RODI 101 – Where to Begin and What You Need to Know

Tap Water Database

The water in our tanks is the lifeblood.  If the water isn’t right, the tank won’t be right.  Starting off with the “purest” water possible is essential if you want to avoid issues down the road.  There’s a lot of confusion amongst new and old reefers alike about RODI systems, how they work and what they should look for in quality RODI units, membranes, filters and DI cartridges.  This article has been in the works for a long time and it’s my goal to stick to the important basics and not get too technical.  Having said that, there’s a lot to cover and I hope you learn something!


What is RODI water?

Over 30 years ago Charles Mitsis the owner of Spectrapure, introduced RO water to the aquatics hobby.  From there, RODI was introduced.  But what is RO and RODI water?  RO (reverse osmosis) forces water through a semi-permeable membrane.  The brine (waste water) is expelled and what’s left travels to the DI stage.  DI (de-ionization) is the process in which the RO water travels through special DI resins which absorb ions the membrane couldn’t reject.  The result is “pure” product water that we put in our tanks.

Why is “pure” RODI water important to us?

Depending on your water source, your water could have contaminants such as chlorine, chloramines, pesticides, nitrates, phosphates and heavy metals.  You do not want this stuff in your tank.  There are also good elements in our water, but it’s best to strip everything from your water to ensure you have a “pure” starting point.  Your salt mix will add the beneficial elements back to you water, that are removed through the RODI process.  TDS (total dissolved solids) is the measurement of organic and inorganic substances in your water – both good and bad.  Our goal is to have product water that reads 0 TDS.  Until this point I have been using the term “pure” water because not all 0 TDS water is equal.  There are measurements beyond 0 TDS and they should be addressed.  What is commonly defined as “ultra-pure water” is water that has a measurement of 0.05 micro-siemens (18 megohm resistivity).  What our TDS meters read is “indicated TDS” and they are not able to detect micro-siemens; this requires special equipment we don’t have.  To ensure you are getting “ultra-pure water” you will need to use the appropriate equipment and buy your RODI supplies from a trusted source who has such equipment.


The RODI unit.

There are many different RODI units out there and a lot of them function in the same manner.  You will want to pay special attention to the features of each unit and if those features will meet your needs both now, and in the future.

What to look for:

  • Flush valve – There are two types of flush valves: manual and auto. This valve bypasses the flow restrictor, forcing more water through the membrane and out the waste line, after you have made your product water.  (Note: this is not as beneficial as installing a manual valve pre-DI and diverting the TDS soiled membrane water down the drain, before collecting product water).
  • Pressure gauge – This tells us the PSI going into the RODI unit. It is important to operate your unit within the manufacturer’s specifications.  The pressure gauge will also be an indicator of fouled sediment and carbon filters (see below for more info on filters).
  • Booster pump – The recommended operating PSI is generally 40-80psi. Some water sources such as wells, may not produce enough PSI for the RODI unit to run efficiently.  A booster pump increases the PSI.
  • TDS meters – TDS meters will read indicated TDS and will help at different steps in the RODI process. How many TDS meters will depend on your setup.  There are inline TDS meters which come standard on a lot of units and there are handheld meters.  Handheld meters are much more accurate as they can have built-in auto temperature compensation (ATC), which inline meters do not.  Handheld meters can also be calibrated and most inline meters cannot, or it’s difficult to do. It’s best to calibrate your handheld meter at least once every 12 months.

Ideally you want:

5(+) stage RODI – Dual TDS meter to read incoming (tap) and post membrane (RO water).  Another dual TDS meter to read post “roughing” DI stage and product water.

4 stage RODI – Triple TDS meter to read incoming (tap), post membrane (RO water) and product water.

The proper TDS meters are important because you will use them to determine membrane rejection rate and when to replace your DI cartridge(s).  More info on that below.

  • Construction – Some RODI units have mounting brackets made of metal and some are made of plastic.


Membranes, filters and cartridges… oh my!

Sediment – The first stage in a typical RODI unit is the sediment pre-filter.  This traps certain sized particulate matter that are in your water.  An appropriate sediment filter will ensure you do not clog your carbon filter or membrane.  It is common for the sediment filter to become discolored over time and is not necessarily an indication it is bad.

Carbon – A good carbon block pre-filter will remove chlorine, organics, heavy metals, trihalomethanes, pesticides and many other chemical pollutants. It will also break-up chloramines.

Both the sediment and carbon filters are rated in microns.  This determines what size particles the filters will block/allow.  Micron ratings are commonly: 10, 5, 1, .5, .2.  The higher the number, the larger particles will pass through (less efficient).  It is extremely important to note that not all micron readings are the same.  There is absolute and nominal micron ratings.  Absolute micron will remove all particles larger than the specified micron rating.  Nominal will only remove a certain percentage of particles larger than the specified micron rating.

What to look for:  You can determine the micron size you need by experimenting, since water quality is different from area to area.  Keep in mind the more particles you let through, the faster you may deplete your carbon and/or membrane.  Also look for filters that are rated absolute/near absolute micron rated, not nominal.

When to replace: Generally, after four to six months of use or if you see a PSI drop of 15-20% (that means one of them could be clogged).  Replace the carbon filter earlier if you detect chlorine, test kits are available and they’re very cheap.

Membrane – There’s a lot of confusion and misinformation out there about membranes and their performance, or perceived performance.  The membrane is the heart and soul of the RODI unit.  Membranes are rated in GPD (gallons per day of product water) and rejection rate (in percentages).  The % rejection rating for a membrane is an indication of how much of these impurities will pass through the membrane and on to the DI cartridge(s). The higher the rejection rate, the more efficient your membrane is and that results in a longer lifespan of your downstream DI cartridge(s).  By starting with a membrane with a 99% rejection rate, you are ensuring cost savings down the road.  You will typically replace more DI then membranes.  A 2% increase in your rejection rate will double the life of your DI.

What to look for:  99-98% rejection rate and a company that tests their membranes for performance and can stand by their rejection rate.  Bonus points if you can find membranes that are specially treated for enhanced performance.  Membranes that are marketed with rejection rates below 98% will cost you more money in the long run.

When to replace:  Typically once the rejection rate drops below 95%, but this may vary depending on your membrane and how well you want to protect your DI.  Checking your rejection rate is pretty easy.  Write down your incoming TDS (X) and post membrane TDS (Y).  Then follow this simple equation:

X – Y = Z

Z / X = Rejection rate.

*  Waste ratio – This is the ratio of waste water to product water.  Most units we use are designed to operate within 3:1 to 4:1. Special, high efficiency units can operate at a 1:1 ratio.  The ratio is controlled by a capillary flow restrictor that installs inside the waste hose, which you can cut down to size to adjust the waste ratio.  Waste ratio is effected by water temperate and membrane GPD, so it’s good to check your ratio if you switch membrane GPD or when your tap water temperature changes.

* Dual membranes – Need more product water, faster?  Buy an RODI unit with dual membranes and that can double your output.  Or, add a piggy-back kit to your existing single membrane RODI unit.  You’ll be able to produce water faster and may reduce the waste ratio, but it’s going to come at a price.  Possibly a heavy one at that.  The adverse effects are that the effective rejection rate will go down and you’ll burn through DI faster.  Remember, just a 2% drop in rejection rate will cut the life of your DI in half.

Deionization – The DI cartridge(s) is the last stage in the RODI process, which removes leftover TDS that was passed on from the membrane.  The result should be 0 TDS water.  There are different types of DI cartridges, and some units are designed to use different number of DI cartridges.  In a two stage DI RODI unit the first cartridge is the cation (roughing stage) and the second is anion (polishing).  Cation removes almost all positively charged ions except hydrogen, and anion will remove negatively charged ions.  What you have left is hydrogen hydroxide.  There is also a DI cartridge called “mixed bed” which contains both cation and anion resins.  This would typically be used in an RODI system that only has one DI cartridge.  Some people do run more than two DI cartridges because of their specific needs, but I’m only going to review the two basic models that are most commonly used (4 stage and 5 stage).

What to look for:  A company that hand sources, mixes the resins in-house and diligently tests their resins.  DI is our last line of defense and if it’s not performing efficiently, you’re not going to have high quality product water and you’ll waste money.  Remember earlier I mentioned we strive for “ultra-pure water” that has a measurement of 0.05 micro-siemens (18 megohm resistivity).  That’s our goal.

When to replace:  Replace the last DI cartridge when the product water is above 0 TDS.  In systems with a roughing stage DI, replace that cartridge when the TDS reaches half of the post membrane TDS.  For example – if the post membrane TDS is 10, replace the roughing stage DI when the TDS for it reaches 5.

* Some DI cartridges are “color-indicating” and these will gradually change color to indicate when they need to be replaced.

* You can also buy bags of mixed bed resin and refill your own cartridges.

Replacing filters and cartridges – It is good practice to ensure you do not foul anything when replacing new filters or cartridges.

Sediment and carbon: Disconnect the line leading up from the carbon block to the membrane at the membrane housing end and stick this line in a bucket.  Install just the sediment filter and very slowly turn on the water supply for a couple minutes.  Then, shut the water off and install the carbon block and repeat the rinsing process.  Reconnect the membrane hose, turn on the water and let it run until the product reaches 0 TDS.

DI: Install the new DI cartridge(s) and run water until the product reaches 0 TDS.



ASO valve – Isn’t it convenient to fill an RODI container and not have to worry about it flooding?  Installing as ASO (auto shut off) valve combined with a float valve (installed on your RODI container), can do just that.  There are various kits you can buy and they are easy to install.

Sanitizing – It’s good practice to sanitize your RODI once a year.  This can extend the life of all components.

  • Remove all filters, membrane and DI cartridges. Rinse the empty filter housings in warm, soapy water and rinse well.  Very well. Also clean the RODI unit itself.
  • Reassemble all the empty housings placing two tablespoons of regular unscented bleach in the sediment filter housing, then turn the water on and let the housings fill up until water starts to exit the product line. Turn off the water and let it disinfect for a few minutes, then turn the water back on and flush the chlorinated water through until all scent of bleach is gone.
  • Install everything back as normal. Ensure that you do not run any bleach through the membrane!

Well water – This can present different issues not commonly found in city water.  Contaminants such as radon, arsenic, hydrogen sulfide, iron, nitrates, nitrates, volatile organic compounds (VOCs), bacterial contamination and a slew of others can be found in well water, along with high CO2.  High CO2 levels in well water can severely reduce the lifespan of your DI.  If you are on a well system, it’s good to have your water tested periodically for contaminants and high CO2.  If you have high CO2, you can use a degassing system in between your membrane and DI.  This will aerate the RO water before it is pumped into the DI stages.

Water softener – Hard water passes through the softening system’s resin bed, the calcium and magnesium (hardness) ions are removed through an ion exchange process, and only softened water passes through.  This will greatly extend the life of your membrane.

Environment – It is best to store your RODI unit in a climate controlled area that is not exposed to extreme temperatures.  Extreme low temperatures can actually freeze certain components and cause the unit to burst.  Extreme high temperatures or even direct sunlight can cause bacteria, algae and viruses to grow inside the housings.


Did you make it through all of that?  GOOD!  But this is just the tip of the iceberg when it comes to RODI.  There are two basic philosophies behind choosing your RODI equipment – Pay a little more upfront and reap long-term savings, or spend a little less upfront and a lot more in the long run.

There are many other details not covered in this article so please discuss below.


©Matt Coddington

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