Bottled purified water usually starts as ordinary municipal tap water and is put through a treatment train before bottling. The processes are the same ones available to a household, run at scale and to a defined specification, which is worth knowing because it means the same result is achievable at a tap for a fraction of the cost.
It starts as tap water
Purified water is defined by the treatment it has received rather than by its source, and the common source is a municipal supply, which has already been treated and monitored. The EPA notes that every community water system publishes an annual Consumer Confidence Report and maintains a finder for them, so the starting quality is documented. That is different from spring or mineral water, which is defined by its source and is bottled with minimal treatment to preserve its mineral character.
The treatment train
Prefiltration removes particles. Carbon removes chlorine, taste and organic compounds, and protects what follows. Then the separation stage, which is usually reverse osmosis, sometimes deionization, sometimes distillation, and this is what does the actual purification by removing dissolved material. Some plants use more than one. Finally the water is polished, often with a further carbon stage, and disinfected before bottling, commonly with ultraviolet or ozone, since the treated water has no disinfectant residual of its own.
Why minerals are sometimes added back
Water stripped of dissolved content tastes flat and is mildly aggressive to the materials it contacts, so many bottlers add a controlled blend of minerals back before filling. That is the same function a remineralization cartridge performs on a household reverse osmosis system, and it is why bottled purified water tastes better than water straight from a domestic membrane without that stage. The added minerals are a taste and stability decision rather than a health claim.
Doing the same thing at home
An under sink reverse osmosis system with a carbon prefilter and a remineralization cartridge is the household version of that train, and it produces water on demand rather than in bottles. For a household already buying bottled water regularly, it is cheaper within a couple of years and removes the carrying and storage entirely. What it needs in return is prefilters changed on schedule, since those protect the membrane. Our softener installation guide covers the separate question of hardness across the house.
Questions people ask about how is purified water made
Where does bottled purified water come from?
Usually a municipal supply. Purified describes the treatment received rather than the source, unlike spring or mineral water.
What processes are used?
Prefiltration, carbon, then reverse osmosis, deionization or distillation, followed by polishing and disinfection before bottling.
Why are minerals added back?
Demineralized water tastes flat and is mildly aggressive to materials, so a controlled blend is added for taste and stability.
Can I produce the same water at home?
An under sink reverse osmosis system with carbon prefiltration and a remineralization stage is the household equivalent of that train.