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Common Problem

There are mainly two kinds of breakdown in the RO system:

(1)The permeate flow and rejection rate become abnormal since the initial running (adjustment) of RO system.

(2)The RO runs normally during the initial period, and the permeate flow and rejection rate decrease after a period of running. Following are the analysis of these two kinds of breakdown.

Troubleshooting of RO System During the Initial Running (Adjustment) Period

At the initial adjustment of RO system, you can compare the actual performance of system with the results figured out using the VONTRON ROdesign software (with fouling coefficient = 1) so as to determine whether the initial performance of system is normal.

Low Permeate Flow and High Pressure

This situation may be brought about by the following causes:

⑴Deviation in Readings of Instrument and Meter

The pressure gauge or flowmeter not calibrated prior to use may bring about inaccurate readings. The pressure gauge is mounted at the position far away from the ends of pressure vessel, and its reading contains the pressure loss within pipings. If said reading is used as the pressure of feedwater, it will be caused that the feedwater pressure is inadequate and the permeate flow is relatively low.

⑵Temperature

The feedwater temperature is lower than the initially designed value. In each temperature drop of feedwater by 3℃, the permeate flow can decrease by approximately 10%.

⑶Feedwater Conductivity (or TDS)

The feedwater conductivity (or TDS) is much higher than designed value. For NaCl solution, if the TDS increases by 1000ppm, then the osmotic pressure increases by around 11.4psi (0.8bar), which means that the permeate flow will decrease under same pressure of feedwater.

⑷Pressure at Product Water Side

Under same feedwater pressure, since pressure suppression is set at the product water side or the product water pipe is relatively small while the destination is far away and is at a high position, an obstacle is formed, resulting in the drop of net pressure and decrease of permeate flow.

⑸Pressure Difference

Under regular circumstance, for the RO system with each housing containing 6 pieces of 8040-sized elements, the pressure difference between two stages is 3~4 bar. The improper design of piping which may cause considerable pressure loss or the incomplete closing of concentrate discharging valve in the second stage may result in the decrease of loss, thus causing the decrease of permeate flow.

⑹Flux Attenuation of Membrane Element

- The wet-type membrane element not preserved properly or not protected properly after being installed in the system can become dry, resulting in the drastic attenuation or even complete losing of permeate flow and thus causing the lower water yield of the system.

- Before the membrane elements are installed in the system, the feedwater hasn’t been confirmed to be acceptable, and the feedwater used for soaking and flushing the membrane elements may contain cation, neutral or amphoteric surfactant or other chemicals incompatible with membrane, thus causing the attenuation of flux of membrane element and then resulting in the low permeate flow of system.

Low Rejection Rate and High Conductivity of Product Water

⑴Deviation 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.

⑵Leakage 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 reinstalling the membrane element, or the membrane element must be replaced if the leakage happens at the membrane element.

⑶pH value of Feedwater

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

⑷The feedwater 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 à HCO3£­+ H+) moves rightward, resulting in the decrease of pH value of product water and the rise of conductivity.

⑸Oxidation of Membrane Element

 If the water after pretreatment is not inspected before the membrane elements are mounted into the system, the feedwater 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.

 Troubleshooting of Failures of RO System Occurring after Regular Running

Such failures usually exhibit at least one of the following situations:

◆ The permeate flow decreases after standardization, and it is usually necessary to increase the operating pressure for the purpose of maintaining the rated permeate flow.

◆ The rejection rate decreases after standardization, and the conductivity of product water rises in the RO system.

◆ The pressure drop increases, and with the flow rate of feedwater remaining unchanged, the pressure difference between feedwater and concentrated water increases.

In case any of the above failures occurs with RO system, following steps shall be taken to analyze and dispose of it:

⑴Preliminarily determine the type of contaminations (fouling, scaling or microbial contamination, etc.) based upon the symptom and position of failure as well as the data and records of system routine running. In case there is no record of system routine running, it is necessary to analyze the water quality of raw water and concentrated water and to check the controlling indexes of pretreated water for the purpose of analyzing the possible cause of failure.

⑵Further determine the cause of failure by means of visual inspection, weighing and on-site dissection, etc.

- Visual inspection: Open the feedwater endplate of the first stage and the product water endplate of the second stage of the pressure vessel, and check the cross section of membrane and the internal wall of pressure vessel. If the internal wall feels velvety and smells odorous, microbial fouling has been formed. In case the internal wall feels relatively rough, it means there is scaling.

- Weighing: Measure the weight of the first membrane element in the first stage and the last membrane element of the second stage. If the first membrane element in the first stage has a relatively heavy weight, it means that there may be contamination by suspending or colloidal substances. If the last membrane element in the second stage has a relatively high weight, it means that there probably exists contamination caused by scaling.

- On-site Dissection of the Membrane Element: Observe the contaminants on the membrane surface, and add acid or alkali on the membrane surface and then observe what will happen.

⑶If the cause of problem cannot be determined from the above two steps, you can take out a membrane element from the system and conduct overall analysis, upon which the cause of failure can be basically determined. Overall analysis includes visual inspection, weighing, performance test, dissection, analysis of contaminants on membrane sheet, pressurized dying, chemical treatment, etc.

⑷The cause of failure can be basically determined after the above analysis, and the cleaning programs and corrective measures can be properly formulated in accordance with “Guide to Cleaning of RO System”. 

The following paragraphs deal in detail with the major causes of the three failures mentioned above, the method to identify such failures and the preventive measures:

Decrease in Permeate Flow after Regular Running

In case the permeate flow of RO system decreases after standardization, the cause of failure can be sought according to the following situations:

1.If the decrease of permeate flow occurs in the first stage of RO system, it means there is deposition of granular contaminants.

2.If the decrease of permeate flow occurs in the last stage of RO system, it means that there exists contamination of scaling.

3.If the decrease occurs in all stages of the RO system, it means there exists fouling.

Determine the cause of failure based upon the symptom and exact position of failure, take corrective measures accordingly, and clean the system in line with Guide to Cleaning of RO System.

In addition, the decrease in permeate flow of RO system is always accompanied by such situations as the decrease or increase of rejection rate, etc.

(1) Decrease in both Permeate Flow and Rejection Rate after Standardization

The decrease in both permeate flow and rejection rate after standardization is the system failure most frequently seen, and is possibly caused by the following:

①Colloidal Fouling

The causes of colloidal fouling:

1.Insufficient amount of flocculant is dosed in the pretreatment process, the jar test hasn’t been carried out for determining the optimum dosage, and on-line flocculation hasn’t achieved a satisfactory result.

2.The multi-media filtration and activated carbon filtration have been overloaded, and the designed velocity of filtration flow is a little higher than necessary, and the system is not backwashed and flushed in time. The designed pore diameter of microfiltration or ultrafiltration membrane is a little larger than necessary.

3.The SDI and turbidity values are not monitored during regular routine operation and management owing to that sufficient importance has not been attached.

 In order to identify the colloidal fouling, it is necessary to:

1.Measure the SDI of raw water

2.Analyze the retained substances on the surface of membrane for SDI test.

3.Inspect and analyze the sediments on the end surface of the first membrane element in the first stage.

②Fouling by Metallic Oxides

The fouling by metallic oxides mainly occurs in the first stage, usually caused by the following reasons:

1.The feedwater contains such ions as iron, manganese and aluminum, etc.

2.The feedwater contains H2S and air enters the system, thus generating the sulphide salt.

3.The fouling is formed by the erosive products from the pipe, pressure vessel, etc.

Following are the methods for identifying the metallic oxides:

1.    Observe the contaminants retained inside the safety filter, and also observe the end surface of the first membrane element and the internal wall of pressure vessel.

2.Take out the first membrane element, and dissect it to analyze the composition of metallic ions on the membrane surface.

③ Scaling

 Scaling is formed by the slightly or hardly soluble salts depositing on the membrane surface, and generally takes place in the brackish water system that operates under high hardness and alkalinity of raw water and requires high recovery rate. It occurs especially in the last stage of RO system, and gradually spreads toward the preceding stages. Scaling may possibly occurs in those raw water containing calcium, bicarbonate radical or sulfate radical within several hours of running, which can block the RO system. Other scaling may be formed slowly. Following situations can cause the fouling:

1. The quality of raw water has not been analyzed, and insufficient amount of antiscalant has been dosed or the antiscalant has inferior effect.

2. The raw water has high hardness and the recovery rate is excessively high, therefore the precipitation and separation cannot be inhibited by simply dosing the antiscalant.

 

The method to identify whether scaling occurs:

1.Check the concentrate side of system to make sure whether scaling is formed, which may cause the roughness feeling of internal wall and end plate of pressure vessel.

2.Take out a membrane element from the system and measure its weight. The membrane element with serious scaling generally has a heavy weight.

3.Analyze the quality data of raw water.

(2) Decrease in Permeate Flow and Increase in Rejection Rate after Standardization

 This failure is possibly caused by the following reasons:

①Densification of Membrane

If 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.The pressure of feedwater is excessively high pressure and exceeds the permitted limit.

2.The feedwater is at a relatively high temperature and there is serious phenomenon of water hammer, in which case the instantaneous pressure exceeds the permitted limit.

The method to identify the densification of membrane: Take out a membrane element and dissect it. Take out a piece of membrane for analysis of its microstructure.

②Contamination by Organic Substances

The organic substances contained in the feedwater can be adsorbedto the surface of membrane element and result in loss of permeate flow, which occurs generally at the first stage. The reasons causing organic contamination are basically similar to the ones causing the colloidal contamination.

Following are the methods for identifying the contamination caused by organic substances:

1.Analyze the retained substances on the filtering element of safety filter.

2.Inspect the flocculant for pretreatment, especially the cation polymer.

3.Analyze the oil and organic contaminants in feedwater.

4.Inspect the detergent and the surfactant.

Decrease in Rejection Rate after Normal Running

(1)Permeate flow remains regular and rejection rate decreases after standardization.

The causes of this failure include:

① Leakage in O-ring

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.

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

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.

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

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

Methods for Preventing the Membrane Surface Damage:

1.Replace the fitering elementof 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.

④Back Pressure

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

(2) Rejection rate drops and permeate flow rises after standardization. 

The causes of this failure include:

① Membrane Oxidation

When the membrane contacts the oxidative 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.

 The membrane can be oxidated possibly by the following reasons:

1.The residual chlorine or other oxidative substances contained in the feedwater of membrane system exceed the provided standard; or

2.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.

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

② Leakage

If the O-ring is seriously damaged or the central pipe is broken, the feedwater 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.

Pressure drop increases after regular running, and it causes the decrease in permeate flow or rejection rate.

Pressure drop refers to the pressure difference between feedwater and rejected water. The upper limit of pressure drop is 3.5bar for each pressure vessel containing a number of membrane elements, or 1bar for each membrane element held in FRP casing. Excessive pressure drop will cause the telescoping phenomenon to occur in the membrane element and the FRP casing to be broken, thus resulting in the mechanical damage of membrane element. If the flow rate of feedwater remains constant, the increase of pressure drop is caused generally by the contaminants or scaling substances contained in the flowing passage of feedwater mesh. Once the flowing passage of feedwater is blocked, the permeate flow will decrease as a result.   

Following are the reasons frequently seen that causes the increase of pressure drop.

① Microbial Fouling

Microbial contamination can usually cause obvious increase of pressure drop in all stages of RO system. Microbial contamination occurs mostly in those systems using surface water or reused wastewater as raw water.

 Solutions to Microbial Contamination

1.Dose germicide in feedwater. Be sure to prevent the oxidative germicide from entering the membrane element so as to prevent the oxidation of membrane element.

2.Change to use the oxidation-resistant membrane element and dose germicide in the whole system (including RO system)

②Scaling Contamination

Since the scaling that fouls the feedwater flow passage can usually cause the increase of pressure drop in the membrane elements of the last stage, it must be ensured that suitable measures be taken to control the scaling, that proper chemicals be used for cleaning the membrane element and that the recovery rate be kept at reasonable level.

Summary of Failure Symptoms, Causes and Corrective Measures of RO System

Symptoms

 

 Possible Causes

 

Corrective Measures

 

 Flux

 

 Salt Passage

 

Pressure Difference

 

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 Scaling Contamination

 

Chemical cleaning; controlling the scaling

 

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 Colloidal Contamination

 

Chemical cleaning; improving the pretreatment

 

 

 

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 Biological contamination

 

Chemical cleaning, disinfection; improving the pretreatment

 

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 Organic Contamination

 

Chemical cleaning; improving the pretreatment

 

↓●

 

 

 

 Densification

 

 Replace or add new membrane element

 

 

↑●

 

 

Oxidative Destruction

 

Replace membrane element and dose reductant; Replace with oxidation resistant element

 

 

↑●

 

 

Leakage of damaged membrane sheet; O-ring leakage

 

Replace the element

 Improve the filtration effect of safety filter

Replace the O-ring

Replace the element.