What Reverse Osmosis Actually Does

Reverse osmosis pushes water through a membrane fine enough to hold back dissolved minerals and most contaminants, not just the particles a carbon filter catches. It is the most thorough treatment available for drinking water, which is why it goes under the kitchen sink rather than on the main: treating the whole house to that standard would be expensive and largely wasted on the washing machine.

A system runs sediment and carbon pre-filters, then the membrane, then a final polishing filter at the tap. The pre-filters protect the membrane, which is the expensive part, so keeping to the replacement schedule is what makes the system last.

What to Know Before You Buy One

RO produces waste water. Some of what goes in leaves down the drain carrying the contaminants the membrane rejected, and how much depends on the system. It also needs a drain connection and a dedicated tap at the sink.

Tank or Tankless

The traditional design stores treated water in a tank under the sink, because the membrane produces slowly. We tend to favour the tankless kind instead. It gives you reverse osmosis water on demand rather than from a reservoir, it frees up most of the cupboard, there is no tank to keep sanitary, and the filters are simpler to change. That is what we fitted on a recent Hendersonville job.

Hard water shortens membrane life, so on a house with significant hardness it is worth handling that first. We often fit an RO system alongside softening or conditioning, and a matching faucet at the sink. On a recent Lebanon job the RO went in with a new kitchen faucet to match the finish.

If your concern is chlorine taste at every tap rather than drinking water specifically, a whole-home filter is the cheaper answer and the two are often fitted together.