Common Issues and Solutions for Nanofiltration Membranes

Nanofiltration membranes are a common product in the water treatment industry, capable of filtering, separating, and purifying a wide range of substances. As such, they are widely used across many sectors and deliver excellent performance. Recently, while compiling user feedback, we’ve noticed that many users lack a thorough understanding of nanofiltration membranes, leading to certain issues during operation that can disrupt system performance. To address this, we’ve compiled a summary of some common problems with nanofiltration membranes and their corresponding solutions.

1. Sudden increase in electrical conductivity

Reason one: the water source has been suddenly contaminated. Solution: immediately halt water production and promptly implement biological cleaning.

Reason two: the membrane elements were damaged after chemical cleaning. Solution: If the conductivity is not excessively high, the elements can continue to be used; if the conductivity is too high, they must be scrapped.

Reason three: Excessive water‑production pressure has caused the membrane to rupture. Solution: Replace the membrane with a new one.

2. The finished water exhibits white flocculent particles, floating matter, and green algae.

The reason is not a membrane issue, but rather the lack of disinfection and cleaning of the piping system and the finished‑water tank over an extended period.

Solution: Cleaning the piping and the finished‑water tank will resolve the issue.

3. Sudden reduction in water output

Reason one: the membrane is fouled. Solution: we recommend biological cleaning.

Reason two: the pre‑treatment water filtration system is clogged, resulting in insufficient water supply. Solution: We recommend replacing the quartz sand, activated carbon, and polypropylene (PP) cotton filter cartridges.

4. The membrane exhibits abnormally high electrical conductivity or fails to produce permeate after installation of a new membrane or following biological cleaning.

Cause: The water seal at the membrane end or the indirect water seal is damaged, and no water may be discharged if the membrane has been installed in reverse. Solution: Replace the water seals and install the membrane correctly.

5. Causes of Nanofiltration Membrane Damage

Microbial Contamination: When microbial contamination occurs, both the permeate flow rate and the total bacterial count in the concentrate are elevated, which is closely linked to inadequate routine maintenance and disinfection.

Colloidal fouling: When colloidal fouling occurs, it is typically accompanied by two characteristics: first, the microfiltration unit in the pretreatment stage becomes rapidly clogged, with a sharp increase in pressure; second, the SDI value exceeds 2.5.

6. How long do nanofiltration membrane elements typically last?

The service life of a nanofiltration membrane depends on factors such as the membrane’s chemical stability, the physical stability of the membrane elements, cleanability, feedwater quality, pretreatment, cleaning frequency, and the level of operational management.

7. New Storage Method for Nanofiltration Membrane Modules

The new nanofiltration membrane module is filled with glycerol as a protective fluid. Before use, it should be rinsed with an alkaline solution, followed by an alkali/chlorine wash to remove the glycerol. The new membrane module must remain in its original packaging until installation and should be stored indoors under the following conditions, avoiding direct sunlight: at a storage temperature of 10–30°C, with relative humidity below 70%, and laid horizontally.

8. Storage Method for Used Nanofiltration Membrane Modules

Before storage, used nanofiltration membrane modules shall be thoroughly cleaned and preserved in one of the following solutions.

1. 80–100% glycerin.

2. Phosphoric acid with a pH in the range of 2–3.

3. Benzoic acid or sodium benzoate at a concentration of 1000 ppm.

4. A concentration of 1,000 to 5,000 ppm of sodium bisulfite or sodium metabisulfite.

The above summarizes several issues encountered during the operation of nanofiltration membranes, along with their corresponding solutions, in the hope that this will be helpful. During routine use, it is still important to maintain the membrane elements to more effectively extend their service life.