What is Reverse Osmosis (and How Does It Actually Work)?

If you've shopped for a water purifier in the last few years, you've seen "RO" stamped on almost everything premium. Reverse osmosis is the dominant technology in home water purification — and also the most misunderstood. People worry it strips minerals, wastes water, makes water "too pure", or is just expensive marketing.

Some of those concerns are real. Some are myths. This guide walks through what reverse osmosis actually is, how it works, what it removes, what it doesn't, and the trade-offs that matter when you're deciding whether you need it.

What Reverse Osmosis Actually Is

Osmosis is something nature does on its own. If you have salty water on one side of a membrane and fresh water on the other, the fresh water naturally moves toward the salty side until both sides have the same concentration. That's how plant roots draw water up from soil. It's how your kidneys filter blood. Osmosis is everywhere.

Reverse osmosis is osmosis run backwards. Instead of letting water move from "clean" to "contaminated" naturally, you push it the other way — from contaminated water, through a membrane, to the clean side — using pressure. Pure water passes through. The dissolved solids, heavy metals, and other contaminants get left behind on the contaminated side.

That's the entire concept. The clever bit is the membrane.

How Reverse Osmosis Works

The RO membrane is the heart of the system. It's a thin, semi-permeable film with pores measuring around 0.0001 microns (one ten-thousandth of a micrometre). For scale: a human hair is about 70 microns thick, a bacterium is around 1 micron, and a virus is roughly 0.1 microns. The RO membrane's pores are a thousand times smaller than a virus.

That's why it works on dissolved contaminants that other filters miss. A regular carbon filter catches chlorine and improves taste, but it does nothing to dissolved lead, sodium, fluoride, or nitrates — those molecules pass right through. The RO membrane is fine enough to block them at the molecular level.

To force water through openings that small, you need pressure. Some RO systems rely on your home's mains water pressure (typically 3–4 bar in Singapore HDB blocks); higher-end systems include a built-in booster pump that runs the membrane at the pressure it was designed for, which produces purer water and lasts longer.

What comes through the membrane is, in chemistry terms, very close to pure H₂O.

The Reverse Osmosis Filter Stack

In practice, "an RO water purifier" is never just a membrane on its own. The membrane is the third or fourth stage in a stack. Each stage protects the next one and handles a different category of contaminant.

A typical home RO system looks like this:

Stage	What it does	Why it matters
1. Sediment filter	Removes sand, rust, dirt particles	Stops the carbon and RO stages from clogging up early
2. Pre-carbon filter	Removes chlorine and chemicals that taste/smell bad	Critical: chlorine destroys RO membranes — the carbon filter sacrifices itself to protect the membrane
3. RO membrane	Removes dissolved solids, heavy metals, microorganisms	The actual purification stage
4. Post-carbon filter	Polishes taste and removes any residual odours	Final cleanup before the water reaches your glass
5. (Optional) Remineraliser	Adds back small amounts of calcium and magnesium	For taste and to raise pH slightly
6. (Optional) UV	Sterilises any bacteria that survive storage	Belt-and-braces for tank-stored water

Skipping the pre-carbon stage is the most common reason cheap RO systems die early. Singapore tap water contains chlorine for disinfection, and unfiltered chlorine will degrade an RO membrane within weeks. The whole stack exists to make the expensive membrane last.

If you want to see what a complete stack looks like in a real product, the HydroFirst+ is a countertop RO system that runs all five stages in a unit you plug into the wall — no installation, no plumbing changes.

What Reverse Osmosis Removes

This is where RO genuinely earns its reputation. Independent testing of properly maintained RO systems consistently shows the following removal rates:

Contaminant	Typical removal
Total dissolved solids (TDS)	95–99%
Lead	95–98%
Mercury	95–97%
Arsenic	94–96%
Chlorine	98%+ (carbon stage)
Fluoride	85–95%
Sodium	85–95%
Nitrates	80–90%
Pesticides & herbicides	90%+
Microplastics	99%+
PFAS ("forever chemicals")	90–99%
Viruses	99%+
Bacteria	99%+

In Singapore, the headline tap-water risks are low — PUB water leaves the treatment plant cleaner than the regulations require. The case for RO here isn't "the tap water is dangerous". It's that RO is the only home filtration technology that handles everything: heavy metals from older building plumbing, microplastics from the distribution network, and emerging contaminants like PFAS that aren't yet regulated.

What Reverse Osmosis Doesn't Remove (and Other Common Myths)

A few honest caveats worth knowing.

Dissolved gases pass through. The RO membrane filters out solid contaminants but does nothing to gases dissolved in water. CO₂, in particular, comes through unchanged. This is harmless but explains why RO water sometimes tastes slightly "flat" — the gases that give tap water its character are mostly still there.

Volatile organic compounds (VOCs) need carbon. The pre-carbon stage handles these, not the membrane itself. If a system advertises an "RO membrane" but skips serious carbon filtration, it'll do worse on VOCs than a basic carbon-only filter.

The membrane doesn't last forever. Even with proper pre-filtration, RO membranes degrade slowly and lose performance. Most home membranes are designed for 2–3 years of typical use. If you stop changing the carbon pre-filter, that drops to months.

"Anti-bacterial" claims are about the housing, not the water. Some marketing implies RO actively kills bacteria. It doesn't — it physically blocks them at the membrane. The water on the clean side has the bacteria filtered out, but the system itself can grow biofilm in the post-membrane storage if it's not maintained. This is what the optional UV stage is for.

The Waste Water Problem

Here's the trade-off nobody mentions in the showroom: RO is not efficient by nature.

Because the membrane only lets pure water through, everything that doesn't pass — the concentrated rejected water — has to go somewhere. In a traditional under-sink RO system connected to your plumbing, that rejected water is flushed straight down the drain. The industry standard ratio is around 1 litre of clean water for every 3–4 litres flushed away. For every glass you drink, three or four go to waste.

This is the single biggest legitimate criticism of RO. On a typical Singapore household water bill it's not catastrophic — but in absolute terms, it's wasteful in a way most filtration is not.

The good news: it's a solvable problem. Modern countertop RO systems address it through recirculation — instead of dumping rejected water down a drain, the system cycles it back through the membrane multiple times until almost all of it has been recovered as clean water. The HydroSpark+ achieves a roughly 4:1 recovery ratio (4 litres of clean water for every 1 litre of waste) using exactly this approach. We wrote a detailed explanation of how the recirculating tank works if you want the mechanical details.

If you're considering an under-sink RO system, ask the seller specifically about the recovery ratio. Anything worse than 1:2 is a 2010s-era design.

The Mineral Question

This is the most-asked, most-misunderstood concern with RO.

It's true that the membrane removes calcium, magnesium, and other naturally-occurring minerals along with the contaminants. RO water is, by design, very low in dissolved minerals. The question is whether that matters.

The honest answer: not very much, for most people. The minerals in tap water contribute a small amount to your daily intake — but only a small amount. Calcium from a glass of mineralised water is dwarfed by what you get from a single serving of yoghurt, broccoli, or almonds. The same is true of magnesium. If your diet is reasonable, drinking RO water doesn't move the needle on your mineral intake.

That said, there are two reasonable positions:

1. You don't worry about it. If you eat a varied diet, RO water is fine. Many people prefer how it tastes — clean, neutral, slightly sweet.
2. You add a remineraliser. Most modern RO systems include an optional remineralising cartridge that adds back small amounts of calcium and magnesium for taste and to raise pH slightly. If "tap-water taste" matters to you, this is the way to get it back.

Either is defensible. What's not defensible is paying a premium for "alkaline ionised" water as a way to "fix" RO water — that's marketing, not chemistry.

Quick Answers

Is RO water safe to drink? Yes. The World Health Organisation has reviewed low-mineral drinking water repeatedly and found no health risk for adults eating a normal diet. Major bottled water brands sell RO water by the billion litres.

Is RO better than alkaline water? They solve different problems. RO removes contaminants. Alkaline water raises pH. The two aren't comparable, and neither is a substitute for the other. If you want both, you need an RO system with a remineralising or alkalising stage downstream.

Is RO water "dead water"? No. "Dead water" is a marketing term used by sellers of alkaline ioniser machines. Water with low TDS isn't biologically inert; the cells in your body don't care whether the water you drink had 10 ppm TDS or 100 ppm. They use it the same way.

Is RO overkill for Singapore tap water? It depends on what you're optimising for. PUB tap water is clean enough that you don't need RO for safety. But if you live in an older HDB block (lead pipes were phased out but residual concerns exist in some buildings), or you want maximum reduction of microplastics and emerging contaminants, RO is the most thorough option available. For most households, a good multi-stage carbon system is sufficient and cheaper. RO is a step up from sufficient.

The Bottom Line

Reverse osmosis is the most thorough water filtration technology in the home market. The membrane works at a molecular scale and removes contaminants that no other home filter touches. The two real trade-offs — waste water and mineral removal — are both solvable: recirculating tank designs handle the first, and a remineralising stage handles the second.

Whether you actually need RO depends on your tap water source, your priorities, and your budget. If you're trying to decide between RO and simpler options, our guide to choosing a water purifier in Singapore walks through the practical decision in detail.