Water Purification System Maintenance Manual

V. Decrease in Permeate Flow after Regular Running

Permeate flow remains regular and rejection rate decreases after standardization. The causes of this failure include:

1. Leakage in O-ring

A. If the O-ring is damaged by lubrication with certain incompatible chemicals or by mechanical stress (movement of membrane element caused by the effect of water hammer, for example), leakage will occur in the O-ring. If the O-ring is not installed or is improperly installed, leakage may also occur.

B. The method for preventing the leakage of O-ring: Use the prepared glycerin as the lubricant; Insert a pad during installation to prevent the membrane element from shifting to and fro.

2. Telescoping Phenomenon

A. The telescoping phenomenon is caused by the excessively large pressure difference between feed water and concentrated water. Comparatively serious telescoping phenomenon can cause mechanical damage to the membrane element.

B. The method to prevent the telescoping phenomenon: Reinforce the recording and analyzing of running data. When the pressure difference is 15% higher than the original value, effective corrective measures shall be taken and chemical cleaning shall be done immediately. Particularly, be sure that the pressure difference shall not exceed the permitted limit.

3. Membrane Surface Damage

A. Membrane surface damage is mostly caused by the combined effect of sharp granules in water, the crystals and the water hammer.

B. Methods for Preventing the Membrane Surface Damage:

1) Replace the fitering element of safety filter in time, and prevent the sharp and hard granules or activated carbon granules contained in the water from entering the membrane element.

2) Evacuate the air from membrane system before starting the high-pressure pump, or additionally install an electrical slow-open gate or start up the high-pressure pump with variable frequency in order to prevent the water hammer.

3) When cleaning the scaled membrane element, the initial flow rate shall be as small as possible to avoid the damage caused by the impact of excessively large flow rate.

4. Back Pressure

Whenever the pressure of product water is 0.3 bar higher than that of feed water or rejected water, inter-layer peeling may occur in the compound membrane, thus damaging the membrane element.

5. Rejection rate drops and permeate flow rises after standardization. The causes of this failure include:

A. Membrane Oxidation

1) When the membrane contacts the oxidant substances in the water, the oxidation can damage the membrane and result in irreversible chemical damage, in which case all membrane elements must be replaced.

2) The membrane can be oxidant possibly by the following reasons:

a. The residual chlorine or other oxidant substances contained in the feed water of membrane system exceed the provided standard; or

b. The system is not cleaned and sterilized in strict accordance with relevant requirements or procedures, and the cleaning time or temperature exceeds the limits thus causing the oxidation of membrane.

c. Pressurized dying test can be performed to determine whether the membrane has been oxidant.

B. Leakage

If the O-ring is seriously damaged or the central pipe is broken, the feed water and rejected water will seep into the product water, and the problem will become more serious especially when the system works under relatively high pressure.

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