Determination method of reverse osmosis membrane fouling density index SDI

The pollution density index SDI value is an important indicator to characterize the influent water quality of the reverse osmosis system.

The standard method for determination of SDI value by reverse osmosis membrane fouling, the basic principle of which is to measure the time required to filter a certain amount of raw water with a 0.45 μm microfiltration membrane under a feed water pressure of 30 psi.

Assembly of test equipment

  1. Assemble the test device:
  2. Connect the test device to the sampling point of the RO system inlet pipe;
  3. Adjust the inlet water pressure to 30psi after loading the filter membrane. In actual testing, a new filter should be used.

In order to obtain accurate test results, special attention should be paid to the following:

  • When installing the filter, flat tweezers should be used to prevent puncturing the filter
  • Make sure the O-ring is clean and properly installed
  • Avoid touching the filter membrane with your hands
  • Flush the test unit beforehand to remove contaminants from the system

Test steps for reverse osmosis membrane fouling

  1. Record the test temperature. During the test time from the beginning to the end of the test, the temperature of the system should not change by more than 1 °C
  2. Remove the air pressure in the filter. Depending on the type of filter, when the feed water ball valve is open, either open the exhaust valve above the filter, or loosen the jacket thread of the filter to fully discharge.
  3. Use a graduated 500ml graduated cylinder to receive the filtered water to measure the amount of water permeating the filter membrane.
  4. Fully open the ball valve, measure and record the time required to fully open the ball valve to fill 100ml and 500ml (Note 1) water samples.
  5. After five minutes, measure the time required to collect the 100ml and 500ml water samples again, and repeat the same measurement after ten minutes and fifteen minutes, respectively.
  6. If the time required to take a 100ml water sample exceeds 60 seconds, it means that about 90% of the filter membrane area is blocked, and no further experiments are required at this time.
  7. Measure the water temperature again to ensure that it does not change by more than 1°C from the water temperature at the beginning of the experiment.
  8. After the experiment is over and the filter is opened, it is best to save the filter membrane after the experiment for future reference.

Calculation formula:

SDI = P 30 / T t = 100 × ( 1 – T i / T f ) / T t
In the formula:
SDI – Pollution Density Index
P 30 —— Percentage of filter membrane blockage under 30psi feed water pressure
T t ——Total test time, in minutes. Typically Tt is 15 minutes, but if 70% of the membrane area is blocked within 15 minutes (Note 2), the test time needs to be shortened
T i ——The time required for the first joint sample
T f —— Time required for sampling after 15 minutes (or less)

Note 1: The time required to take a 500ml water sample is about 5 times the time required to take a 100ml water sample. If the time required to dispense 500ml is much greater than 5 times, the time used to dispense 100ml should be used when calculating SDI.
Note 2: In order to accurately measure the SDI value, P30 should not exceed 75%, if P30 exceeds 75%, the test should be re-tested and the Tf value should be obtained in a short time

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Further reading:

LP21 4040 ro membrane

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