Reverse osmosis is a method of water purification where water under pressure is forced through a semi-permeable membrane. The water molecules are small enough to pass through the holes in the membrane, however dissolved salts and other impurities are not. The output is two streams from the RO filter, a purified water stream and a waste stream that contains the impurities at a higher concentration than the input stream.
Typical household units produce around 2-4 litres of wastewater for every litre of purified water. The waste water can be used for washing clothes or watering plants, but can be quite wasteful if it is discarded to the drain in areas where water restrictions apply.
There are various membranes available, although TFC or Thin Film Composite membranes appear to be the most appropriate for aquarium applications, and are capable of reducing contaminants to very low levels of Total Dissolved Solids (TDS).
RO filters will normally utilise pre-filters to remove fine particulate solids, and carbon filters to remove chlorine prior to entering the RO stage in order to extend the life of the membrane. They can also be used with a final deionisation column, with such a system referred to as an RO/DI. This allows much lower levels of contaminants to be reached.
- Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria by Randy Holmes-Farley - Reefkeeping Magazine
- What is TDS? by Randy Holmes-Farley - Reefkeeping Magazine
- Reverse Osmosis - WetWebMedia
- Animation: How Osmosis Works (note that is for osmosis, not reverse osmosis, which is the reverse process)