What Is RO Water? Reverse Osmosis Explained

Reverse osmosis water, or RO water, is tap water that has been pushed through a membrane so fine that almost nothing except water molecules gets through. It is the most thorough filtration you can install in a home, and it is what most bottled drinking water in Malaysia is made from. This guide explains how it works, what it removes, what it does not do, and how to decide whether your household needs it.

Quick Answer: What is RO water?

RO water is water forced through a semi-permeable membrane with pores around 0.0001 micron across. Dissolved salts, heavy metals, bacteria, viruses and most chemicals are held back, and what comes out the other side is very close to pure water. Home RO systems add carbon stages before and after the membrane, which is what makes the water taste good as well as test clean.

How reverse osmosis works

Osmosis is what happens when water passes through a membrane from the purer side to the saltier side, which is how plant roots drink. Reverse osmosis uses pressure to push it the other way: from the tap water side through the membrane, leaving the dissolved solids behind.

A home RO purifier does this in four stages:

• Sediment filter. A PP cotton cartridge catches rust, sand and grit from the pipes.
• Pre-carbon filter. Activated carbon absorbs chlorine and organic compounds, which protects the membrane and takes away the swimming-pool taste.
• RO membrane. The 0.0001 micron membrane rejects dissolved salts, heavy metals such as lead and arsenic, bacteria, viruses and microplastics.
• Post-carbon filter. A final carbon stage polishes the taste of the water as it leaves.

The rejected solids are flushed away to the drain with a small stream of waste water, which is how the membrane keeps itself clear.

What RO removes

That last row is the trade-off. RO does not know the difference between a harmful dissolved solid and a useful one, so minerals go too. The water is perfectly safe; most of the minerals in a diet come from food, not water. Some people find very pure water tastes flat, which is why post-carbon and mineral stages exist.

What RO does not do well

• Flow rate. Water passes the membrane slowly. Cheaper systems store it in a tank so a glass fills at a normal speed. A high-capacity membrane, rated at 600 gallons a day, delivers about 1.5 litres a minute straight to the tap, which is roughly six seconds for a glass.
• Waste water. Every litre purified sends some water to drain. Modern membranes waste far less than older ones, but it is not zero.
• Filter costs. The carbon cartridges last about a year and the membrane about two. Budget for both; a system with old filters is worse than no system.
• Boiling is not a substitute. Boiling kills bacteria but does nothing about metals, chemicals or sediment, and it concentrates them as water evaporates.

Do you need RO water in Malaysia?

Tap water in Malaysia is treated to a safe standard when it leaves the plant. What reaches your glass depends on the pipes, the building's storage tank, and the age of the plumbing, which is why the same water can taste and test differently across one city.

RO is worth it if any of these apply:

• The water tastes or smells of chlorine, metal or earth.
• The building is older, or has a rooftop tank you cannot see cleaned.
• There is an infant, an elderly person or someone with a weak immune system in the house.
• You currently buy bottled water, which RO replaces at a fraction of the cost.

If none apply, a simpler carbon filter at the tap will improve taste and remove chlorine. Our guide to water purifiers versus water filters explains where that line sits. For the health case in full, see the benefits of a reverse osmosis water filter.

What RO costs to run

For the VATTI system below, the four-layer carbon filter is RM189 and lasts about 12 months, and the RO membrane is RM499 and lasts about 24 months. That works out at roughly RM440 a year, or about RM1.20 a day, for unlimited purified water at four temperatures. A family that buys bottled water spends more than that in a month. Our guide to how often to replace a water filter explains the signs that a cartridge is due.

The VATTI One Tap purifier is an RO system

The VATTI One Tap water purifier is a four-stage RO purifier that fits under the sink and feeds a single tap on the counter. The membrane is the 0.0001 micron kind described above, the post-carbon stage handles the taste, and the membrane is rated at 600 gallons a day so the tap runs at 1.5 litres a minute with no storage tank. A separate heater beside it delivers hot water in three seconds, and the tap gives four temperatures: 100°C for tea, 85°C for coffee, 45°C for a baby's milk, and room temperature. You change the filters yourself by lifting the lid.

RO water FAQs

Is RO water safe to drink every day? Yes. It is the standard for bottled drinking water and for hospitals. The minerals it removes are ones you get from food.

Does reverse osmosis remove minerals? Most of them, yes. That is inherent to the method. It does not make the water unhealthy, and a post-carbon stage brings the taste back.

Is RO water the same as distilled water? No. Distilled water is boiled and condensed, which removes almost everything including dissolved gases. RO water keeps a trace of minerals and tastes more like ordinary water.

How often does an RO membrane need changing? About every two years in normal home use, with the carbon filters changed yearly. A drop in flow or a return of the chlorine taste means one of them is due.

Why does RO water taste flat? Because it is very pure and low in minerals, and minerals are most of what water tastes of. Most people stop noticing within a week, and a post-carbon stage softens the difference.

Final thoughts

RO water is tap water with the dissolved solids, metals and microbes taken out. It is the right choice for a household that wants bottled-water purity from the kitchen tap, and the running cost is lower than bottles. The VATTI One Tap purifier delivers it hot, warm or cold from one fixture, with filters you change yourself.