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Silt Density Index (SDI) Measurement

Purewatercloud is the source for your Silt Density Index (SDI) measurement needs. SDI measurement is one of the best methods for monitoring the performance of pretreatment equipment relative to the potential for downstream membrane fouling.  SDI testing is performed using a 0.45 micron filter, which is sensitive to the presence of biological particles, unlike turbidity monitors or most particle counters (because the well hydrated bio-particles do not refract much light).


SDI Equipment

Correlation Between SDI and RO Fouling

Normalized SDI RO Cleaning Rate*
< 2 1 or 2 times/year
2 - 3 2 to 3 times/year
3 - 4 3 to 4 times/year
4 - 5 4 to 5 times/year
> 5 More than 5 times/year

*Estimated...RO units should be cleaned prior to a normalized permeate flow rate decline or a normalized pressure drop increase of 15%.

SDI Index Calculation

The SDI test is based on measuring the change in water flow rate through a 0.45 micron filter over time.  The manual test requires first measuring the time for 500 mL of water to pass through a 0.45 micron filter at a regulated pressure of 30 psi, continuing the stopwatch for an interval of 5, 10, or 15 minutes, and again measuring the time for the 500 mL after each interval.

The SDI index is calculated for each of these intervals by first finding the filter %plugging with a ratio of that measured time over the initial time.  Subtract the %plugging from one-hundred and divide by that time interval in minutes.  The most representative SDI result is the one that uses the longest time interval with a %plugging of less than 50%.

The SDI results are heavily dependent on the relative permeability of the filter and the water temperature.  Use our SDInormalizer™ tool to normalize for their effect.  Better yet, automate the entire test measurement procedure with the SDIcube™.  You will quickly see how easy SDI testing can be while also obtaining far more repeatable results.

SDIcube™ Accessory to a Manual SDI Device

Automates the SDI measurement for significant time savings

Normalizes the SDI results for temperature and filter permeability

Mobile - battery included capable of operating the (optional) SDIpump™ for boosting pressure, or for batch SDI or suspended iron measurement

Click to download the SDIcube™ specs/options/instructions

Millipore SDI Replacement Filters

The original Silt Density Index test procedure was developed using a Millipore™ 0.45 micron, 47 mm nitrocellulose filter pad. SDI results can vary dramatically based on the particular filter.  The Millipore 0.45 micron filter is the standard for SDI measurement.


SDI Equipment Options and Replacement Filters

SDI Equipment
Manual Silt Density Index (SDI) Kit with filters and carrying case

SDIcube Automator and Normalizer
SDIcube Automation/Normalization Unit

SDIcube Kit with Filters and SDItripod
SDIcube Kit, with test device, filters, carrying case, & SDItripod (SDIpump not included)

SDIpump with inlet screen
SDIpump & strainer (powered by SDIcube)

Millipore SDI 0.45 micron filters
SDI (Millipore) 0.45 micron filters, 100 each