How Seven-Stage Water Filtration Works, Stage by Stage

The principle: coarse to fine

Every stage in a well-designed water treatment line exists to protect the stage after it. That is the whole logic, and once you see it the sequence stops looking like marketing and starts looking like engineering.

A reverse osmosis membrane is a precision component with pores measured in fractions of a nanometre. Send raw water at it and it fouls within days: sand abrades it, silt blinds it, chlorine chemically degrades the polyamide film it is made from. Everything upstream of the membrane is there so the membrane sees water it can survive.

So the sequence runs from the crudest filtration to the finest, with the expensive and delicate elements last. Skipping a stage does not just remove that stage’s benefit — it shortens the life of everything downstream.

The seven stages

In the order the water passes through them.

1. 01 — Back-Wash Sand Filter — Removes large suspended particles — sand, dust, rust and mud.
2. 02 — Double Y-Strainer — Two passes, 10–20 micron then 5 micron, clearing finer suspended solids before the sensitive filters.
3. 03 — CTO Carbon Block — Chlorine, Taste & Odour. Removes chlorine, pesticides and organic chemicals.
4. 04 — Sediment Filter — 5 then 1 micron. Removes fine silt, rust and particulate matter, protecting the RO membrane downstream.
5. 05 — RO Membrane — Reverse osmosis strips dissolved solids, heavy metals and microbiological contaminants.
6. 06 — Activated Carbon — Adsorbs dissolved organic compounds — removes residual odour and colour, and improves taste.
7. 07 — Ozonation — Ozone (O₃) is injected into the purified water, destroying bacteria, viruses and fungi and preventing microbial growth inside the sealed bottle.

What reverse osmosis actually does

Reverse osmosis is the only stage in the sequence that removes dissolved substances rather than suspended ones. Everything before it is filtration in the ordinary sense: physically catching particles too large to pass. RO is different.

Osmosis is the natural tendency of water to move across a semi-permeable membrane from the less concentrated side to the more concentrated one. Reverse osmosis applies pressure to drive water the other way — from concentrated to dilute — so that water molecules pass through the membrane and the dissolved solids do not. What comes through is the permeate; what is left behind, carrying the concentrated dissolved load, is the reject stream.

This is why RO removes things no filter can catch: dissolved sodium, nitrates, fluoride, arsenic, lead. They are in solution, not in suspension, so there is nothing to strain out. The membrane excludes them by size and charge at the molecular level.

It is also why RO produces a reject stream at all — a genuine cost of the process, and one reason RO treatment is more expensive per litre than filtration alone.

Why ozonation is last

The final stage is the one that matters most for a sealed bottle sitting in a warehouse for months, and the reason it comes last is specific.

Ozone (O₃) is an unstable, powerful oxidant. Injected into the water immediately before filling, it destroys bacteria, viruses and fungi — and then decomposes back into ordinary oxygen within hours, leaving no residual taste or smell. The bottle is sealed while the ozone is still active, so the water is being sterilised inside the sealed container rather than before it.

That timing is the point. Water sterilised and then exposed to a filling line, a cap and the air has been recontaminated in transit. Ozonating at the fill means the last thing to touch the water is a disinfectant that then disappears.

The alternative in common use is UV, which sterilises water passing a lamp but leaves no residual protection inside the bottle. Ozonation is the more robust choice for packaged product with a months-long shelf life.

Putting the minerals back

A membrane that removes dissolved solids removes the good ones too. Calcium, magnesium and potassium are dissolved solids, and RO does not distinguish them from anything else.

The result is water that is exceptionally clean and tastes empty. Almost every commercial packaged drinking water operation therefore adds a controlled mineral blend back after the membrane, which is why a bottle can legitimately list minerals without being natural mineral water — they are there by design rather than by geology.

The trade-off is deliberate: strip everything, then add back exactly what you want, and you get the same water every single day regardless of what the source did that season. Consistency is the product.

What to ask a bottler

1. How many stages, and in what order — If sediment filtration comes after the RO membrane, ask why. The order tells you whether the line was designed or assembled.
2. Ozonation or UV — Both are legitimate. Ozonation gives residual protection inside the sealed bottle; UV does not.
3. Is the water remineralised — If not, expect it to taste flat, and expect half-finished bottles.
4. When was the membrane last changed — A tired membrane still produces water; it just stops removing what it is there to remove.
5. Can I see the plant — The best answer is yes.

Frequently asked questions