Friday, April 10, 2015

How to Replace Your RO/DI Filter Cartridges



A RO/DI system is your reef defender, preventing harmful contaminates from ever entering your aquarium water. That is why it is important to regularly maintain your RO/DI filter so your reef is protected from these unseen killers.

The biggest challenge most hobbyists face is not knowing precisely when to change out their filter cartridges. But you’re in luck! Today we are going to show you how to properly monitor your filter cartridges to help you get the most out of your RO/DI system.


The recommendation you often hear in the hobby is to change your sediment, carbon and DI cartridges every 6-8 months and your TFC membrane every 2 years. While this is a good starting point, it can be misleading. The rate at which cartridges are exhausted actually depends on the level of impurities in your tap water and how much water you produce using your RO/DI system.

If your tap water is fairly clean or you don’t use much water, you might be changing out the filter cartridges prematurely (and therefore wasting money). On the other hand, if the quality of your tap water is poor or you use a substantial amount of DI water, you may inadvertently be allowing harmful impurities into your aquarium.

Fortunately, finding out exactly when you need to change your filter cartridges is quite easy and requires only a few simple tools to monitor your filter usage.



The most important tool is a TDS meter which measures the Total Dissolved Solids in parts per million (or PPM for short). HM Digital Inline TDS Meters are among the most popular because they allow you to take readings while your RO/DI system is operating.

Water exiting the DI should read 0 or 1 ppm. When the TDS reading begins to rise, it means that the DI is exhausted and should be replaced. We recommend changing your DI cartridge when your TDS reading is 3-5ppm. It is a good idea to change out your sediment and carbon filters at the same time because they usually exhaust at about the same rate.

If your tap water has an unusually high amount of sediment, you should swap out the sediment cartridge more often. It is easy to tell when your sediment cartridge is clogged because it will turn color from white to a medium brown. You will also see a drop in water pressure inside the membrane housing. Low water pressure will slow down water production and make the TFC membrane less efficient. You should replace the sediment filter cartridge when water pressure drops by 15-20%.

Most aquarium hobbyists change the carbon block filter at the same time as the DI. The carbon filter’s main job is to remove chlorine and chloramine (if you live in a municipality that uses chloramine), so it is a good idea to monitor your carbon cartridge weekly. An exhausted carbon filter may allow chlorine or chloramine through your system which can damage your TFC membrane and exhaust your DI cartridge prematurely.

To test your carbon filter, you will need a total chlorine or free chlorine test kit. Allow your RO/DI system to run for 10 minutes then test the waste water coming from your RO/DI system for the presence of chlorine. If the chlorine measures above 0.5 ppm, it is time to change out the carbon filter.

The TFC membrane is the heart of your RO/DI system. It removes 95-98% of the impurities from water passing through it. With proper care, a TFC membrane will last anywhere from 1-3 years. However, they are sensitive and can be easily damaged. Running hot water accidentally through the system, allowing the system to freeze or the membrane to dry out are the most common reasons why TFC membranes fail.

In order to test your membrane, you will need to measure the TDS of both the tap water entering your system and the product water coming out of the membrane before it travels through the DI filter. Having a Triple Inline TDS Meter is helpful here because you can install one of the inline probes between your membrane and DI cartridge. Otherwise, you can simply disconnect the water line from your DI cartridge and collect some of the RO water coming out of your membrane.

After passing through the membrane, your water should have no less than 95% of the impurities removed. For example, if your tap water TDS is 100 ppm, then the TDS reading after the membrane should be 5 ppm or less. Once your TFC membrane allows more than 5% of impurities to pass through, it is time for a new one.



Flushing your TFC membrane regularly with a flush valve kit will help to maintain optimal performance and extend the life of your TFC membrane. Be sure to pick one up if it was not included with your RO/DI system. Manual flush valve kits are inexpensive and can double the life of your membrane.

Be sure to turn off the supply water line before replacing RO/DI filter cartridges. Spilling a small amount of water is inevitable since there isn’t any way to magically drain all the water out of an RO/DI system after it has been used, so keep a towel or small bucket handy. Gently remove the filter housing with an RO wrench or plumber’s pliers to access and change your filter cartridges and/or membrane. When reinstalling the canisters, be sure to hand-tighten only! Overtightening the canisters may cause leaks or damage. Whenever you change out your reverse osmosis system’s filter cartridges, it is a good idea to produce a couple of gallons first (you can use it to water your plants) before you start making water for your tank.



Knowing how to properly monitor your RO/DI filter system will save you time and money because you will know exactly when to replace your filter cartridges. More importantly, you will be protecting your aquarium from being poisoned with harmful contaminates.

If you have questions about maintaining your RO/DI system or wish to purchase any one of the products featured in this video, feel free to contact our aquarium experts for fast and friendly service. Thanks for tuning in and until next time… take care and happy reefkeeping.



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