RO stands for Reverse Osmosis and RO/DI is for Reverse Osmosis Deionization which is the same as a Reverse Osmosis system with an extra DI stage of filtration which filters your tap water down to ZERO TDS (Total Dissolved Solids). While RO systems are suitable for freshwater aquariums, it is necessary to utilize an RO/DI system for mixing saltwater and replenishing freshwater lost due to evaporation in a marine aquarium.

A standard 4 stage RO system includes the 4 basic stages necessary to filter your tap water down to 0 TDS which means the product water will be pure H2O with no other dissolved compounds or contaminates. For situations in which the RO/DI system is used heavily or the supply water is heavily contaminated, it may be useful to invest in a 5 or 6 stage system instead in which an extra carbon and/or extra DI stage is included.

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 cold water 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 contaminates 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 contaminates are removed before 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 thin film that remove a majority (up to 98%) of contaminates such as salts, bacteria, heavy metals, and other organics. 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.

From here the water is split into two different water lines: the waste water line and the product water line. The waste water 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 by-pass the flow restrictor and flush your membrane free of particulates before and after each use.

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

RO and RO/DI systems produce pure water at a ratio of 1 gallon of pure product water per 3 gallons of waste water. 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 waste water when employing an RO or RO/DI system in your home.

The product water will be almost pure, but not quite. This is why it travels into the next stage which contains the DI (deionization) cartridge (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 water is ready for use in your aquarium. In 6 stage systems, you will have dual DI cartridges which helps 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 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 supply water pressure is below 40 PSI, you will not get optimal performance out of your membrane and end up producing far less product water per gallon of waste water. In these cases, a booster pump will help boost pressure into the RO membrane to an ideal range.

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 begin to rise, it means that it is time to replace your RO membrane or DI cartridge.

Now that you know how it works, check out How to Maintain your RO and RO/DI System