Reverse Osmosis Membrane Contamination Density Index SDI Measurement Method

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

The standard method for determining the SDI value of reverse osmosis membrane fouling is based on the principle of measuring the time required to filter a certain amount of raw water with a 0.45 μm microfiltration membrane at a water pressure of 30 psi.

1, the assembly of test instruments

1.1 Assemble the test device according to Figure 1:

1.2 Connect the test device to the sampling point of the RO system inlet water line;

1.3 Adjust the feed pressure to 30 psi after loading the filter. In the actual test, a new filter should be used.

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

A. When installing the filter, use flat tweezers to prevent puncture

B. Make sure the O-ring is clean and properly installed

C. Avoid touching the filter by hand

D. Flush the test device beforehand to remove contaminants from the system

2. Reverse Osmosis Membrane Contamination Test Procedure

2.1 Record the test temperature. The temperature of the system shall not change by more than 1°C during the test period from the beginning of the test to the end of the test.

2.2 Exclude the air pressure in the filter. According to the type of filter, when the water supply ball valve is opened, or open the exhaust valve above the filter, or loosen the filter jacket thread, fully discharge.

2.3 Take filtered water with a graduated 500ml cylinder to measure the amount of water that passes through the membrane.

2.4 Fully open ball valve. Measure and record the time required to fully fill the 100 ml and 500 ml water samples from the ball valve.

After 2.5 minutes, the time required to collect the 100 ml and 500 ml water samples was again measured. The same measurements were taken after ten minutes and fifteen minutes, respectively.

2.6 If the time required to receive 100 ml of water sample exceeds 60 seconds, it means that about 90% of the filter membrane area is blocked. No further experiment is required.

2.7 Measure the water temperature again to ensure that the temperature of the water at the beginning of the experiment does not vary by more than 1°C.

2.8 After the end of the experiment and opening the filter, it is best to keep the filter after the experiment for future reference.

3, the formula:

SDI=P30/Tt=100×(1-Ti/Tf)/Tt

In the formula:

SDI - Pollution Density Index

P30 - Percentage of filter clogging at 30 psi feedwater pressure

Tt - total test time in minutes. Usually Tt is 15 minutes, but if 70% of the filter area is blocked within 15 minutes (Note 2), the test time will need to be shortened.

Ti - the time required for the first sample

Tf - Sample time after 15 minutes (or less)

Note 1: The time required to receive 500 ml of water sample is approximately 5 times the time required to receive 100 ml of water sample. If the time required to receive 500 ml is much greater than 5 times, then when calculating SDI, the time used to access 100 ml should be used.

Note 2: In order to accurately measure the SDI value, P30 should not exceed 75%. If P30 exceeds 75%, it should be retested and Tf value obtained within a short time.