RO/DI System

What Is An RO/DI System?

An RO/DI system removes contaminants from our tap water, making it safe for use in your aquarium. When it comes to home water filtration, you're going to either have an RO or RO/DI system. An RO and RO/DI are almost identical except the RO/DI system has an extra filtration stage called the "DI or Deionization" stage which brings the water down to 0 TDS (total dissolved solids). 

  • RO: Reverse Osmosis
  • RO/DI: Reverse Osmosis Deionization

The applicable difference between the two is the level of water purity you get which is then used for different applications. RO systems are used to produce drinking water and purify water for freshwater aquariums. RO/DI systems are used to produce 99.9% pure water for scientific applications and use in saltwater aquariums. For all intents in purposes, this article is focused on RO/DI systems.

How Many Stages?

When choosing an RO/DI system, you're going to be given the choice of how many stages of filtration. Choose the system based on the following criteria, tap water conditions, and demand for pure water production.

4 Stage - The most popular RO/DI system because it is the most affordable and most portable system. The perfect system for your first-time RO/DI system and can easily be upgraded to accommodate a wide variety of source water conditions and tank demands. Will produce 0 TDS water in most all water situations and is great for nano tank owners and those of you starting out on a budget.   

5 Stage - Our most recommended system because it includes an extra carbon block filter.  The extra carbon block filters out a wider variety of disinfectants commonly used in municipal water sources such as Chloramines. When it's all said and done, a majority of reefers are best suited with a 5 stage system because these common disinfectants will deplete a single carbon block filter pretty quickly in many cases. By having two carbon blocks, you can be sure you're not allowing for these contaminants to damage your membrane or worse yet, find their way into your tank.  

6 Stage - All of the benefits of the 5 stage system with dual carbon blocks and the added benefit of two DI resin filters. The extra DI filter ensures contaminants will never sneak their way through your DI resin should you forget to change it out or you're finding the mixed bed resin is depleting quickly. Great for serious hobbyists with larger or multiple tanks resulting in heavier demands for fresh RO/DI water.  

7 Stage - The BEST! The most valuable and the most comprehensive freshwater filtration system we make.  In this case, you get the dual carbon filters along with a total of three DI resin filter stages. This 3 stage DI filter approach is what we call the "Pro Series DI Resin" where you take advantage of single bed DI resin in addition to the mixed bed resin used on all of the others. This is easily the most economical approach to DI resin filtration and allows you to target the contaminants that dominate your particular source of water resulting in less resin waste.  It is best suited for only the most dedicated hobbyists who really want to maximize the use of their RO filters and DI resin.  

RO/DI System Diagram

How An RO/DI System Works

No matter how many stages you get, they all work in a similar fashion where water flows through the filters in a particular order to achieve 0 TDS filtration. The most basic 4 stage filtration will produce 0 TDS water but only contains one of each filtration stage. The larger multi-stage systems simply double up on certain filter stages for additional filtration capacity and help target certain impurities, such as Chloramine. 

Input Water or tap water is fed into the first stage of your RO/DI system. This is typically connected via a garden hose adaptor which connects directly to the threads of your garden hose. You can also use faucet adaptors which allow you to connect with your household faucet or the plumbing underneath your sink. The input water should always be connected to your cold water line because hot water will damage your RO/DI system.

Stage 1 contains the sediment filter which is a mechanical filter that catches the larger free-floating particles of debris or contaminates. This filter does get clogged after some use and should be changed every 6-8 months or as needed based on usage. The filter will discolor from pure white to a brownish-yellow over time.

Stage 2 contains the carbon filter which consists of granulated carbon that adsorbs organics and other dissolved contaminants such as chlorine and chloramines. With a 6 stage system, you will have two carbon cartridges to accommodate heavy usage and target chloramines which exhaust a carbon filter quite quickly.

This carbon filter becomes exhausted over time and should be changed regularly every 6-8 months at the same time you change out the sediment filter. Replacing the carbon and sediment filters on a regular schedule will drastically improve the life of your membrane because it will ensure any possible contaminants are removed before the water enters the next stage which is your RO membrane—the most expensive and critical filter stage of your RO/DI system.

After the carbon stage, water enters the RO membrane which is the heart of your filtration system. Here, water is filtered through multiple layers of a thin film that remove a majority (up to 98%) of contaminants such as salts, bacteria, heavy metals, and other organic impurities. The RO membrane should be monitored via the use of a pressure gauge and/or TDS meter. In most cases, you will need to replace your RO membrane every 12-24 months based on usage.

The RO membrane splits the water, separating it into two different water lines: the wastewater line and the product water line. The wastewater line will contain your flow restrictor which is a small capillary inside the tubing that regulates water flow through the membrane. A flow restrictor is necessary for the RO membrane to function properly. The waste line will also be where you connect the flush valve kit that allows you to flush your membrane free of particulates before and after each use.

The wastewater should be connected to a drain or collection chamber for alternate use. This wastewater will contain concentrated levels of TDS but is perfectly suitable to water your garden, lawn, or even wash your car!

The product water leaves the RO membrane and is almost pure, but not quite. This is why it travels into the next stage which contains the DI (deionization) cartridge (DI is stage#4 in a 4 stage system and stage #5 and #6 in 6 stage systems). Here, the water passes over positively and negatively charged resins that remove any and all leftover traces of contaminates including silicate, nitrate, and phosphate. After leaving the DI cartridge, the product water should measure 0 TDS and be ready for use in your aquarium. In 6 stage systems, you will have dual DI cartridges which simply help to ensure that no contaminates will ever escape your system.

In order to monitor your RO/DI system for proper functionality, you will utilize a pressure gauge and TDS meter. The pressure gauge monitors water pressure going into the RO membrane which lets you know that you are getting the required amount of water pressure (40 to 80 PSI) into the RO membrane for optimal performance. When this input water pressure starts to drop, it is a good indication that your carbon and sediment pre-filters are becoming clogged and should be replaced.

If the tap water pressure is naturally below 40 PSI, you will not get optimal performance out of your RO/DI system and end up producing far less product water per gallon of wastewater. In these cases, a booster pump will help boost pressure into the RO membrane to an ideal range. RO and RO/DI systems should produce pure water at a ratio of 1 gallon of pure product water per 3 gallons of wastewater. So for every gallon of pure water you produce, 3 gallons will be wasted. This is why it is a good idea to find a viable use for the wastewater when employing an RO or RO/DI system in your home.

If you have a heavy demand for water or simply want to reduce the amount of wastewater, you can install a water saver upgrade. This will employ an extra RO membrane making it possible to get a 1:1 product to wastewater ratio.

The TDS monitor measures total dissolved solids via inline probes. These monitors can be plumbed into your system in various ways but it is best to monitor the water after it exits the RO membrane as well as after the DI stage. This will tell you that water exiting the RO membrane is getting filtered properly (< 15 TDS or < 5% of contaminates) and let you know that water exiting the DI cartridge is being filtered down to zero (0) TDS. When either of these numbers begins to rise, it means that it is time to replace your RO membrane or DI cartridge.

Learn More With BRStv: How to Know When to Change your RO/DI Filters.


Reverse Osmosis Systems
  1. Sediment Cartridge for AquaReady RO System
    Sediment Cartridge for AquaReady RO System

  2. Carbon Block Filter for AquaReady RO System
    Carbon Block Filter for AquaReady RO System

  3. 3 Stage 75 GPD RO System
    3 Stage 75 GPD RO System