Reverse osmosis explained in plain English

Reverse osmosis, usually shortened to RO, is one of the most thorough ways of treating drinking water at home. The name sounds technical but the idea is simple: water is pushed through a membrane so fine that most of what is dissolved in it gets left behind.

How it works

Think of the membrane as an extremely fine sieve. Your mains pressure pushes water against it, the water molecules pass through, and most of the dissolved material does not. That left-behind material is flushed away to drain, so the membrane does not clog as quickly.

A home system normally works in stages:

• Sediment filter. Catches sand, rust and other particles.
• Carbon filter. Takes out chlorine, which would otherwise damage the membrane, and improves taste.
• RO membrane. Does the main work of removing dissolved solids.
• Storage tank. Holds the finished water, because RO is slow and the tank means you can fill a glass at normal speed.
• Final carbon filter. Polishes the taste on the way to the tap.

What it removes

A well maintained RO system removes the large majority of dissolved solids. That typically includes hardness minerals, nitrates, many heavy metals such as lead, and fluoride, along with chlorine and its taste through the carbon stages. Exact performance depends on the system, your water and the condition of the membrane, so check the specification of the model you are looking at.

It also removes beneficial minerals. Some people prefer that clean taste, and others add a remineralising filter as a final stage.

What it means for your home

Most systems fit under the kitchen sink, feed a small separate tap and connect to the waste pipe. Many people fit them themselves, and a plumber can do it quickly if not.

RO does send some water to drain as it works. How much depends on the system and your water pressure. Low mains pressure slows it down and makes it less efficient, so a booster pump is sometimes needed.

Looking after it

Upkeep is straightforward. The pre-filters and final filter are usually changed every 6 to 12 months and the membrane less often, commonly every 2 to 3 years. Follow the schedule for your system, as hard or heavily chlorinated water will shorten it.

A TDS meter is a handy way to keep an eye on things. It gives a reading of total dissolved solids, so if the number from your RO tap starts to creep up, the membrane may be due for a change.

Is RO right for you?

It suits people who want the cleanest drinking water they can get at the tap. It will not protect your pipes or appliances from limescale, because it only treats one outlet. For that, read our guide to water softeners and reverse osmosis.

Browse our reverse osmosis range or call us on 0121 557 5532 for advice.