Analysis of Common Problems in Reverse Osmosis Systems (Part 2)

Iron contamination

The membrane element contaminated by iron has a reddish-brown membrane surface and a light red feed water channel.

The membrane surface of the iron-contaminated membrane element is reddish-brown, and the true appearance of the membrane sheet is revealed after adding acid.

I. Phenomena and Symptoms:

1. Low system desalination rate and reduced water production;

2. After opening the pressure vessel, the end face of the membrane element is reddish-brown;

3. After dissection of the membrane element, the membrane surface is reddish-brown.

II. Causes:

1. RO The feed water contains excessive iron;

2. Corrosion of pipes or pressure vessels in the pretreatment system.

III. Cleaning or Solutions:

1. The membrane system can use acidic NaHSO3 (PH<5) or H3PO4 and citric acid cleaning to restore;

2. Sometimes iron will accelerate membrane oxidation, leading to irreversible damage to the membrane element.

Pressure drop ( Δ P ) is too large

The fiberglass shell is cracked along the axial direction , Severe contamination at the inlet.

Outlet pressure vessel (ATD) Washed away, the concentrate flow channel grid is flushed out.

Destructive analysis of the membrane element found: black viscous liquid flowed out. The entire membrane element is completely contaminated, and the membrane leaves are between ( Inlet flow channel ) The inlet flow channel is filled with black viscous substances. There is no odor, biological contamination is ruled out, and it is confirmed that inorganic pollutants severely clog the inlet flow channel, resulting in high pressure drop.

I. Phenomena and Symptoms:

1. The system desalination rate has dropped significantly, sometimes accompanied by an increase in water production;

2. After the membrane element is dissected, bubbles and delamination appear on the membrane surface.

II. Causes:

1. System design defects, such as unreasonable installation location of the check valve on the product water pipe;

2. Incorrect operation, such as forgetting to open the product water valve after cleaning;

3. Unpredictable mechanical failures.

III. Cleaning or Solutions:

The membrane element is irreversibly damaged and difficult to repair, and the membrane element can only be replaced.

Product water back pressure

A stop valve is installed on the product water pipe. During the system cleaning process, the operator closes this stop valve. After the cleaning is completed, the operator forgets to open it. After the system is shut down and restarted, it is found that the system desalination rate has decreased.

The membrane surface damaged by product water back pressure usually shows scratches parallel to the product water pipe on the outermost side of the membrane, often near the bonding line of the outermost membrane bag. Membrane rupture is most likely to occur near the three bonding seals on the feed water side, the outermost side, and the concentrate side. Other positions are supported by the feed water network, and many bubble-like delaminations will appear in many small grids.

To prevent product water back pressure, the above two measures can be adopted: reasonably install a check valve or a three-way valve on the product water pipe.

I. Phenomena and Symptoms:

1. The system desalination rate has dropped significantly, sometimes accompanied by an increase in water production;

2. After the membrane element is dissected, bubbles and delamination appear on the membrane surface.

II. Causes:

1. System design defects, such as unreasonable installation location of the check valve on the product water pipe;

2. Incorrect operation, such as forgetting to open the product water valve after cleaning;

3. Unpredictable mechanical failures.

III. Cleaning or Solutions:

The membrane element is irreversibly damaged and difficult to repair, and the membrane element can only be replaced.

Membrane oxidation

Membrane elements damaged by oxidation cannot be detected by mechanical methods such as vacuum testing. This type of chemical damage can be detected by testing membrane elements or small membrane samples using Fujiwara test. For example, Fujiwara The experimental solution turns pink in the experiment , proves that the membrane sheet has been oxidized.

Membrane oxidation (caused by residual chlorine) A new membrane sheet that has not been contaminated or oxidized should only consist of C, O, N and H composed, without other elements. For used membrane sheets, the types and contents of added elements in the membrane material can be analyzed to determine whether the membrane sheet has been oxidized or contaminated. Atomic spectroscopy chemical analysis (ESCA) proves that the membrane sheet is chlorinated.

I. Phenomena and Symptoms:

1. The system desalination rate has dropped significantly, while the water production has increased;

2. After the membrane element is dissected, Fujiwara The experimental solution turns pink in the experiment, atomic light;

3. Spectrum chemical analysis (ESCA) Chlorine element was found.

II. Causes:

1. RO Problems with dechlorination measures before the system, such as, activated carbon failure or NaHSO3 insufficient quantity;

2. The membrane element comes into contact with a strong oxidizing agent.

III. Cleaning or Solutions:

The membrane element is irreversibly damaged and difficult to repair, and the membrane element can only be replaced.