What we know about silt density index?
At the beginning of the development of semipermeable membranes for water treatment, it was decided that it was essential to assess the potential for membrane contamination of raw water in order to recognize the need for pre-treatment. In some waters, the membrane can process tens of thousands of gallons before it needs to be cleaned, while in other waters, filtration of thousands of gallons clogs the membrane.
Attempts to correlate the tendency of water pollution to turbidity have been very successful. A test called the silt density index test (SDI test) was developed to better correlate "sludge" or solids in the water with the tendency of membrane fouling.
Water with very low turbidity (e.g.,<0.5 Nephlo-turbidimetric Units, or NTU) is unacceptable due to the large number of very small diameter particles (usually less than 3 microns in diameter) that do not reflect light efficiently and often causes membrane fouling.
Nonetheless, these small particles can quickly contaminate the spirally wound RO membrane. Sources of silt can be organic colloids, iron corrosion products, precipitated iron hydroxides, algae, and fine dust.
The SDI test forces water to pass through a 0.45 micro-meter absolute small diameter (47 mm) membrane filter while maintaining a pressure of 30 psi. It takes the time required to pass 500 ml at the end of 5, 10 and 15 minutes and is used for unitless SDI calculations.
The sludge density index is calculated as follows:
Sludge Density Index Test Description
Three SDI are generally considered acceptable for membrane manipulation without pre-treatment for sludge removal. 35 SDIs can result in frequent cleaning (quarter to weekly), but SDIs above 5 usually require upstream filtration that can reduce submicron particles.
SDI should be measured onsite from a sampling point near the proposed location of the RO installation. Since there can be significant differences in SDI between labs and in large production facilities and proposed installation locations. This may be due to corrosion of old black iron pipes that are often used in processes and drinking water distribution.
The sludge density index is a measure of the ability of water to pollute in reverse osmosis systems. This test measures the rate at which a 0.45-micron filter clogs when exposed to constant water pressure of 206.8 kPa (30 psi). SDI is the rate of decrease in water flow per minute through the filter, for example the average for 15 minutes.
Typically, spiral reverse osmosis systems require less than 5 SDIs. Hollow fibre reverse osmosis systems require less than 3 SDIs. For these types of systems, deep well water (normal SDI is 3) can be used directly from the water source. If supplied by surface water (normal SDI is greater than 6), the water must be filtered before using seawater.
SDI is a common method for determining RO water quality. This is based on the time it takes to filter a large amount of water through a 0.45 micro-m filter mat with a supply pressure of 30 psi.
SDI Test
-Locate a sample tap on the feedwater piping and install the test equipment.
-With the filter pad installed, set the pressure regulator to 30 PSI.
-Use the new filter for actual testing.
For best results:
-When placing the filter, use dull tweezers to avoid sticking the tweezers filter.
-Make sure the O-ring is clean, in good condition and correctly placed.
-Do not touch the filter with your finger.
-Rinse the device to remove any impurities that may be contained with this.
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