Reverse osmosis pushes water through a semi-permeable membrane under pressure, separating out substances too large to pass through it. In homes it's a point-of-use system, typically installed under the kitchen sink with a dedicated faucet, producing water for drinking and cooking. What a given system reduces depends on that specific unit, its membrane and its verified manufacturer specifications — no RO system removes everything.

The name, briefly

Osmosis is water moving naturally through a semi-permeable membrane from a less concentrated solution toward a more concentrated one. Reverse osmosis applies pressure to push water the other way — from the more concentrated side to the less concentrated side — leaving behind substances the membrane won't pass.

That's the whole principle. Everything else is engineering around it.

What's actually under your sink

A residential RO system is a compact assembly. Working in the order water travels:

  1. Pre-filtration. One or more stages that capture sediment and reduce chlorine. These exist primarily to protect the membrane — sediment clogs it and chlorine degrades it.
  2. The membrane. Household pressure forces water through an extremely fine semi-permeable material. Substances too large to pass are carried away to a drain line.
  3. The storage tank. A small pressurised tank holding treated water, so there's usable volume ready when you turn the tap on. RO produces water slowly; the tank is what makes it practical.
  4. A post-filter. Final polishing on the way out of the tank.
  5. The dedicated faucet. Mounted at the sink, separate from your regular tap.

Point-of-use, not whole-house

This is the single most important thing to understand before buying one.

RO treats water at one tap. It doesn't soften your home's water. It doesn't treat what comes out of your shower, your laundry or your water heater. It produces a limited volume for drinking and cooking, and that's what it's for.

If your complaint is the smell in the shower, RO won't help — you'd be looking at point-of-entry filtration. If your complaint is scale on your fixtures, RO won't help either — that's a softener conversation.

RO complements whole-house treatment. It doesn't replace it.

What determines what it reduces

You will see claims that RO "removes everything." Treat those with scepticism.

What a given system actually reduces depends on:

  • The specific unit and how many stages it has
  • The membrane type and its rating
  • The condition of the feed water arriving at it
  • Water pressure, which drives the whole process
  • The manufacturer's documented, verified performance data for that model

There's a legitimate, specific answer available for every real system. If a company can't produce it, that tells you something.

Why your feed water matters

Membranes are consumable, and they're the expensive part. Two things shorten their life significantly:

  • Hardness — scale builds on the membrane surface
  • Sediment — physically fouls it

Which is why, on hard water, softening ahead of the RO system often protects it and extends its working life. It's also why we ask about the rest of your water before quoting an RO system — not to upsell, but because installing one into unsuitable feed water is a short-lived solution.

Installation realities

In a typical suburban kitchen this is straightforward. The system fits in the cabinet, the faucet goes in an existing hole from a soap dispenser or sprayer, and the drain connection is close by.

In a Philadelphia rowhome with a shallow cabinet, a disposal unit and unusual plumbing, the practical question becomes whether everything fits and still leaves the cabinet usable. That's something we look at rather than assume.

Running a line to a refrigerator or ice maker is a common addition. Whether it's practical depends on distance and routing, and it's worth deciding upfront because it affects the output you need.

The drain water question

RO systems send some water to the drain as part of the separation process. How much depends on the system design, your feed pressure and your water conditions.

It's a genuine trade-off, and we'd rather raise it than have you discover it from a water bill. We'll explain what to expect for the specific system we're recommending.

Living with one

  • Pre- and post-filters — changed on a schedule. Letting them go costs you the membrane, which is the expensive part.
  • The membrane — replaced at a longer interval. How long depends heavily on feed-water conditions.
  • Sanitisation — done at service, particularly when the system has been opened.
  • Watch for changes — reduced flow at the faucet or a change in taste usually means something's due.

More: maintenance intervals.

Do you need one?

Probably worth it if:

  • Your main complaint is the taste of drinking water, coffee, tea or ice
  • You're buying bottled water despite having safe tap water
  • Testing has identified something specific you want addressed at the drinking tap

Probably not the right starting point if:

  • Your complaint is scale, or the shower, or laundry
  • You have untreated hard water or sediment that would foul it quickly
  • You're expecting it to treat the whole house