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SDI detection

  • SDI (silt density index)

    The value of sludge density index (SDI) is one of the important parameters of water quality index. It represents the content of particles, colloids and other objects in water that can block various water purification equipment.

    Main application

    By measuring the SDI value, the corresponding water purification technology or equipment can be selected. According to ASTM method 4189-95, this method is a recognized standard test method in the industry.

    In the process of reverse osmosis water treatment, SDI value is one of the important signs for measuring the inlet water of reverse osmosis system; It is the main means to check whether the effluent of the pretreatment system meets the requirements of reverse osmosis inlet. Its size is very important to the operation life of reverse osmosis system.

    The SDI value is to measure the flow rate attenuation through a 47mm diameter, 0.45um aperture membrane. The reason for choosing a membrane with a pore size of 0.45um is that under this pore size, colloidal substances are easier to block the membrane than hard granular substances (such as sand, scale, etc.).

    The attenuation of the flow rate is converted to a value between 1 and 100, that is, the SDI value. The lower the SDI value, the smaller the pollution and blocking trend of water to the membrane. From the comprehensive consideration of economy and efficiency, most reverse osmosis manufacturers recommend that the SDI value of reverse osmosis inlet water should not be higher than 5.

    Determination method


    SDI measurement is based on the measurement of blocking coefficient (PI,%). The determination is in Φ 0.45 of 47mm μ Continuously add the measured water with a certain pressure (30psi, equivalent to 2.1kg / cm) to the microporous filter membrane of M, record the time Ti (seconds) required to filter 500ml of water and the time TF (seconds) required to filter 500ml of water again after 15 minutes, and obtain the reverse osmosis membrane pollution index SDI according to the following formula:


    Where 15 is 15 minutes. When the pollutants in the water are high, the filtered water volume can be 100ml, 200ml, 300ml, etc., the interval can be changed to 10 minutes, 5 minutes, etc., and 15 in the calculation formula can be adjusted to 10 and 5 accordingly.

    Determination steps

    a) Connect the SDI tester to the sampling point (no filter membrane is installed in the tester at this time);

    b) Open the valve on the tester and thoroughly flush the system for several minutes;

    c) Close the valve on the tester, and then use blunt tweezers to close the 0.45 μ M put the filter membrane into the filter membrane clamp;

    d) Confirm that the O-ring is intact, place the O-ring accurately on the filter membrane, then cover the upper half of the filter membrane clamp and fix it with bolts;

    e) Slightly open the valve and slowly loosen 1-2 butterfly bolts under the condition of water flow to remove the air at the filter membrane;

    f) Re tighten the butterfly bolts on the basis of making sure that all the air has been exhausted and the water flow is continuous;

    g) Fully open the valve and adjust the pressure regulator until the pressure is maintained at 30 psi (if the setting value is less than 30 psi, it can be tested under the existing pressure, but not less than 15 psi);

    h) Use a suitable container to collect water samples. When the water samples just enter the container, start recording with a stopwatch. The time required to collect 500ml water samples is Ti (seconds);

    i) After the water sample flows for 15 minutes (including the time for collecting the initial 500ml of water), collect 500ml of water sample in a container again, and record the time spent collecting the water sample as TF (seconds);

    j) Close the sampling water inlet ball valve, loosen the butterfly bolt of the microporous membrane filter container, and take out the filter membrane for storage (as a sample for physical and chemical test);

    k) Dry the microporous filter and microporous filter membrane support orifice plate.

    computing method

    A) When the feed water pressure during the measurement is 30 psi (2.1bar), substitute the data obtained during the measurement into the following formula to calculate the SDI value:


    Meaning of each item:

    Ti: time of first collecting 500ml water sample (unit: s)

    TF: time for the second collection of 500ml water sample (unit: s)

    B) When the pressure in the measurement process is less than 30 psi (2.1bar), the existing pressure can be used, but the measured SDI value must be converted to the SDI value at 30 psi, as follows:

    %PP = (1-ti / TF) * 100 (% PP is the blocking index at non-standard pressure of 30 psi)


    matters needing attention

    I) during each test, the pressure and temperature shall be kept stable, the pressure fluctuation shall not exceed ± 5%, and the water temperature fluctuation shall not exceed ± 1 ℃, otherwise the data obtained from the test shall be invalid and need to be measured again;

    II) the selected water sample collection volume shall be 500ml (or other determined water volume value), and the time interval between two water samples collection shall be 15 minutes;

    III) when the time interval is 15 minutes, the maximum SDI value (the membrane hole is completely blocked during the test, and there is no water production after 15 minutes) is 6.7. In this case, the test interval can be adjusted to 10 minutes and 5 minutes, and the corresponding maximum SDI values are not greater than 10 and 20 respectively.

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