RO and RO/DI systems

The water you add to your reef tank is one of the most important factors for long-term success. Municipal and well water sources often contain copper, manganese, lead, nitrate, phosphate, disinfection chemicals and other substances that make it harder to keep a reef aquarium full of fish, corals and other invertebrates. The best defense against these contaminants is a reverse osmosis filtration system. Reef enthusiasts rely on their RO system to ensure the saltwater they mix up is consistently safe for their tanks. The same goes for top-off water. Your reverse osmosis system needs periodic maintenance to keep it working properly. If not, it will allow contaminated water through the system, causing water quality problems in the short and long term. Here’s what you need to know on how to maintain your RO or RO/DI system.

Sediment Pre-filter

The sediment pre-filter cartridge is designed to protect the filtration system from clogging or damage caused by fine particles of iron oxide, sand and mineral scale. These gritty particulates can clog the tiny flow restrictors and, and clog activated carbon filters. The pre-filter is made of spun polyester fibers designed to capture fine particles. The fibers eventually clog with sediment and become coated with naturally-occurring bacteria slime. Clogged pre-filters reduce the flow of water to the rest of the reverse osmosis system. How to tell when to change the RO or RO/DI pre-filters is if the pressure gauge readings decline, otherwise, can be replaced every six months to maintain the maximum water production.

RO/DI cartridges that should be switched out according to use
Replacement filters: DI Resin, Carbon Block, and Sediment Pre-filter

Activated Carbon Cartridge

The activated carbon cartridge follows the prefilter. Your RO may use a granular carbon or a solid carbon block cartridge.  The primary role of the activated carbon cartridge is to remove chlorine and chloramine disinfectants. Activated carbon also removes organic compounds like pesticides, carcinogenic disinfection by-products and naturally-occurring organics.  Carbon cartridges should be changed about every six months when maintaining your RO or RO/DI system. Just like the prefilter, activated carbon will eventually become coated with slime and stop removing chlorine and chloramine.

Reverse Osmosis Membrane (TFC)

Reverse osmosis/TFC membrane for RO and RODI systems
Replacement membrane: Puratek Replacement Membrane Filter Cartridge 100 GPD - AquaMaxx

The heart of the RO system is the reverse osmosis membrane also called the TFC membrane. A semi-permeable membrane is wound around the center core. As water is forced through the membrane, minerals, nitrate, phosphate and metals are stripped from the water and sent to the wastewater line.  The membrane works by “rejecting” the contaminants from the water stream. But they must be flushed away to keep the membrane working.  Over time all RO membranes become fouled with calcium carbonate, bacteria and other contaminants.  How to maintain and extend the life of the membrane is with an RO Flush Kit.  The flush kit loosens and washes away minerals and contaminants that clog the membrane.  The easiest way to monitor the status of the RO membrane is with a TDS meter. Whether hand-held or in-line, the meter will show when minerals and salts begin slipping past the membrane. This signals that the RO membrane needs to be replaced.

Deionization Cartridge

Reverse osmosis alone does not produce extra-pure water. RO removes a percentage of the contaminants. Certain substance like nitrate and silicate are hard for the RO to remove. That’s why a deionization cartridge (DI) is often the last stage in the water purification system. DI uses ion-exchange resins to scavenge out any trace of silicates, phosphate other undesirable substances. Once the resin is used up, it stops working. DI resin cartridges will cause the water to have a “fishy” odor when used too long. If your DI cartridge use color-changing resin to indicate when it needs to be changed, change it as soon as the cartridge changes color. You can also use a hand-held TDS meter to monitor the quality of your water after the DI cartridge.

Infographic of typical maintenance schedule for RO/DI and RO water purifying systems
Recommended RO/DI System maintenance schedule under normal use.

Hoses and Fittings

Tee fitting, locking clip and hosing example.
Tee union, locking clip, and RO tubing

The cause of nearly all drips and leaks are loose fittings. After performing maintenance, hand-tighten all the fittings. If you see a leak, loosen the fitting, reseat the tubing and hand-tighten once again.  Deformed or cracked tubing should be replaced immediately.

Filter Canisters and Housings

RO/DI systems use heavy-duty plastic canisters to hold the cartridges while water flows through the filter media. The RO membrane is contained in a plastic housing that operates under high pressure. When you open the canisters or replace the RO membrane it is important to check the O-rings. They should be smooth with no nicks or distortions. Over time the O-ring will lose its flexibility and ability to seal the canister. This can cause a leak. Replace the O-rings every year. It’s a good idea to always have a spare in case you spot a slow drip.