How Does Reverse Osmosis Work?

As is so often repeated, high water quality is needed in a marine aquarium. Reef systems need higher quality than fish only, but both need to be maintained at the highest level possible. This is done by regular monitoring of water parameters.

A good way of enhancing water quality is by preparing the routine water change mix (and the initial fill) with a high grade salt mixed into reverse osmosis (R/O) water. This water is obtained by filtering tap water to remove undesirables therefore increasing the water quality in the first place.

R/O devices usually come in a unit containing three pods: there is a pre-filter to remove larger debris, a carbon filter to remove chlorine, and the R/O membrane itself. Some units have a fourth pod, which hold resins that take out any remaining undesirables. There isn’t any harm in having a fourth pod, they will increase the water cleanliness somewhat,  but they are not strictly necessary.

The R/O unit is connected to the mains supply in a convenient out of the way place (simple connectors are normally supplied with the device). The water is driven through the unit by the mains water pressure. They will not work if, say, supply water is gravity fed.

The water first meets the sediment pre-filter, which, as said, removes larger particles so that the next filter doesn’t clog.

Next in line is a carbon filter. This is very important as it removes chlorine. Chlorine is very detrimental to the R/O membrane.

The water now reaches the main point of the exercise, the R/O filter. This is a semi-permeable membrane. In other words, it will allow water to pass but not other unwanted substances. These substances could be insecticides, viruses, spores of fungi, metals, bacteria etc.

Finally, if the device has four pods, the final filter is a ’polishing’ unit. As said, it contains resins which will take out further pollution giving a slightly higher purity.

As has been said, the R/O membrane will only allow nearly pure water to pass through. Therefore there are two water outlets, one for the purified water (known as permeate) and one for the waste (known as concentrate). The aquarist runs the permeate outlet into a container for storage, and the concentrate to a convenient point for disposal.

R/O units come in various ’gallons per day’ outlets. The aquarist should purchase one that will provide sufficient water for the routine water change in a reasonable time. This will cover daily top-ups as well.

It is possible to connect an R/O unit to an automatic top-up device. This means when the water level falls it will be filled with purified water in the absence of the aquarist. For smaller systems, a daily top-up is usually sufficient.

It is very important to follow the manufacturer’s instructions when using the R/O device. The sediment pre-filter should be replaced or cleaned at the appropriate time. It is essential to replace the carbon filter when required, as failure to do so could allow chlorine to reach the R/O membrane and cause damage. This could allow impure water to pass through the membrane. This would mean obtaining a new membrane, which will be in the region of half the price of the entire unit.

R/O devices will provide 1/5th of pure water in relation to the water input. In other words, for every 5 parts of tap water, 1 part of purified water will be obtained. The purity on a three pod system is usually between 95 to 98%.

Using R/O water is highly recommended. As already stated, it sets the seawater quality at a high level from the start. From a logical point of view, manufacturer’s of dry salt mixes provide salt of a very high standard nowadays, with nothing undesirable present, such as nitrate and phosphate. Why pollute this salt with tap water?

If the aquarist has been using tap water to date, changing to R/O water is a good idea. Better, use it from the very start.


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