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Currently, aromatic polyamide is the primary material used in the makeup of semipermeable membranes in reverse osmosis filtration systems. Unfortunately, these membranes are easily damaged by chlorine, which is found in most municipal water supplies.
Over time, the chlorine attacks cause the membranes to lose their performance, leaving your water susceptible to inadequate filtration.
Today we are going to look at how exactly chlorine can damage an RO membrane, what needs to be done if your RO membrane does become damaged, and possible alternatives the industry is trying out to prevent this from happening entirely.
When a reverse osmosis membrane is exposed to chlorine, oxidation occurs. This then causes the membrane’s salt rejection rate to decrease. The speed at which this happens depends on the amount of chlorine that is in the water, but it usually takes between one and several weeks.
The damage causes corrosion of the mesh in the membrane, which makes the size of the pores more prominent. This means the membrane cannot prevent all the impurities in the water from getting through.
To prevent this, dechlorination of the water before it reaches the RO membrane is necessary, such as with a carbon pre-filtration step or a point-of-entry carbon filter.
After exposure to 1 mg/L of free chlorine for between 200 and 1000 hours, an RO membrane is going to decrease in performance. To prevent this from happening, the feed water must be dechlorinated before reaching said membrane, as it has very little chlorine tolerance.
Also, the damaging process is, unfortunately, irreversible. An acceptable limit of free chlorine would be well under 1 mg/L.
To protect your RO membrane from a chlorine attack, the best option is to ensure your RO system comes with a carbon pre-filtration step. This will trap the free chlorine in its pores before it has a chance to get to the RO membrane.
Luckily, most if not all RO systems already come with this feature. Another option would be to have a point-of-entry carbon filter for your whole house, to dechlorinate all the water coming into your home.
Studies are also underway to try and find alternative RO membrane materials that are resistant to chlorine or use alternative disinfection methods for source water.
If the damage is already done, and your RO system’s membrane has been oxidized, it will need to be replaced, as repairing it is not possible. This is a relatively simple procedure, but be sure to wear rubber gloves when replacing the membrane so the oils on your skin do not damage the new one.
You will also need to replace your filters to make sure that the water is being appropriately pre-filtered, and so that it doesn’t immediately begin damaging the replacement membrane.
As chlorine remains the most widely used method to disinfect water supplies, the design of a reverse osmosis membrane that is not vulnerable to damage from chlorine, yet works just as well as the aromatic polyamide, is very much needed.
Good news is, for the home water filtration industry, many different membrane types are being designed and tested.
The areas being looked into include surface protection of the membrane, coating the membrane with hydrophilic materials, and using different materials to build the membrane altogether.
Research is being performed to find alternative materials for parts of the RO membrane that are vulnerable to chlorine attacks. Materials currently being studied include variations of the current polymeric material, hybrid materials, or polyamide with polymers and sulfonated poly(arylene ether).
They have been found to improve the membrane’s chlorine tolerance but reduced its salt retention percentage to an unacceptable level for a one-pass membrane.
Membrane coating involves immersing the membrane in a casting polymer solution. This creates permanent membrane surface changes that protect the areas that are sensitive to chlorine. The challenge is to find a way to maintain the level of water permeability and salt retention.
UV, ozone, and silver are all being considered as alternatives to chlorine for water disinfection.
UV and ozone disinfection, while effective, each have their own drawbacks. UV disinfection’s success very much depends on the water being clear of particulate. Ozone disinfection can be corrosive and needs corrosive-resistant materials in any system it passes through, which adds considerably to expenses. Silver seems a viable alternative, though, and without the harmful toxic by-products.
The results of studies using this 5-step process for water disinfection have been very promising, showing almost 99% microbial removal from water. Its compounds can be made at a low cost, but it needs to be activated by a high-energy UV source.
Swimwear material is even being tested. Chlorine-resistant polyester has been used to create an alternative RO membrane with comparable performance. The work on these is still in the early stages, though, and needs to be commercialized.
If you have any questions about the effect of chlorine on an RO membrane please don’t hesitate to leave a comment below!
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