The reverse osmosis membrane is the most important part of any RO system. The RO membrane with other filter cartridges determines the overall performance of the reverse osmosis process when it comes to filtering water.
What is a reverse osmosis membrane?
A reverse osmosis membrane is a Thin-film Composite membrane (TFC) which is made up of three or more layers of semi-permeable materials that work as a molecular sieve that allows water molecules to pass through while stopping wide variety impurities and contaminants.
This three-layer configuration is what gives the RO membrane a high rejection and filtration rate against undesired materials as well as good mechanical strength.
The pores of the membrane are sized from about 0.001 to 0.0001 micron which basically excludes most of the dissolved contaminants in the water as it passes through the membrane.
Aside from the Thin-film Composite (TFC) membrane, another common type of RO membrane used in household RO systems is the Cellulose Triacetate (CTA) membrane. What sets these two types apart is their filtration ability and chlorine tolerance.
CTA membranes are chlorine tolerant but tend to be more susceptible to organic fouling plus they have a rejection rate of only 93% against standard contaminants. TFC membranes, on the other hand, have an average rejection rate of 98% and much less susceptible to organic fouling.
However, they may require carbon pre-treatment filters when dealing with chlorinated municipal water as they are mostly quite effective at purifying chlorine-free water.
How does an RO membrane work?
As mentioned earlier, the RO membrane is made up of several layers of materials. On one side there is the polysulfone layer, on the other side is the impenetrable polyamide layer which is the actual purifying material and in the center is a perforated collecting tube.
All these components of the membrane are then placed in one housing chamber which is designed to separate the filtered water from the waste or rejected water.
After the initial filtration through sediment and carbon filters, the feed water pressure forces the partially filtered water through the membrane chamber where it flows in a spiral direction starting from the top layer down to the center tube.
Inside the chamber, the polysulfone filtration layer allows the pure water to pass through leaving behind contaminants and other undesired particles.
The polyamide film is nanofiltration membrane high levels of dissolved solid rejection and water permeability. It possesses the properties that facilitate selective water permeation while restricting most other soluble contaminants.
From the polyamide layer, the purified water is channeled into a collecting tube in the center of the spiral wounds of membranes. The perforated collection tube then directs the purified water into a pressurized storage tank where it’s stored.
By pushing the feed water through the RO semi-permeable membranes, the natural osmosis process is reversed and overcame thereby leaving impurities behind down to 0.0001 microns while the filtered water molecules continue through the system.
The water is separated into two pathways.
The wastewater containing the dissolved compounds and other organic and inorganic impurities is subsequently flushed down a separate drain line while clean, purified water goes into the RO tank for storage.
The polyamide layer is selected primarily due to its high permeability to water as well as relatively high impermeability to various kinds of dissolved substances like salts and other impurities including small, unfilterable molecules.
It’s responsible for the high rejection rate which is almost 98.99%.
One innovative thing about the design of RO membranes is that it utilizes the same water pressure and flow to clean and rinse the membrane layers throughout the process.
The contaminants are flushed out and away on a separate line hence reducing waste build-up and accumulation inside the membrane chamber.
This feature protects the membrane from potential clogs and obstructions that often caused by bio-fouling, contaminant saturation, and scale hardness.
The lifetime of an RO membrane
What is the lifetime of an RO Membrane and when should you change it for each stage?
The RO membrane is actually the component that does all the work. The pre-filters prepares the water by removing most of the larger impurities that may cause damage to the RO membrane.
Typically, the lifetime of an RO membrane can be between three to seven years based on the water quality as well as application.
For softened water (with a water softener), you can get about 5 years out of your RO membrane provided you replace the other pre-filters regularly.
For hard water or if you never flush the membrane, it may last a little shorter than 3 years. Even while the RO membrane is capable of rejecting most of the minerals, some of it can come out of solution and end up clogging several parts of its surface.
Therefore, you may need to replace the membrane in one or two years.
A TDS meter can help you test your membrane and when the TDS starts to climb, then you will know that your RO membrane is beginning to go bad.
To be on the safe side, we would recommend changing the membrane at least every three years.
This video will show you how to determine when you should change the RO membrane.
GPD of an RO Membrane
The GPD (gallons per day) of an RO membrane will determine the amount of water you can get on average. It’s the actual production rate of the RO membrane in terms of how many gallons of purified water it can produce in a day.
For under-the-sink and countertop home RO membranes, the common production rate you will find are 50, 75, 90 and 100 GPD.
There are also other RO membranes with much more capacity which are meant for large RO systems for the whole hose. These can have a GPD of 200, 300 or 500.
A 50 GPD-rated membrane is ideal for a common household of around 2 to 4 people while 75 90 and 100 GPD RO membranes are best suited for families of 5 to 8 people.
A 300 GPD or 500 GP RO membrane will be adequate if you are using the RO water for other purposes other than for drinking like daily laundry, for mopping the floor, and to wash your utensils.
The GPD performance can dramatically go up or down depending on such factors as water pressure, water temperature, the total amount of dissolved solids in the water being filtered, condition of pre-filters, etc.
If your RO system uses a standard-sized membrane, then you can change it to increase the GPD in order to increase the daily production capacity of your RO system plus the speed at which the purified water is produced.
However, one thing you should note is that if you change your RO system membrane from the original size to a higher capacity, then you must change the flow restrictor as well because it is sized to go together with the RO membrane.
The size of the flow restrictor should properly match with the RO membrane capacity or rather the GPD. For instance, if you change from a 36 GPD membrane (uses a 350 ml per minute flow restrictor) to a 50 GPD membrane, then you will need a 500 ml per minute flow restrictor.
Flow restrictors help create pressure throughout the Ro membrane as well as maintain a proper ratio of permeate (product) flow and reject flow (to the drain).
Therefore, the RO membrane GPD much match the flow restrictor to ensure that a proper balance is maintained between pure water and wastewater.
Not matching them properly could lead to excessive wastewater flow to the drain line or even cause premature membrane fouling.
Replacing the RO membrane will depend on the quality of your input water and the type of membrane you are using in your system.
Types of RO membrane replacement
Universal RO Membrane
Universal RO membranes are standard TFC RO membranes found in most common household RO systems. For this kind of membranes, they can last for about 2 to 3 years.
Therefore, you will have to replace them every 3 years, but they can go up to 5 years if you are dealing with softened water and take good care of your filter cartridges by replacing them on time.
You will probably need a TDS meter to help you check the TDS level in the purified water produced. This is the best way to determine when you should replace the RO membrane.
Branded RO Membrane
When it comes to replacing branded RO membranes, you will need first to figure out the brand and model of your RO system. You have to determine the brand and model number so that you can get the right replacement membrane or filter cartridges.
You can find this information on the plastic or metal manifold which holds the filters in place or on a small sticker on the back part of the manifold. However, the labels on the unit may not indicate the brand or model of the RO system.
The good thing though is that most brands of RO systems usually provide great customer support and even send the customers reminders of when it’s time to replace the membrane or filters on your system.
They may not also come with clear instructions on how to replace the RO membrane or filters but we are going to help you know how to do that now.
RO Membrane Replacement Procedures
Before starting, you need to turn off the water supply going into the RO system, close the ball valve on the storage tank and then turn on the RO faucet so that the water pressure can bleed out.
Once, you’ve done that, follow the procedure given below to replace your old RO membrane.
Disconnect all the tubing from the RO membrane housing cap.
Unscrew the RO membrane end cap from the housing and slide out the old membrane. You may need a needle-nose plier to pull it out as it may be a little difficult.
Rinse the inside of the membrane housing using warm water without soap (you can remove all the other cartridges and sanitize the entire system at this point if necessary)
Insert the new membrane inside the housing making sure that the O-ring sits securely and correctly otherwise your system won’t work. The O-ring end should go first.
Screw the end cap back onto the membrane housing while also ensuring that the O-ring is still securely in place.
Re-connect each tubing into their correct location making sure they are properly secured as well. You should re-install the tubing to correct fittings as far as possible.
Now, open the water supply valve and turn on the RO faucet. Let it on until there’s a steady dripping. Check for leaks.
Turn off the RO faucet and let water flow into the storage tank and check for leaks again. If there’s none, then your RO system is ready for use (don’t drink the initial water produced after the membrane replacement)
Side notes: RO membrane and filter cartridge replacement
Like mentioned earlier, the reverse osmosis membrane and filter cartridges are the most vital part of an RO system. The RO membrane is the component that actually does all the work.
The other filter cartridges like sediment and carbon filters are equally important as they prepare the water for the actual filtration by the RO membrane. They protect the RO membrane from getting damaged to ensure optimal performance.
Therefore, you need to take good care of both the RO membrane and the filter cartridges and replace them at the right intervals. For filter cartridges, you are required to replace them every 6 to 8 months.