Troubleshooting of RO System

 

The breakdown of RO system is generally characterized by at least one of the following occurrences:
1. The permeate flow decreases after standardization, and it is usually required to increase the operating pressure to maintain the designed permeate flow.
2. The rejection rate decreases after standardization, which is characterized by the increase in conductivity of permeate.
3. The pressure difference is enlarged, i.e. the pressure difference between feed water and concentrated water increases with the feed water flow kept unchangded.

These three kinds of breakdown mentioned above will be detailed as follows:
A. Decrease in Permeate Flow after Standardization
In case of decrease in permeate flow after standardization, the cause can be traced back to three instances as follows:
A) If the decrease in permeate flow takes place at the first step of the RO system, then there exists the precipitation of granular contaminants.
B) If the decrease in permeate flow takes place at the last step of the RO system, then there exists scaling.
C) If the decrease in permeate flow takes place at all steps of the RO system, then there exists the contamination blockage.
The aforesaid symptoms and the positions where problems take place should be depended on to determine the causes of breakdown, and relevant measures should be taken to carry out cleaning according to the Guide to Cleaning.
In addition, the decrease in permeate flow of RO system is also accompanied coincidently by the decrease or increase in rejection rate.
1. Decrease in both permeate flow and rejection rate after normalization, which is a kind of system breakdown most frequently seen, is possibly caused by the following:
1) Blockage by Colloidal contamination. For discriminating the blockage by colloidal contamination, it is necessary to:
(1) Test the SDI of feedwater.
(2) Analyze the matters retained on the surface of membrane for SDI testing.
(3) Inspect and analyze the deposits existing in the end of the first membrane element at the first step.
2) Blockage by metallic oxides. It takes place mainly at the first step, and is usually caused by

the following：
(1) The feedwater contains iron and aluminum colloid.
(2) The feedwater contains H2S and incoming air, which produce the salfide salt.
(3) The corrosive is produced in the pipe, pressure vessel and other components.
3) Scaling. Scaling is the deposition of slightly or hardly soluble salts on the membrane surface, which exists generally in the brackish water system that has the comparatively poor pre-treatment and quite high recovery rate, and often takes place in the last step of RO system, and then spreads to the preceding step. Scaling will possibly take place in the the system fed by water containing Ca2+, HCO3- or SO42- within several hours and block the system, while the scaling containing Ba2+ and F- is formed quite slowly. Following is the method for discriminating whether scaling has formed:
(1) Check the side of concentrated water in the system to make sure whether scaling is formed.
(2) Take out the last membrane element and weigh it up. In general case, the membrane element with serious scaling is comparatively weighty.
(3) Analyze the data of raw water quality.

2. Decrease in permeate flow with increase in rejection rate after normalization
The possible reasons for decrease in permeate flow accompanied by increase in rejection rate after normalization are as follows:
1) Densification of membrane.
After the membrane is densified, it will usually come into being that the permeate flow decreases and the rejection rate increases. The densification of membrane will likely occur in the following situations:
(1) Excessively high pressure of feed water
(2) High temperature of feed water.
(3) Hydraulic hammer.
2) Contamination of organic matters
The organic matters contained in the water will be adsorbed to the surface of membrane and result in the loss to permeate flow, which occurs generally at the first step. Following are the methods for discriminating the contamination of organic matters:
(1) Analyze the matters retained in the filtration element of safety filter.
(2) Inspect the flocculant of pretreatment, especially the cationic polymer.
(3) Analyze the oil and organic contaminations contained in the feed water.
(4) Inspect the cleaning agent and surfactant

B. Decrease in rejection rate after normalization
1. Decrease in rejection rate with regular permeate flow after normalization
The causes for this symptom include the following:
1) O-ring leakage
When coming into contact with certain chemicals or being exerted the mechanical stress, for example, the movement of membrane caused by the effect of hydraulic hammer, the O-ring will suffer the leakage. And failure and improper installation of O-ring will also cause leakage.
2) Telescope phenomenon
The telescope phenomenon is caused by the excessively large pressure difference between feed water and concentrated water. Comparatively serious telescope phenomenon can cause mechanical damage to the membrane element.
3) Abrasion of membrane surface
This occurrence is generally caused due to that the membrane elements in the front end of RO system is abraded by the crystals or metallic suspended matter with keen edge.
4) Back pressure of permeate
At any time when the pressure of permeate is 0.3bar greater than that of feed water or concentrated water, the peeling off among the multiple layers can possibly take place in the emembrane, thus damaging the membrane element.

2. Decrease in rejection rate with increase in permeate flow after normalization
The causes for this symptom include the following:
1) Oxidation of membrane
After the membrane comes into contact with the oxidative matters contained in the water, the membrane is destroyed by the oxidation, which is an irreversible chemical damage. In case of this occurrence, you cannot but replace all the membrane elements.
2) Leakage
The serious mechanical damage of membrane element or permeate tube will cause thefeed water or concentrated water to infiltrate into the e, and this problem will become more serious especially when the operating pressure is comparatively high.

C. Enlargement of Pressure Difference
The pressure difference between the feed water and the concentrated water is called pressure difference. The upper limit of pressure difference for each pressure vessel containing several membrane elements is 3.5bar, and the upper limit of pressure difference for the membrane element reinforced with FRPis 1bar. When the feed water flow is steady, the enlargement of pressure difference is generally due to that the contaminants or scalings exist in the feed spacer, and thus the permeate flow decrease.
Following are the causes for enlargement of pressure difference commonly seen:
1. Scaling
Scaling will generally cause the enlargement of pressure difference in the last-step membrane element, and therefore, it must be ensured that proper measures have been taken to control the scaling, that proper chemical reagents have been used for cleaning the membrane elements, and that the recovery rate of the system will not exceed the design value.
2. Blackage of biological contamination
The blockage of biological contamination can usually cause the obvious increase in pressure differencee of the front end of the RO system, and bring about extremely high resistant force to the feed water.

 

 

Troubleshooting of RO systmem

Symptom 1: The salt permeability becomes increasingly high, while the permeate flow decreases, the pressure differences between each step increases, and the weight of membrane elements also increases obviously.

Possible causes	Position of breakdown and method for discrimination	Method for settlement
contamination by metallic oxides	◎         Analyse the matters retained in the membrane for routine SDI testing. ◎         Analyze the metallic ions in the cleaning solution. ◎         Anatomize and analyze the contaminated elements.	◎         Clean off the contaminants of metallic oxides. ◎         Improve the technologies and running conditions of pretreatment
Colloidal contaminationpollution	◎       Analyse the matters retained in the membrane elements for routine SDI testing. ◎       Anatomize and analyze the contaminated elements.	◎       Clean off the contamination with the use of cleanning agent containing lipid. ◎       Improve the technologies and running conditions of pretreatment.
Scaling contamination by inorganic salts	◎       Calibrate the LSI of concentrated water, and test the solubility product of hardly soluble matters possibly produced ◎       Anatomize and analyze the typical element contaminated.	◎       Select the suitable cleaning agent depending on specific circumstanes. ◎       Add the more effective antiscaling / dispersing agent. ◎       Improve the pretreatment system
Sludging contamination	◎       Inspect the NTU of feed water following the pretreatment system ◎       Anatomize and analyze the typical element contaminated.	◎       Improve the pretreatment system ◎       Cleaning the contaminated with the use of HF and colloidal cleaning solution

 

Symptom 2:  Both the salt permeability and the permeate flow increase, while the pressure differences between feed water and concentrated water is regular.

Possible causes	Position of breakdown and method for discrimination	Method for settlement
Contamination of organic matters	◎         Disassemble the pressure vessel, and examine the contamination symptom of feed end of RO element. ◎         Analyze the quality of feed water and conentrated water.	◎         Select the alkaline cleaning solution to clean the system. ◎         Improve the pretreatment technology of the system

Symptom 3:  The salt permeability remains unchanged at the beginning or even decreases to some extent. After a certain period of running, the salt permeability becomes continuously increasing, coincidently with the pressure difference between feed water and concentrated water becoming enlarged and the permeate flow  of the system becoming lower.

 

Possible causes	Position of breakdown and method for discrimination	Method for settlement
Biological  contamination	◎         Disassemble the pressure vessel, and examine the contamination symptom of feed end of RO elements. ◎         Analyze the biological and bacterium indexes of concentrated water and permeate of the system.	◎         Above all, use the alkaline cleaning solution to clean off the contamination for the first time. Then, formulate the cleaning agent with using the germicidal cleaner permitted for use to clean the system. ◎         Improve the pretreatment technology of the system

 

Symptom 4:  The salt permeabibility is high, while the permeate flow is satisfactory or even slightly higher than expected and the pressure differences among each steps are comparatively great.

 

Possible causes	Position of breakdown and method for discrimination	Method for settlement
◎         Unreasonable design or operation of the system that causes excessive polarization of concentration difference.	◎       Calibrate and Examine the proportion of concentrated water and permeate and the operational recovery rate of the system ◎       Inspect the pressure vessel and pressure piping in the RO system to make sure whether they are properly fixed, and whether the pressure vessel presents warp or deformation. ◎       Inspect the U-ring for sealing of concentrated water	◎       Enlarge the  feedwater flow and reduce the water recovery rate of RO system. ◎       Replace the broken sealing U-ring in the RO elements ◎       Improve the fastening of the pipings.

 

Symptom 5: Salt permeability increases and thepermeate flow increases while the pressure difference decreases.

Possible causes	Position of breakdown and method for discrimination	Method for settlement
◎         The membrane surface is scratched by the granular matters contained in the feedwater or by the scaling crystals formed by the polarization of concentration difference of system	◎       Granular contaminants ◎       Analyze the scaling contamination of inorganic salts at the last step, calibrate the LSI of concentrated water, and test the solubility product of hardly soluble matters.	◎       Improve the pretreatment system. ◎       Adjust the water recovery rate of the system ◎       Select the more effective antiscaling / dispersing agent. If the permeate flow from the firtst step of RO system decreases, there possibly exists the precipitation of granular  contaminants.

 

Symptom 6: Salt permeability is high and the permeate flow is satisfactory or slightly higher than

 

expected, while the pressure differences among every step remain basically unchanged.

Possible causes	Position of breakdown and method for discrimination	Method for settlement
O-ring of membrane element or pressure vessel suffers leakage of water.	Take sample from the sampling pipe of pressure vessel for testing and analysis so as to determine the specific position of the breakdown.	Replace the O-ring in the membrane element or the pressure vessel that has been broken or suffers leakage.
The glue line of membrane bag is broken, the permeate tube of element is broken, or the element suffers mechanical damage.	◎         Take sample from the pressure vessel for testing to determine the specific position of the breakdown. ◎         Hold a vacuumized test of the element to determine the specific position of breakdown. ◎         Protrude the coil of membrane element for anatomizing and analyzing the causes of breakdown.	◎         Replace the broken element. ◎         Examine the feedwater pressure, the pressure of permeate and the pressure difference of element in running so as to make sure whether they are suitable and to make necessary adjustment.
Hydraulic hammer is formed during the running of system.	◎           Inspect the equipment driving program to make sure whether it is suitable, and find out the reason resulting in hydraulic hammer.	Modify the design, the running condition and the system-driving program.

Symptom 7: Both the salt permeability and the permeate flow increase while the pressure difference between feed water and concentrated water is low or remains regular.

Possible causes	Position of breakdown and method for discrimination	Method for settlement
The RO membrane is oxidated , thus causing the degeneration of membrane performances.	Lay stress on monitoring the quality and quantity of the RO membrane assembly at the first step, normalize the measured values, and compare these values with the data indicated in the trial-run report.	◎       The element with comparatively serious degeneration must be replaced on a selective basis. ◎       Improve the pretreatment technology of the system. ◎       Establish additionally the monitoring of oxidation-reduction potential (ORP).