RO/DI Water System Guide
How a reverse osmosis and deionization system produces ultra-pure water, what it removes, and why it suits marine and planted tanks.
What it is
An RO/DI system combines reverse osmosis and deionization to produce ultra-pure water free of chlorine, heavy metals, silicates and phosphates. It is used where precise water chemistry control is required, such as saltwater aquariums and demanding planted tanks.
How it works
Reverse osmosis uses a semi-permeable membrane to separate water molecules from other substances. In ordinary osmosis water moves toward the more concentrated side; reverse osmosis applies pressure that overcomes this osmotic pressure, forcing purified water through the membrane while leaving dissolved and suspended chemical species behind on the reject side. The water that passes through is called the product or permeate, and the concentrated stream that is flushed away is the reject or wastewater. A deionization stage then strips remaining ions to push purity even higher, which is the DI part of an RO/DI unit.
What it removes
- Dissolved solids including salts and minerals
- Heavy metals such as copper
- Nitrates, nitrites, phosphates and silicates
- Chlorine and chloramines (removed by the carbon prefilter, not the membrane itself)
Prefilter stages
Two stages prepare water before the RO membrane. A sediment filter removes particles and suspended solids, and an activated carbon filter traps organic chemicals and chlorine — important because chlorine degrades certain thin-film composite membranes.
Why aquariums use it
Tap water can carry copper, nitrates, nitrites, phosphates, silicates or other chemicals detrimental to marine organisms, and these vary with the local supply. Starting from RO/DI water gives a known, near-zero baseline so that salt mix or remineralizing additives can be dosed to exact target values, which is why it is favored for reef systems and tanks keeping soft-water species.
Wastewater and purity
RO produces highly purified water but also generates reject water: household purifiers typically produce one liter of usable water for every 3 to 25 liters of wastewater. The exact ratio depends on membrane type, feed pressure and water temperature. The resulting purified water reaches the high purity needed for sensitive aquarium applications.
Installation and maintenance
The unit connects to a pressurized water supply, with product water collected and reject water drained. Prefilters and the DI resin are consumables: sediment and carbon cartridges are replaced on schedule, and the DI resin is changed when output conductivity rises. Because chlorine degrades thin-film composite membranes, the carbon prefilter must be kept fresh; the membrane lasts longer when fed properly dechlorinated, sediment-free water.