Silt density index (SDI) analyzers

Silt density index (SDI) analyzers

Many attempts have been made to identify the parameters that make it possible to effectively evaluate the compatibility of a fluid with membrane treatment, typically  in relation to reverse osmosis applied on "difficult" primary waters (e.g. pre-treated surface water) or wastewater to be re-used.

The use of classical measurements of turbidity (e.g. NTU) has proved unreliable, although sometimes useful to identify variations in the quality of input water in a very timely manner.

An experimental procedure, now ASTM (American Society for Testing and Materials) regulation D 4189-95, was thus developed for the calculation of a dimensionless parameter called the Silt Density Index (SDI), which makes it possible to gather information on the quantity of particles in water to be treated with membranes.

It consists in a filtration test on a disk with 0.45 micron porosity extended over a time interval (normally 15'), long enough to identify a "fouling potential". Most manufacturers of reverse osmosis membrane elements use the control of this parameter to maintain process guarantees; normally, the accepted limit values are in interval 3-5.

A filtering disk having a diameter of 47 mm with porosity of 0.45 micron is fed at the constant pressure of 2,07 bar. The time needed to collect 0.5 litres of filtered water at the beginning of the test (Ti) is identified; after a reference time (Ttot), normally equal to 15 ', the time needed to collect 0.5 litres of filtered water (Tf) is measured again. The Silt Density Index value is determined as follows:

SDI = (1-Ti/Tf) * 100' / Ttot

The values detected are an indicator of the possibility of feeding the water in question to a reverse osmosis system. Purely for example purposes, a compatibility scheme might be the following:

 

The test in question gives results that might be generally correlated to fouling phenomena occurring in the medium/long-term on reverse osmosis membranes. It should however be noted that there are significant limitations to the interpretation of the SDI data and that an expert on the subject should always be consulted before making decisions about the type of pre-treatment to be adopted prior to reverse osmosis membranes.

• The hydrodynamic conditions of the SDI test are very different from those prevailing in the cross-flow  filtration processes.
• The porosity of the filter used is much higher than that of the reverse-osmosis membranes.
• The test does not take into account the concentration factor, which may be of significant relevance in certain cases

Instruments for the calculation of the SILT DENSITY INDEX (SDI):

SEPRA provides automatic and semi-automatic instruments for SDI measurement that may be used by plant engineers during process design and by those who manage reverse osmosis systems for the optimization of operations (e.g. choice of the best pre-treatment options).

Automatic Instrument - QUICK SDI:

The automatic instrument is formed by:

• a feed pump (optional);
• a case containing the complete measurement kit and an LCD monitor showing results.

The user simply inserts the filter disc, connects the instrument to a sample outlet and presses the start button. The instrument automatically performs all measuring cycles and indicates the SDI value on the display at the end of the test.

Semi-Automatic instrument - EASY SDI:

The semi-automatic instrument is formed by:

• a feed pump (optional);
• a pressure reducer with pressure gauge;
• a filter housing;
• connection tubing in polypropylene;
• 0.5 liter container for the measurement of the sample.

The user simply inserts the filter disc, connects the instrument to a sample outlet and presses the start button. The user must then detect the two times (Ti and Tf) with an interval of 15' and calculate the SDI value, using the above mentioned formula. 

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