The amount of chlorine that remains in the water has a significant impact on how quickly membranes degrade. Depending on the water quality, even a modest chlorine content of 1PPM (parts per million) can degrade the system in 200–1000 running hours.
Maintaining the effectiveness and lifespan of reverse osmosis membrane components requires an understanding of how residual chlorine affects these components. Water treatment systems can assure constant water quality and extend the membrane elements’ service life by efficiently controlling residual chlorine levels. This article will show reverse osmosis and Chlorine as friends and foes.
RO Membranes and Their Role in Water Purification
In terms of operation, reverse osmosis membranes work by using pressure to force water through a semi-permeable membrane, leaving behind the impurities and contaminants. This process ensures that water is purified effectively and efficiently.
Comparing reverse osmosis membranes to other water treatment methods, such as activated carbon filtration or UV disinfection, shows the superiority of these membranes in terms of removing a wide range of contaminants.
Chlorine’s Effect on Membrane
- Salt rejection rate will decrease when oxidation forms in the reverse osmosis membrane and residual chlorine. Within 200 to 1000 hours, about 1 PPM of chlorine will cause membrane elements to degrade; the rate at which this happens is dependent on the water’s quality. The membrane will degrade more quickly in alkaline settings than in acidic ones, which will cause the membrane to degrade more slowly and moderately.
- Dichlorination should be used to stop any remaining chlorine from damaging the membrane film. One common method for lowering residual chlorine in feed waters is activated carbon dechlorination, however sodium bisulfite (SBS) can also be used: NaHSO3+ HOCl→HCl+ NaHSO4.
What is a Residual Chlorine Meter?
A residual chlorine meter is a handheld device that measures the amount of residual chlorine in water. It works on the principle of amperometric titration or colorimetry. They are compact, easy to operate, and provide precise measurements.
To measure residual chlorine, the device uses an electrode probe that reacts with the chlorine in the water. The resulting current is proportional to the chlorine concentration, and the meter displays the reading in ppm (parts per million) on a digital screen.
Common Problems with Residual Chlorine Meters
Calibration
- One of the most common issues with residual chlorine meters is incorrect calibration. Calibration is the process of adjusting the meter to ensure that it provides accurate readings. If not calibrated correctly, the meter might show incorrect results.
- Calibration is especially necessary when using a new meter for the first time or after a certain period of usage. To calibrate, refer to the meter user manual and follow the calibration steps precisely. Ensure that you have the right calibration standards and solutions and also check the expiry dates of the calibration solutions.
Probe Contamination
- Probes are the most sensitive parts of a residual chlorine meter, and proper care is necessary to maintain their accuracy. Probe contamination is a common issue that could lead to incorrect readings.
- Contamination can occur when the probe is exposed to organic matter or other impurities, resulting in poor contact with the solution. To avoid contamination, clean the probe after each use with distilled water or use alcohol swabs. Also, replace the probe periodically as per the manufacturer’s recommendation.
Electrical Interference
Electrical interference is another issue that could impact the meter readings. Interference could occur from nearby electronic devices or wiring leading to data accuracy issues. To check if there is electrical interference, use the meter in an area with minimal electronic devices. If the problem persists, contact the manufacturer support team for further guidance.
Temperature Fluctuation
Temperature affects the accuracy of residual chlorine meters, and significant temperature fluctuations could lead to inaccurate readings. Follow the manufacturer’s guidelines to store, operate, and maintain the meter at the optimal temperature. Also, ensure that the meter’s temperature compensation feature is activated.
Low Battery
A low battery is another issue that could impact the accuracy of the residual chlorine meter. Low battery power could lead to incorrect or zero readings. To avoid this, regularly check the battery levels and keep spare batteries on hand.
Case Study: Zero Might Not Be Truly Zero
The client of a particular business discovered this the hard way. Their daily DPD testing showed that the chlorine level in their RO was “zero,” Even though there didn’t appear to be any chlorine present, the new membranes they installed degraded in less than a month.
An inquiry revealed that the bisulfite solution used to remove chlorine had degraded into sulfuric acid and was older than three months. This acidic bisulfite was added to the feed water to reduce its pH. The pH ranged from 3 to 6, but it was frequently nearer 5. When the chlorine test was performed at the proper pH, chlorine was found in the water.
How DPD Measurement Works?
Dr. Palin spent a lot of time standardizing a chlorine measurement technique in the 1950s, which led to the development of the DPD colorimetric method. A straightforward, trustworthy technique to measure the amount of chlorine in treated water was needed since chlorination is becoming a more and more popular way to disinfect drinking water. Dr. Palin was a pioneer in the development of the DPD method for determining chlorine and in the work of breakpoint chlorination for water.
When chlorine and other oxidizing chemicals are present, a colorless substance known as N,N-diethyl-p-phenylenediamine (abbreviated DPD) will turn a vivid pink.
Best Practices for Protecting RO Membranes from Chlorine Damage
There are no widely accepted guidelines that direct membrane producers and end users to provide appropriate RO system feed water. The membrane’s lifetime may be significantly reduced if the chemistry of the feed water is not controlled. Overfeeding will degrade TFC RO elements if chlorination is employed to regulate microbiological development in the pretreatment. As a result, extreme caution must be used to shield the membrane from oxidizing chemicals.
Conclusion
Maintaining a functioning reverse osmosis system requires regular chlorine testing. Monitoring chlorine aids in preventing undesired oxidation of reverse osmosis membranes. Chlorine test kit makers have made the test highly dependable and simplified the process. So more attention should be on reverse osmosis and chlorine.
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References
How to Troubleshoot Common Problems with a Residual Chlorine Meter
Chlorine attack on reverse osmosis membranes: Mechanisms and mitigation strategies
https://www.sciencedirect.com/science/article/abs/pii/S0376738817308517
Measuring Chlorine using DPD
https://www.palintest.com/measuring-chlorine-using-dpd/
Impact of Residual Chlorine on Reverse Osmosis Membrane Elements
https://www.axeonwater.com/blog/impact-of-residual-chlorine-on-reverse-osmosis-membrane-elements/
