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 identified again. The Silt Density Index value is determined as follows:
SDI = (1-Ti/Tf) * 100' / Ttot
The values detected are indicative of the possibility of feeding the water in question to a reverse osmosis system. Purely by way of an example, a compatibility scheme might be the following:
SDI<1 excellent water quality (e.g. UF/MF permeate )
1: good quality water
3: water to be thoroughly assessed
SDI>5: water that needs further pre-treatment
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 fluid-dynamic 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 from case to case may vary considerably.