RO/DI systems
The membrane is the heart of your reverse osmosis systems and is where a majority of the contaminants are rejected from the water. Based on the frequency of flushing, how much water you produce, and the quality of that source water, you will need to change your RO membrane every 3 - 5 years.  This is a big window because the usage and system maintenance play such a big role in how long our membrane lasts.  

How To Monitor RO Membrane Performance

In order to monitor membrane performance, you want to calculate the rejection rate. Most RO membranes have an expected rejection rate of 96-98% meaning they will remove 96-98% of total dissolved solids from the water.  When a membrane can no longer meet that 96-98% rejection rate, it is time to replace it. 

Some membranes are rated for 99% rejection while others may be rated for 97% rejection. The point is that once the membrane falls below the expected rejection rate, it's time to replace it. It should also be noted that water pressure effects membrane performance so you can only expect to get the level of rejection when operating under ideal pressure.  Reduced water pressure going into your RO/DI system will result in less efficient rejection out of your membrane.  

How to Calculate Rejection Percentage Rate

  1. Measure the TDS of your incoming source water (TU - Total Units)
  2. Measure the TDS of the RO water exiting the membrane (before it goes through the DI resin). (LU - Leftover Units)
  3. Subtract the TDS of the RO water (LU) from the TDS of the source water (TU) to get the total number of rejected PPM (TR - Total Rejects)
  4. Divide the Total Rejects (TR) by the Total Units (TU) and then multiply by 100 to get the rejection percentage rate.

Rejection Rate % = Total Rejects / Total Units * 100

For example, if the source tap water measures 285 ppm and your RO water coming out of the membrane measures 8 ppm using a TDS meter.

  1. 285 - 8 = 277
  2. (277 / 285 ) x 100 = 97.2% rejection Rate

In the above example, the membrane is performing below the expected 98% rejection rate but is within a reasonable range.  It is reasonable to give your RO membrane a 2% +/- variance around the rated rejection rate but when it starts to drop consistently, you know the membrane needs to be changed out. Just keep in mind that the lower the rejection rate, the faster you will exhaust your DI resin media because that means more contaminants are escaping the membrane. This effect will get progressively worse over time if the membrane is not replaced so allowing your system to operate with a clogged or exhausted membrane costs more in the long run.  

Required Tools

  • Towel
  • Bucket
  • Plumbers Pliers (optional)
  • Needle Nose Pliers
  • New RO Membrane

RO membrane

1. Remove Old Membrane

Shut down the source water and disconnect the three tubes from your RO membrane housing. Before removing the tubing, take a picture of the RO Membrane Housing so you know where to reconnect the tubing after you have replaced the membrane. 

BRS RO/DI systems utilize quick-disconnect Mur-Lok fittings for easy removal. Just push down on the plastic collar, then pull the tube out of the fitting while keeping pressure on the collar. If you have trouble pushing the collar down with your fingertips, you can use a pair of needlenose pliers to push down on the collar while pulling out the tube. 

Changing RO membrane

It is normal for some water to leak out of the tubes and membrane housing, this is why you have the towel. 

Once the tubes are disconnected, you’re going to remove the RO membrane housing from its clips. You do not need to remove the clips themselves from the RO/DI unit (they are attached to the system via screws). Just grab the membrane housing firmly and pull up and away from the unit and the housing will snap out.

Changing RO membrane

2. Unscrew The Cap

Once the membrane housing is removed you want to unscrew the cap from the membrane housing. Remember, righty-tighty, lefty-loosey. These caps can be difficult to remove, especially after 3-5 years.  Ideally, you can unscrew it with your hands, using a towel or rag for friction can help you get a grip on the cap. Worst case, use plumbers pliers with towel wrapped around the plastic housing so as not to damage it. The pliers will give you some leverage to break the cap free which can then be unthreaded using your hand.  

Once the cap is removed it will reveal the membrane inside. Make sure to keep track of the two black O-rings on the membrane housing. One of them should be in the cap and the other on the main body of the housing.

Changing RO Membrane

3. Remove the Old Membrane

RO Membranes are simply held in place with pressure and o-rings. There are no clips or threads meaning all you have to do is pull it straight out of the housing.  If you have small enough hands to reach in and pull it out then great! For the rest of us, needle nose pliers will do the trick. It’s best to remove it by grabbing the plastic spindle in the middle. Keep in mind that the membrane itself is being disposed of so don’t worry about damaging it with the pliers. 

Grip the white spindle firmly and pull straight out.  The membrane should release and easily slide right out so long as you have a good grip on it. 

RO Membrane Orientation

4. Install the New Membrane

Installation of the new membrane will be the opposite of removal. Remove your new membrane from its packaging and insert it into the membrane housing. In case you forgot the orientation, the end of the membrane with the 2 small O-rings on the end of the plastic goes in first. See the image above.

Push the membrane in by hand until it is snug and pushed all the way into the housing as far as it will go. It is important to seat the RO membrane correctly inside the housing and push it all the way into the housing before screwing on the cap.  DO NOT USE THE CAP to force the RO membrane down into the housing. 

Once the membrane is seated correctly, screw the cap back onto the housing as tight as you can by hand. 

5. Reconnect the Tubing

At this point, it is easier to reconnect the tubes before placing the housing back into the clips.  Reference that picture you took at the beginning.

  • The red tube connects to the inlet of the RO membrane (cap).
  • The black tube is the wastewater port (off-center port)
  • The blue tube is the product water. (near-center port on the end of the housing)

Push the RO membrane back into the housing clips and you are all set! Be sure to check your tubing connections for any leaks and you will want to rinse the membrane free of any residue before collecting the water. Removing your DI resin while rinsing the membrane is also recommended to avoid unnecessary exhaustion of the DI resin.

How To Rinse A NEW RO Membrane:

  1. Remove the DI Resin filter(s) from the final canister of your RO/DI system. Be sure the empty canister is securely screwed back onto your RO/DI system so it won't leak before proceeding.
  2. Turn on the source water and let the RO/DI System produce 1-3 gallons of product water without the DI resin. This will rinse any residue from the new membrane.
  3. Dispose of that water.
  4. Replace DI resin back into the final canister.
  5. Flush the RO membrane using the Flush Valve for 1-2 minutes.
  6. Start collecting water for use in your aquarium.