Top 5 Frequently Asked Questions About RO/DI Systems
Keeping your reef healthy starts with the water you use. Even the best salt mix cannot make up for poor source water filled with impurities, chlorine, and dissolved solids. That is where RO/DI systems come in. If you are setting up a reef tank or looking to improve your maintenance routine, here are answers to the five most common questions about RO/DI systems and how they keep your tank thriving.
1. How Many RO/DI Stages Do I need?
How many stages your RO/DI system should be is going to mostly depend on the quality of the water you're starting with and which contaminants you're trying to remove.
City Water
- Treated with Chlorine: A 4 Stage or larger system will usually do the trick. 4 Stage systems typically feature a single carbon block, designed to remove chlorine.
- Treated with Chloramines: A 5 Stage system or larger. These systems have two carbon blocks designed to remove both chlorine and chloramines, so you're covered either way.
Well Water
- Private wells vary widely from one to the next. A 5 Stage system is usually a pretty safe bet if you don't know what sort of contaminants you're trying to remove. If you know your well water is on the cleaner side, a 4 Stage system is a great option as you won't have any chlorine or chloramines to remove.
A Note About CO2: Private wells can have higher levels of dissolved CO2, causing rapid DI resin consumption. In extreme cases, a 7 Stage system is the best choice due to the Pro Series DI Resin configuration.
2. Which is Best for Saltwater Aquariums: RO or RO/DI?
Unless you know exactly what's in your source water, RO/DI is best for saltwater and reef aquariums.
Think of RO as the heavy lifter and DI as the final polish.
- Reverse Osmosis: Water is pushed through a semi-permeable membrane that filters out up to 98% of dissolved solids and contaminants.
- Deionization: The DI resin helps by removing nearly all of what might slip past the RO membrane, such as silicates or phosphates.
Together, they create a two-stage purification process that is far more effective than either one alone.
3. How Often Do I need to Replace the Filters?
Filter replacement frequency depends on three main factors:
- How much water you're running through the system
- The quality of your source water
- The type of contaminants you're trying to remove
A Filter-by-Filter Guide:
Sediment Filter: The pressure gauge measures the pressure of the water entering the RO membrane. As the sediment filter clogs, the pressure will drop. Once you see this starting to happen, it's time to swap out that sediment filter.
- Tip: If the pressure is low even when the filters are brand new, you may want to consider installing a booster pump to improve performance and efficiency.
Carbon Block: This one is harder to measure, so we recommend changing out carbon blocks on a regular schedule. Usually once every 6 to 12 months. Opt for every 6 months if your source water contains chloramines or are producing large volumes of RO/DI water.
RO Membrane: The membrane usually lasts upwards of 3 years if the pre-filters are maintained properly. You can also compare the incoming water's TDS to that of the water leaving the RO membrane to determine the rejection rate. Most RO membranes have a rejection rate upwards of 98%. In other words, if the incoming water is 100 TDS, the water leaving the membrane should be close to 2 TDS.
- Did You Know? Cold water temps in the winter and low pressure can reduce RO membrane rejection rate.
DI Resin: TDS measures the total dissolved solids in your water. You should see 0 TDS water leaving your RO/DI system. Anything higher indicates exhausted DI resin. If you don't have a TDS meter, use color-changing DI resin and change it just before the color indication on the resin reaches the top of the cartridge
- Tip: The color change starts at the bottom and works its way up the cartridge as the resin exhausts.
4. Do I Need a Booster Pump?
It...depends. Booster pumps do exactly what their name suggests. They boost the water pressure entering the RO system. Higher water pressure generally means better RO membrane performance, faster water production, and a better product to waste water ratio.
Ideal water pressure for an RO system is somewhere in the range of 50-90 PSI.
You should consider a booster pump if the pressure entering your RO system is below 50 PSI.
Learn More: How to Install a Booster Pump on an RO/DI System
5. How Much Water Does an RO/DI System Waste?
If you're new to RO systems, you might not know that an RO membrane has two outputs on it. One for filtered "product" water and another for rejected "waste" water.
Assuming the water pressure to the system is within the optimal range:
Single RO Membrane: 1 gallon of product water to 3 gallons of waste water.
Two RO Membranes: By installing a second membrane, you can cut this ratio down to 1 gallon of product water to 1.5 gallons of waste water.
We call this a "Water Saver" system because it takes the waste water from the first membrane and re-filters it through a second membrane. Not only does this cut down on waste water, but it also produces filtered product water about twice as fast as a single membrane RO system.
It's important to note that using a water saver dual-membrane setup requires slightly higher minimum water pressure of 65 PSI vs the 50 PSI of single membrane systems.
Learn More: How to Install a Water Saver Upgrade Kit
Ready to Get Your Own RO/DI System?
Adam has been reef keeping since 2010 and got his start on the Bulk Reef Supply Customer Service team. He believes the best tanks are built around what you enjoy, not what’s trending. Adam is especially interested in simplifying the hobby, exploring the latest gear, and helping others focus on long-term success over chasing perfection.
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