How a Reverse Osmosis System Actually Works

Reverse osmosis is the deepest filtration technology used in homes. It can pull out things that ordinary filters can’t touch — dissolved salts, metals, even some pharmaceutical residue. Here’s the simple version.

Start with osmosis

Osmosis is what plants do. Water naturally moves through a thin membrane from a low-mineral side to a high-mineral side, trying to balance the two. It’s a one-way trip in nature.

Now reverse it

An RO system uses your home’s water pressure to push water the wrong way through that membrane — from the dirty side to the clean side. The membrane is fine enough that water molecules pass through, but most contaminants are too big and get left behind.

The full stack

A real-world RO system has multiple stages because the membrane is delicate. A typical setup:

• Sediment pre-filter — keeps grit out of the membrane
• Carbon pre-filter — strips chlorine that would damage the membrane
• RO membrane — the deep filtration step
• Storage tank — holds clean water for instant use
• Carbon post-filter — final polish before the faucet

What it removes — and what it doesn’t

RO removes the vast majority of total dissolved solids: salts, lead, fluoride, nitrates, arsenic, and many other contaminants. It does not sterilize water — you still need pre-treatment for bacteria. And because it removes minerals along with contaminants, some people pair RO with an alkaline post-filter to add minerals back.