Silt Density Index Standard Test Method
Scope
This test method covers the determination of the silt
density index (SDI) of water. This test
method can be used to indicate the quantity of particulate matter in water and
is applicable to relatively low (<1.0 FTU) turbidity waters such as well
water, filtered water, or clarified effluent samples. Since the size, shape, and nature of
particulate matter in water may vary, this test method is not an absolute
measurement of the quantity of particulate matter.
This test method is not applicable for reagent grade water
Types I, II and II of Specification D1193, or effluents from most reverse
osmosis and ultra-filtration systems.
This standard may involve hazardous materials, operations
and equipment. This standard does not
purport to address all of the safety problems associated with its use. It is the responsibility of the user of this
standard to establish appropriate safety and health practices and determine the
applicability of regulatory limitations prior to use.
Referenced Documents
ASTM Standards
D 1229 Definitions of Terms Relating to Water
S 1193 Specification for Reagent Water
D 3370 Practices for Sampling Water
E 1 Specification for ASTM Thermometers
Terminology
Definitions - For
definitions of terms used in this test method, refer to Definitions D 1129
Descriptions of Term
Specific to This Standard Silt Density Index (SDI) - An index calculated from
the rate of plugging of a 0.45 µm membrane filter.
Summary of
Test Method
Water is passed
through a 0.45 µm membrane filter at a constant applied gauge pressure of 207
kPa (30 psig), and the rate of plugging
of the filter is measured.
The SDI is calculated
from the rate of plugging.
Significance & Use
1. The
SDI can serve as a useful indication of the quantity of particulate matter in
water.
2. The
SDI can be used to determine effectiveness of various processes such as
filtration of clarification used to remove particulate matter.
3. The
SDI has been empirically correlated with the fouling tendency of some water
treatment equipment such as reverse osmosis devices.
4. The
SDI may vary as a function of water temperature and values obtained at
different temperatures may not necessarily be comparable.
5. The
SDI will vary with the membrane filter manufacturer. Thus, SDI values obtained with filters from
different membrane manufacturers, cannot be comparable.
Apparatus
1. SDI
Assembly, as schematically described in Fig. 1.
All wetted parts should be made of high quality stainless steel or
plastic to prevent contamination by corrosion produced. Do not use reactive materials such as carbon
steel or iron. Suitable filter holders,
designed to withstand an operating gauge pressure of 350 kPa (50 psig) can be
obtained from suppliers of membrane filters.
2. Membrane
filter, 47 mm in diameter, gridded, with a mean pore size in the range of 0.45
± 0.02 µm. Use only filters that are
packaged in the same orientation.
3. Graduated
cylinder, 500-ml capacity.
4. Stopwatch,
graduated in hundredths of a minute.
5. Thermometer,
liquid-in-glass, suitable for measuring the temperature of the water sample;
capable of being read within ± 1ºC and conforming to the requirements as
described in specification E1.
Fig. 1: Apparatus for
Measuring the Silt Density Index
Procedure
Assemble the apparatus as shown in Fig. 1 and set pressure
regulator at 207 kPa (30 psig).
Before installing the
membrane filter, flush the water to be tested through the apparatus to remove
entry contaminants. For sampling, follow
the procedure given in practices D 3370.
Discrete samples can be used with appropriate pressurizing apparatus
such as a booster pump to obtain a feed supply gauge pressure of > 276 kPa (>40 psig).
Measure the temperature of the water.
Open the membrane filter holder and place a 0.45 µ membrane
filter (47mm in diameter) on the holder.
Place the filter grid side up.
Handle the membrane filter only with dull tweezers to avoid
puncture. Avoid touching the membrane
filter with the fingers.
Note 1: Other membrane filter sizes, that is, 25 mm
or 90 mm diameter can be used.
Note 2: Record the manufacturer of the membrane
filter and the manufacturer's identification for the membrane filter.
Make sure the o-ring is in good condition and properly placed. Replace the top half of the filter holder and
close loosely.
Bleed out trapped air y cracking the ball valve. Close the valve and tighten the filter
holder.
Open the ball valve.
Simultaneously, using a stopwatch, begin measuring the time required for
the flow of 500 mL. Record the time (ti). Leave the valve open for continued flow.
Note 3: Time (ti)
to collect 500 mL should be within ± 10% of the time to collect 500 mL using
non-plugging reference water at the same water temperature. The non-plugging reference water can be
obtained by filtering distilled water through a 0.2 µm pore size membrane
filter.
Note 4: If it
is less than 90% of the non-plugging time, the filter may be cracked and new
filter should be used. If it is more
than 110% of the non-plugging time, then a smaller sample size, that is 250 mL
or 100 mL should be used.
Note 5: The
500 mL sample size is based on a 47 mm diameter filter. If a different filter size is used, adjust
the sample size in direct proportion to the filter area.
Measure and record the times to collect additional 500 mL
(Note 6) volumes of sample, starting the collectoin at 5, 10 and 15 min of
total elapsed flow time. Measure the
water temperature and check the pressure as each sample is collected (Notes 7
& 8).
Note 6: If the
initial size was not 500 mL, sue the same sample size as previously used.
Note 7: The pressure must remain at 207 ± 7 kPa (30
±1 psig) through the test.
Note 8: The
water temperature must remain constant (± 1ºC) throughout the test. This is necessary as flow rate changes by
about 3% per ºC.
After Completion of the test, the membrane filter may be
retained for future reference.
Calculation
Calculated the Silt
Density Index (SDIT) as follows:
SDIT = (%P30 ÷ T) =
[1-(ti÷tf)] • 100 /T
Where:
%P30= percent at 207 kPa (30 psig) feed pressure
T = total elapsed flow time, min (usually 15 min, see Note
9)
ti =
initial time required to collect 500 mL of sample (s)
tf =
time required to collect 500 mL of sample after test time T (usually 15 min,
see Note 9)
Note 9: For this test method, %P30 should not exceed
75%. If %P30 exceeds this value, use a
shorter time for T, that is, 5 or 10-min measurements. If %P30 exceeds 75% after 5 min, other test
methods should be used to analyze for particulate matter.
Report
Report the following
information:
1. The
SDI, with a subscript indicating the total elapsed flow time (T) in minutes.
2. The
water temperature before and after the test, and
3. The
manufacturer of the 0.45 ±m membrane filter used for the test, as well as the
manufacturer's identification for the membrane filter.
Precision
& Bias
Based on round-robin testing, the pooled single operator
standard deviation of this test method is 0.43 for water sources of choice with
a silt density index (SDI) of 0.4 to 15.
Nine operators from five laboratories participated using the test water
of their choice.
The bias of this test method cannot be determined because
the test method is based upon waters of choice, which may differ with each
source.
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