SDI Kits
SDI Kits

SDI Kits

Sterlitech offers two brands of SDI (Silt Density Index for water and wastewater) Kits; the GE Auto SDI kits and the Simple SDI kits; which are available  in manual, 115 V, and 220 V versions. Each SDI Kit includes 1 pack of 100 filters (PN: MCE4547100).

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The MCE membrane used for either fo the SDI kits is our part number MCE4547100.

Click here to purchase additional the membrane (0.45 micron, 47mm, 100/box).

To learn more about how ASTM methods for SDI testing, please refer to ASTM Standard 4189-07. 

SKUProduct NamePrice
Simple SDI: Automated Portable Test Instrument, dual voltage182-10025Simple SDI: Automated Portable Test Instrument, dual voltage
Manual SDI Kit | Sterlitech Corporation182-00105Manual SDI, Portable Test Kit
Manual SDI Kit | Sterlitech CorporationSDIKIT4701Manual SDI Kit
Auto SDI Tester Kit - 115 VSDIKIT115AAuto SDI Tester Kit - 115 V
Auto SDI Tester Kit - 220 VSDIKIT220AAuto SDI Tester Kit - 220 V
SDI Booster Pump - 110 V182-40005SDI Booster Pump - 110 V
SDI Booster Pump - 230 V182-40025SDI Booster Pump - 230 V
Auto SDI Booster PumpSDIPUMPAuto SDI Booster Pump
Simple SDI International Power Supply/Battery Charger182-11006Simple SDI International Power Supply/Battery Charger

What is the Silt Density Index

Silt Density Index (SDI) testing quantifies the amount of particulate contamination in a water source. SDI is widely accepted for estimating the rate at which colloidal and particulate fouling will occur in water purification systems – especially in applications using reverse osmosis (RO) membranes. Water sources often change their water quality and this test often needs to be done weekly or monthly.

The Sterlitech SDI tester automatically calculates a relative value for the amount of suspended matter in feedwater streams. Measured values reflect the rate at which a 0.45-micron membrane filter will plug with particulate material when feedwater is flowing through it. The ASTM chose the 0.45 micron filter because it is more likely to clog from colloidal matter than from hard particles such as sand or scale. SDI testing is commonly used as an “early alert” to ensure that particulates in feedwater do not plug the micropores in RO membranes.

How is SDI Calculated?

SDI is calculated based upon the decay in flow rate during a 15 minute period across a new 0.45 micron filter installed in the built-in holding fixture. Flow rate measurement accuracy is ensured by automatically maintaining a constant 30 psi (2.07 bar) in the feed stream to the filter.

Flow rates are measured during the collection of 500 mL samples at the start of the test and after 5, 10, and 15 minutes. SDI units (percent decay per minute) represent the degree of plugging that may occur due to the feedwater stream particulate and colloidal levels. The 15 minute SDI (SDI 15) is defined by ASTM D 4189-95 as the interval required for accurate and standardized testing. The 5 and 10 minute SDI values are only estimates of the 15 minute value.

In addition to calculating SDI 15, the measured decay in flow rate is also converted from an SDI value to plugging factor (PF) value, a number between 1 and 100%. 100 percent means the filter is completely plugged. This provides an understandable point of reference for the potential of your RO membranes to plug — the higher the percentage, the faster your membranes may become fouled with particulates.

Information on silt density index (SDI) testing

What's new in the 2007 version of ASTM D4189?
An update, to the standard for Silt Density Index testing (ASTM D4189-07) was released in July of 2007. The update addresses a deficiency and in our opinion corrects an error that has existed since the standard was first released in 1989. 

Perhaps the most critical component of the SDI test is the membrane filter used in the test. Previous versions of the standard had defined the membrane as follows:
6.2 Membrane Filter, 47 mm in diameter, gridded, and with a mean pore size in the range 0.45 +/- 0.02 μm, inclusive. Use only filters that are packaged in the same orientation.
Note that the type of membrane, that is, the polymer, was never specified. While standard practice has been to used mixed cellulose ester membranes, the lack of a specification meant that membranes of other polymers could be used. Since membranes of different polymers have widely varying permeability characteristics this is a potentially huge source of variation in test results. 

The error in the old standard was the designation to use a "gridded" membrane. This never made sense and in conversations with water professsionals all around the world I never encountered one who was using "gridded" membranes. This is significant because the process of "gridding" the membrane affects the surface of the membrane making it more permeable. This obviously introduces an unwanted and unneeded variable. 

Both of these issues have been corrected in the new standard. Section 6.2 has been expanded to include the following definitions of the membrane filter properties:
6.2 Membrane Filter: 
6.2.1 Membrane—white hydrophillic, mixed cellulose nitrate (50–75 %) and cellulose acetate (MCE). 
6.2.2 Mean Pore Size—0.45 μm. 
6.2.3 Diameter—47 mm nominal, plain. 
6.2.4 Thickness—115–180 μm. 
6.2.5 Pure Water Flow Time—25–50 seconds/500 mL. 
6.2.6 Pressure—91.4–94.7 kPa (13.3–13.8 psi). 
6.2.7 Bubble Point—179–248 kPa (26.0–36.0 psi)
How do the membranes from different manufacturers compare?
  • For many users, the Millipore membrane has been the "standard" membrane for silt density index testing. If you Google silt density index you'll even find a reference or two that actually states that the test is to be performed with a Millipore membrane. Since the characteristics of the membrane has a profound effect on the results, we prepared a comparison of the leading membranes. 


This comparison reveals some interesting differences. Especially interesting is the relatively "loose" characteristics of the Millipore membrane which is so widely used. The "loose" designation is supported by the relatively low bubble point and a corresponding high flow rate of the Millipore membrane. For use in SDI testing, "looser" translates to less consistency and less reliable results. Given the many variables in the silt density index test, adding membranes as a variable doesn't make sense. We don't have any experience with the Whatman membranes, but based on their specs they look like a viable option. Note: Sterlitech (MCE4547100) and Advantec MFS are equivalent products. 

Which side of the membrane is the feed side?
The membranes used for silt density index measurement are asymmetrical. This asymmetry means that one side is slightly shinier than the other. The shiny side of the membrane has pores that are smaller than the "dull" side. For consistent and the most accurate results, you should always use the "shiny" side of the membrane as the feed side. 

Why does it matter which side of the membrane is used as the feed side?
Because the membranes are asymmetrical, the pores are smaller on the shiny side. Using the side with the smallest pores ensures that the particle size retained by the membrane are more consistent with the rating of the membrane. In practical terms, the difference is small, but if you want consistent results, its worth the effort.

See User Guides for specific details on power, weight, and dimensions. 

Simple Vs. Auto SDI

GE Auto SDI Kit User Guide PDF

SimpleSDI Kit User GuidePDF

ManualSDI Kit User GuidePDF