Guide
Reverse osmosis water filters in the UK: what they do, need and cost
A reverse osmosis (RO) water filter works differently from a carbon filter. It pushes water through a very fine membrane that holds back most of what is dissolved in it, and sends the rest to waste. That has practical consequences for space, water use, upkeep and the water you end up drinking, so it is worth understanding them before you buy a reverse osmosis system.
How reverse osmosis works
Water is forced, under pressure, against a semi-permeable membrane. Treated water passes through; most dissolved material is held back and flushed away in a separate stream, usually called reject or waste water. Most systems add other stages around the membrane, such as sediment and carbon filters before it, and sometimes a further stage after it.
Two things follow. First, an RO system always sends some water to a drain or collection tray, so there is a waste connection or a waste routine to plan for. Second, it changes the water more than a carbon filter does, which is the point and also the trade-off, covered below.
What the official guidance says
Before you decide, read what the Drinking Water Inspectorate says. Its guidance for England and Wales, domestic water filters and softeners, makes three points that matter here:
- Public water supplies in England and Wales don’t need treatment at home to protect health, so a home filter is a choice you make for your own reasons.
- Reverse osmosis water is not generally suitable for drinking and cooking without minerals being added back.
- Your water company should be notified before a reverse osmosis unit is installed.
This page paraphrases that guidance; the original is the authority, and it covers England and Wales only. Scottish and Northern Irish households should check the equivalent advice for their supply.
The water fittings rules point the same way. In England and Wales a reverse osmosis unit connected to the supply is one of the installations that must be notified: your water supplier has to be told and give consent before the work starts. WaterRegsUK sets out the list in its England requirements. That is about units connected to the supply; if you are considering a refill-style appliance that isn’t plumbed in, ask your supplier whether anything applies.
What an RO system needs
Requirements are specific to each model, so read the manual for the one you are considering. The usual questions are:
| Requirement | What to find out |
|---|---|
| Water pressure | Many membranes work to a stated pressure range. Check what the manual asks for, and whether the model includes a pump if your pressure is low. |
| Electricity | Some units have a pump or electronics and need a socket; others don’t. Countertop plug-in units run from the mains. Find out what happens in a power cut. |
| A waste connection | Under-sink systems usually send reject water to the household drain, using a waste fitting. Some countertop units use a tray or a hose to the sink. Where will it go in your kitchen? |
| A storage tank | Some systems store treated water in a tank; others are tankless and treat as you draw. Tanks take cupboard space; tankless units may need more pressure or power. |
| A tap | Under-sink RO usually uses its own small tap, which may need a hole in the sink or worktop. Check this against your tenancy and surfaces. |
The amount of water sent to drain differs between models and conditions, so ask the maker for the stated figure for the model rather than relying on a general ratio. If you have a water meter, water sent to drain still counts as water used, which is a running cost to consider.
Countertop or under-sink reverse osmosis
| Countertop | Under-sink | |
|---|---|---|
| Space | Worktop footprint, plus a socket and somewhere for waste water. | Cupboard space for the membrane, filters and possibly a tank. |
| Plumbing | Often none; some connect to the tap by a hose. | A connection to the cold supply and to the drain, by a qualified plumber, with your water company notified first. |
| Renting | More likely to be possible without permanent changes. | Usually needs the landlord’s permission, and may need drilling. |
| Waste water | A tray, a reservoir or a hose to the sink, depending on the model. | A fitting on the waste pipe. |
| Upkeep | Cartridge and membrane changes, plus cleaning the reservoir or tray. | Cartridge and membrane changes, usually in the cupboard. |
For the wider fit questions, see the under-sink guide and the countertop guide.
Minerals and remineralisation
Because RO holds back most dissolved material, the water that comes out is lower in minerals than your tap water, and that includes the calcium and magnesium behind hardness. People sometimes buy RO for exactly that reason; it is also why the official guidance cautions about drinking and cooking with it.
Some systems include a remineralisation stage that adds minerals back. What it adds, and how much, differs between products, so read the maker’s statement for the model, and check whether any independent certificate covers it. We can’t tell you how a particular product’s water will compare with your tap water. If limescale is your main concern, the hard water guide sets RO alongside the other options.
Cartridges, membrane and upkeep
RO systems have several stages, and each has its own change schedule. A typical arrangement is one or more pre-filters, the membrane, and sometimes a post-filter or remineralising stage. The pre-filters are usually changed more often than the membrane, but the intervals differ between models, so use the manual’s figures. Some stages have a time limit, some a litre limit, and some both.
In the running-cost calculator, add each stage as its own cartridge with its own cost and interval. If a price is missing, the total stays marked incomplete rather than guessing. Read how change dates work for the details, and check that every stage’s replacement is available before you commit to a model.
Costs to add up
- The system, and delivery.
- Installation: a plumber’s quote for the cold connection, the waste connection and the tap, or your own time.
- Every cartridge and the membrane, each on its own schedule, over one, three and five years.
- Water sent to drain, if you are on a meter.
- Electricity, if the model has a pump or runs from the mains.
Questions to ask before you buy
- Which exact UK model and variant is this, and what does its manual say about pressure, power, waste and tank?
- What is the replacement schedule for each stage, and what does each cost today?
- What does the maker say about minerals, and is there a remineralisation stage? What exactly does it do?
- Does an independent certificate name this exact model and the RO standard, NSF/ANSI 58? How to check.
- Has my water company been told and consented before work starts, and has a qualified plumber checked the installation meets the requirements for my area?
Details vary by model; the maker’s instructions and a qualified plumber have the final say.