How Does an RO Water Purifier Work? – A Complete Step-by-Step Explanation
You see it mounted on your kitchen wall every day, producing clean drinking water at the push of a tap — but have you ever wondered what actually happens inside your RO water purifier? Understanding how it works helps you use it better, recognise problems early, and make sense of the maintenance schedule. This guide explains every stage of an RO purifier in plain, simple language.
What is Reverse Osmosis (RO)?
Reverse Osmosis is a water purification process that pushes water through an extremely fine semi-permeable membrane — one so dense that it allows water molecules through but blocks dissolved salts, heavy metals, bacteria, and other contaminants.
The word "reverse" refers to the reversal of osmosis — the natural process where water moves from a less concentrated solution to a more concentrated one across a membrane. In RO, we use a pressure pump to force water in the opposite direction, pushing it from concentrated (impure) water through the membrane, leaving contaminants behind.
📏 Just How Fine is the RO Membrane?
The pores in an RO membrane are approximately 0.0001 microns in size. To put that in perspective: a human hair is about 75 microns wide. The RO membrane's pores are 750,000 times smaller than a human hair — small enough to block individual dissolved salt molecules.
The Multi-Stage Purification Process – Explained
A modern home RO purifier doesn't just have one filter — it has 5 to 8 stages, each doing a specific job. Here's what happens to your water from inlet to tap:
Stage 1 – Sediment Filter (PP / Spun Filter)
This is the first stage your tap water enters. The sediment filter (usually a polypropylene spun cartridge) traps visible particles: sand, silt, rust, mud, and suspended solids. This protects the more delicate filters in later stages from getting clogged by large particles. It has no effect on dissolved substances or bacteria.
Lifespan: 3–4 months in Indore's water conditions.
Stage 2 – Pre-Carbon Filter (GAC or CTO)
The water now passes through a bed of granular activated carbon (GAC) or a compressed carbon block (CTO). Activated carbon has an enormous surface area at the microscopic level and absorbs chlorine, chloramines, organic chemicals, pesticides, and compounds responsible for bad taste and odour. This stage is critical because chlorine — used by municipalities to disinfect water — would otherwise damage the RO membrane in the next stage.
Lifespan: 6 months. When this filter expires, chlorine bypasses it and can permanently damage the RO membrane.
Stage 3 – The RO Membrane (The Heart of the System)
This is where the magic happens. The pre-filtered water is pressurised by a pump and forced through the semi-permeable RO membrane. Water molecules pass through easily, but the vast majority of dissolved solids — salts, fluoride, nitrates, arsenic, lead, chromium, and bacteria — are blocked and diverted away as "reject water" (also called brine or wastewater).
A quality RO membrane removes 90–99% of Total Dissolved Solids (TDS), bringing water from 400–600 mg/L (typical Indore tap water) down to 20–60 mg/L.
Lifespan: 12–18 months in Indore. The membrane is the most expensive component to replace (₹1,800–₹2,550) and the most important to protect through timely pre-filter changes.
💧 What is Reject Water?
For every litre of purified water an RO produces, it also generates 2–4 litres of "reject water" — water concentrated with the contaminants that were filtered out. This reject water flows out through a drain pipe (you can hear it draining when the RO runs). It is not wasted — many households collect it for mopping floors, watering plants, or flushing toilets.
Stage 4 – Post-Carbon Filter (Polishing / Inline Carbon)
The purified water from the membrane flows into the storage tank. When you open the RO tap, this stored water passes through one final carbon filter before reaching your glass. This "polishing" stage removes any residual taste or odour that may have developed while water was stored in the tank, ensuring the water tastes fresh every time.
Lifespan: 12 months.
Stage 5 – UV Sterilisation (in UV models)
Some RO models include an ultraviolet (UV) lamp. UV light damages the DNA of any bacteria or viruses that may still be present after RO filtration, rendering them unable to reproduce. This adds an extra layer of biological safety — particularly useful during monsoon season when bacterial contamination spikes.
Lifespan: 12 months (UV lamps lose intensity over time even if they appear to still glow).
Stage 6 – UF Membrane (in UF models)
Ultrafiltration (UF) membranes have slightly larger pores than RO membranes — large enough to allow minerals through but small enough to block bacteria and protozoa. UF is often added alongside RO as a redundant biological safety layer.
Stage 7 – TDS Controller / Mineraliser
Pure RO water can have a TDS as low as 10–20 mg/L, which some find bland or slightly acidic. A TDS controller allows a small amount of pre-filtered (but not RO-processed) water to blend with the purified water, raising the output TDS to a healthier 50–150 mg/L. A mineraliser adds back specific beneficial minerals like calcium and magnesium.
What Does an RO Remove — and What Doesn't It Remove?
| Contaminant | RO Removes? |
|---|---|
| Dissolved salts (calcium, sodium, magnesium) | ✅ 90–99% |
| Heavy metals (lead, arsenic, chromium, cadmium) | ✅ 95–99% |
| Fluoride | ✅ 85–95% |
| Nitrates | ✅ 85–95% |
| Bacteria & protozoa | ✅ 99.9%+ |
| Chlorine & chloramines | ✅ Removed by pre-carbon |
| Viruses | ⚠ Partially (UV better for this) |
| Volatile organic compounds (VOCs) | ⚠ Partially (activated carbon more effective) |
| Dissolved gases (CO₂, Radon) | ❌ Not removed |
| Pesticides and herbicides | ✅ Mostly removed |
RO vs UV vs UF – What's the Difference?
- RO — Removes dissolved solids, heavy metals, bacteria. Needs electricity and produces reject water. Best for high-TDS water like Indore.
- UV — Kills bacteria and viruses using light. Does NOT remove dissolved chemicals or TDS. Best used alongside RO, not as a replacement.
- UF — Removes bacteria and suspended particles but NOT dissolved salts. Better than UV alone for biological purity, but cannot reduce TDS. Works without electricity — good as a backup or for lower-TDS water sources.
✅ For Indore's Water: Always Choose RO
Given Indore's high TDS (300–700 mg/L) from municipal supply and groundwater, only RO can adequately reduce dissolved contaminants. UV and UF alone are insufficient. Look for RO + UV + UF combination purifiers for comprehensive protection.
Why Does My RO Need Regular Service?
Now that you understand each stage, it's clearer why regular service matters:
- A clogged sediment filter reduces water pressure for all downstream stages, forcing the pump to work harder
- An expired carbon filter lets chlorine reach and damage the RO membrane — turning a ₹400 filter replacement into a ₹2,000 membrane replacement
- A worn membrane lets high-TDS water through, silently degrading your family's water quality
- A dirty storage tank can reintroduce bacteria into otherwise pure water