How to Know When to Change your RO/DI Filters

Reverse Osmosis System with filter stages labeled

Your RO/DI system is your front-line defense against contaminants that could make their way into your reef tank via your water supply. It is critical to change out the filters on time before those harmful contaminants and toxins can sneak through and wreak havoc in your aquarium.

Monitoring the performance of each of the individual RO filter stages is going to be the best way to get the most bang for your buck and ensure your changing out the filter cartridges before putting your tank at risk. Being everybody has different source water quality, there is no generic timeframe we can give you and exactly why you should be monitoring performance closely. Thankfully, monitoring your individual RO/DI system filters is pretty simple with just a couple of tools and a bit of know-how.

Required Supplies

Dirty RO/DI sediment filter

When To Change Your Sediment Filters

When you see a pressure drop going into your RO Membrane via the inline pressure gauge, it's time to swap out the sediment filter.

When the sediment is clogged, you will notice a pressure drop on your inline pressure gauge that is installed between your carbon block filter and RO Membrane. It will also change color or start to darken over time as it becomes clogged

If your RO System does not have a pressure gauge, it's extremely useful and easy to install. We have complete Pressure Gauge Install Kits and replacement gauges available as well as a great video with detailed instructions to show you how to do it.

Read Instructions: How To Install A Pressure Gauge On Your RO/DI System

The exact time frame in between necessary changes will vary based on your particular source water quality but changing these out no less than every 6-12 months is typical. We recommend the ROSAVE.Z Depth Sediment filters because they contain truly progressive filtration fibers meaning they will last longer and outperform almost any other sediment filter in its price range.

When To Change Your Carbon Block Filters

Test for the presence of Chlorine and Chloramines using Total Chlorine Test Strips in your RO System's wastewater.

To test for total chlorine, first, run your RO system for 30 minutes and then collect a sample of your wastewater from your RO system wastewater line. Submerge the test strip into your sample for 2 minutes and wait another 10 minutes for the color indication to develop. If you show more than 0.5 PPM of Total Chlorine in your wastewater, swap out your carbon block filters right away.

Watch Video: Learn How To Test Your RO/DI Water For Chloramines

Carbon block filters are the 2nd stage of filtration in your RO/DI system with the most important function of removing chlorine and chloramines. This is critical to ensure long-lasting DI Resin and eliminate the risk of toxic ammonia making its way into your aquarium.

Being pesky Chloramine use is on the rise in municipal water supplies, using two carbon block filters is becoming quite common to ensure complete chloramine removal. On that note, not all carbon block filters are created equal and some will not remove those harmful chloramines. We recommend the use of our BRS Universal Carbon Block filter which targets both chlorine and chloramines.

The same advice applies in terms of a typical time frame in between changes, everyone's situation will be different. On average you should be changing out carbon block filters no less than every 12 months. Carbon block filters do become clogged over time which will also be indicated by a drop in input pressure, just like a clogged sediment filter. Therefore, monitoring that input pressure is also a great indicator of your carbon block filter performance alongside the presence of chlorine/chloramines.

When To Change Your RO Membrane

Monitor your RO Membrane rejection rate using an inline or handheld TDS meter. When product water TDS starts to climb, your RO Membrane should be changed out.

You will need an inline TDS meter to test your membrane. Watching the TDS reading of water coming out of your RO Membrane, before it goes through the DI Resin, is the key. When this number starts to climb, you need to change out the membrane. Calculating the "rejecting rate" is a good way to gauge how well your membrane is performing.

Rejection Rate = Percentage of TDS removed by your RO membrane.

For example, if your tap water measures 100 PPM using a TDS meter and your RO water coming out of the membrane measures 2 PPM, your membrane is performing with a 98% rejection rate. RO Membranes are typically rated for 96%-99% rejection rates and when you start to see this performance drop below the rated 96%, it is time to get a new membrane. Having the inline TDS Meter installed on your RO/DI system is extremely valuable for active monitoring.

The RO Membrane is the heart of your RO/DI system and is doing most of the work, it also the most expensive and critical filter in your RO/DI System. How long it lasts is based on tap water quality and how much RO/DI water your producing so monitoring that TDS is critical. It is not uncommon for an RO Membrane to last for 2+ years with proper pre-filter changes (carbon & sediment) and the regular use of a flush valve.

When To Change Your DI Resin

Monitor TDS of your product water and use color-changing DI resin to indicate when it is time to swap out the DI Resin.

Using an inline TDS meter to measure the TDS of your DI water coming out of the DI Resin filters is your best way to directly monitor performance. Anything other than 0 PPM on the TDS meter is not safe for use in your reef tank. We recommend the use of color-changing DI Resin which allows you to get ahead of the resin and change it out before it has been exhausted 100%. When 75% of your color-changing DI resin has changed color, it's time to swap out the resin. DO NOT WAIT UNTIL 100% OF THE DI RESIN HAS CHANGED COLOR!

Watch Video: How To Replace DI Resin

Your DI Resin is the final stage of filtration that absorbs any and all contaminants leftover by the RO membrane. This is what gives you that 0 TDS purified water that is safe for use in your saltwater aquarium. If you allow for even 1-2 PPM of contaminants to slip through your DI resin filters, this can stack up over time in your display tank causing a myriad of problems pending the exact contaminant.

If you're using refillable DI resin, be sure to pack the resin as tight as you possibly can inside the canister to avoid channeling of water and ensure proper performance.

RO/DI systems will have 1, 2, and 3 stage DI Resin filters which are chosen based on your particular demand for 0 TDS product water and the quality of your source water.

  • Mixed Bed DI Resin: 50/50 blend of cation and anion removal resins for RO/DI systems with 1 or 2 stage DI filters.
  • Single Bed Pro Series DI Resins: Contain only Cation or only Anion resin in a single cartridge and are used in the 3 Stage Pro Series DI Systems.

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