RO Plant Maintenance Manual

II. Low Rejection Rate and High Conductivity of Product Water

(1) RO Plant Deviation in Readings of Instrument and Meter

The conductivity meter (or TDS meter) not calibrated before use has a considerable deviation of readings, thus figuring out a lower rejection rate.

(2) RO Plant Leakage in the sealing of membrane element connector or the adapter connecting the end plate of pressure vessel.

During the installation of membrane elements, the O-ring of connector may be twisted or fall off, and the high salinity water then enters the product water.

Diagnosis: First measure the conductivity of product water from each pressure vessel. If a certain pressure vessel contains the product water with relatively high conductivity, use the “Probe Method” to determine the exact position of salt leakage. If the salt leakage happens at the connection position, this problem can be corrected by re-installing the membrane element, or the membrane element must be replaced if the leakage happens at the membrane element.

(3) RO Plant pH value of Feed water

The advisable pH value of RO membrane ranges between 6~8. Excessively lower or higher pH value may have influence on the rejection rate.

The feed water is of ground water, with relatively high content of hydrogen carbonate (HCO3－).

The ground water has high alkalinity and high content of hydrogen carbonate (HCO3-), and if the hydrogen carbonate (HCO3－) is removed, the balance of (CO2 + H2O = HCO3- + H+) moves rightward, resulting in the decrease of pH value of product water and the rise of conductivity.

(4) RO Plant Oxidation of Membrane Element

If the water after pre-treatment is not inspected before the membrane elements are mounted into the system, the feed water to membrane system contains excessive residual chlorine or other oxidant and thus results in the oxidation of membrane and the decreased rejection rate of membrane element. In addition, the cation, neutral or amphoteric surfactant may also result in the decrease of rejection rate of membrane element.

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