretest flush.
2. Flush the test kit and supply line for 3 to 5 minutes to remove any possible entrainedcontaminants.
3. Measure the temperature of the water.
4. Make sure the O-ring in the membrane filter holder is in good condition and properly placed. Set the pressureregulator to 30 psig. The setscrew on the regulator should be adjusted while there is asmall flow. Supply pressure to the regulator should be > 40 psig.
5. Open the membrane filter holder and carefully place a 0.45 μ membrane filter shinyside up on the support plate of the holder. Handle the membrane filter only with dulltweezers to avoid puncturing. Avoid touching the membrane filter with thefingers.
6. Replace the top half of the filter holder and close loosely.
7. Open the feed valve slightly and adjust the filter housing to overflow, displacing anytrapped air. Tighten the filter-housing overflow. Open the feed-valve completely andmake final adjustments to the pressure regulator as required. Close the feed valve.
8. Prepare to take measurements. Open the ball valve. Simultaneously, using astopwatch, begin measuring the time required for the flow of 500 ml. Record the time(ti). Leave the valve open for continued flow.
9 Measure and record the times to collect additional 500 ml volumes of sample, startingthe collection at 5, 10, 15 minutes of total elapsed flow time. This value is recordedas (tf) with f being the time used. Measure the water temperature and check thepressure as each sample is collected. The pressure must remain constant at 30 psig(+/- 1 psig) and the temperature must remain constant 1oC.
10 After completion of the test, the membrane filter may be retained for future reference.Record the Date, Sample Location, Time, Operator, SDI Value, and Comments withthe filter pad.
CALCULATIONCalculate the silt density index (SDIT) as follows:
SDIT = [1 – (ti / tf)] * 100
T
Where:
T = total elapsed flow time, minutes (e.g., 15 minutes for an SDI15).
ti = initial time required to collect 500 ml of sample
tf = time required to collect 500 ml of sample after test time T (15 minutes for an SDI15)
Note: The expression [ 1 – ti / tf ] should not exceed 75%. If it does exceed this value,use a shorter time for T; that is 5 or 10 minute measurements. If times shorter than 15minutes are required, the SDI values are too high for membrane.
Comments on Variability
The procedure outlined must be followed exactly for the information to have meaningand be reproducible. Test variability (50-100%) has been a recognized problem with thismethod and operator training in procedural details is a critical factor in obtainingprecision and accuracy.

Ir. Al-hadidi

Reference:- The Silt Density Index (SDI)(ASTM D-4189 -7)