Silt Density Index (SDI) and Modified Fouling Index (MFI)

The world-wide used ASTM standardized SDI (Silt Density Index) is a simple and cheap test to determine the fouling potential of water. Convergence Industry achieved to develop the first available portable and fully automatic SDI analyzer for doing both SDI as wel as Modified Fouling Index (MFI) experiments.


See an instruction movie of our SDI/MFI inspector


Dimensions: 30 x 25 x 18 cm
Weight: 9.5 KG
Data: USB 2.0
Interface: HMI color touchscreen
Pump capacity: 10 – 300 ml/min
Measuring range SDI: 0-6
Measuring range MFI: 0-12

Download datasheet

Silt Density Index (SDI) and Modified Fouling Index (MFI)

SDI: Silt Density Index

To determine the rate at which particulate and colloid matter will foul membranes, the widely accepted method Silt Density Index (SDI, ASTM D4189) is used. Many (membrane) manufacturers have adopted this parameter and use it amongst other chemical/physical parameters as basis for their guarantee conditions.

The simple test compromises a short filtration cycle on a 0.45 um microfiltration (MF) membrane resulting in a indexed number.

A drawback of the SDI test is the fact that it is difficult to obtain a reproducible and accurate SDI value. Due to this there are often discussions on obtained SDI results. The most important known sources of error are:

  • Variable microfiltration (MF) membrane properties
  • Varying testing conditions
  • Artifact causing parameters such as air bubbles

Therefore, it is recommended:

  • To use a standardized membrane filters having uniform and constant properties
  • To correct the SDI for temperature and membrane resistance
  • To use a very accurate equipment
  • To avoid the artifacts such as the effect of the bubbles, filter holder and operation errors

MFI : Modified Fouling Index

The Modified Fouling Index (MFI) is a measure for the fouling capacity of water on (mostly) membrane reactors. The MFI (0.45) has been developed to overcome the main deficiencies of SDI. The advantages of MFI are:

  • Accurate measurement to predict feed- and driving pressure
  • Temperature corrected
  • Linear correlated with colloidal/suspended matter
  • Fouling potential of particles <0.45 um is measured
  • Applicable for highly fouled feed water
  • Not dependent on test membrane permeability
  • Applicable for Ultrafiltration (UF) permeate (feed to Reverse Osmosis (RO))

The obtained MFI value is corrected for temperature and pressure and shows a linear relation with colloidal suspended matter concentration.

Convergence Inspector SDI-MFIA

The Convergence Inspector SDI Analyzer / MFI Analyzer is a test device that has been designed not only to consider the MFI (0.45) advantages, but also to minimize, if not eliminate, human errors. Due to the software installed on the system it is capable of measuring the following parameters in one test: SDI5, SDI10, SDI15, SDIv, SDI+ and MFI.

Specifications Filter Cartridge:
Mean pore size: 0.45 um
Diameter: 25 mm, flat
Thickness: 115 – 180 um

Pure water flow: 25 – 50s per 500 ml
Pressure: 91.4 – 94.7 kPa
Bubble point: 179 – 248 kPa


Download an article on SDI/MFI comparison

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