What is in London tap water, and what reverse osmosis actually takes out

If you live in London, the water in your kitchen has already travelled through chalk before it reaches you. That single fact explains most of what people notice about it: the taste, the kettle, the film on a glass, and why a filter that works in Manchester may behave differently here.

This article is about what is in the water and what a reverse-osmosis machine actually does to it. It is not about health. The machine is a water treatment appliance, and nothing here is a medical claim.

Where London's water comes from

A large share of London's supply is groundwater drawn from the chalk aquifers under the city and the surrounding hills, topped up with river water from the Thames and the Lea. Water that has spent time in chalk is carrying dissolved calcium and magnesium — the two minerals that make water "hard".

That is why the same tap can taste different in two postcodes. The blend of groundwater and river water changes across the supply area and through the year, and hardness changes with it.

What hardness actually is

Hard water contains dissolved calcium and magnesium bicarbonates. They are not contaminants and they are not a fault — they are minerals the water picked up on its way through rock. The visible consequences are the ones everyone recognises:

• Limescale. When hard water is heated, calcium carbonate comes out of solution and deposits on the element, the kettle wall, the inside of the tap. It is the same chemistry in a kettle and in a boiler.
• Soap that will not lather. Calcium and magnesium react with soap before the soap can do its job.
• A dry, chalky feel on glass and skin once the water has evaporated.

Chlorine, and why it is there

Water leaving a treatment works is disinfected, and a small chlorine residual is deliberately maintained all the way to your tap — it is what keeps the water safe as it travels through the mains. The residual is why tap water in London often smells faintly of a swimming pool, and why a glass left on the counter can taste flatter the next day.

Chlorine is easy to remove. It is what carbon is for, and it is the reason a carbon cartridge sits at the front of most filtration trains.

What the four stages in the KW-01 do

The KW-01 uses four stages in series, every litre:

1. PAC composite — pleated PP cotton around a carbon rod. Mechanical filtration for sediment and carbon for chlorine and taste.
2. 200G reverse-osmosis membrane — rated at 0.0001 µm. This is the stage that rejects dissolved material: the calcium and magnesium behind hardness, along with other dissolved salts.
3. CF-Strontium carbon cartridge — a second carbon pass after the membrane.
4. Electrolysis at the spout — the stage that dissolves hydrogen into the water as it pours.

A reverse-osmosis membrane works by rejecting a stream of water to the drain rather than passing everything through. That reject stream is not a fault; it is the mechanism. It is also why the machine needs a drain within reach.

What changes, and what does not

What changes is measurable and dull: the limescale stops forming in the kettle, the chlorine taste goes, and the dissolved mineral content drops. What does not change is just as important — the water does not become medicine, and the machine does not claim to treat anything.

The dissolved hydrogen figure the machine reports — up to 2,000 ppb on the KW-01's own display, 1.0–1.5 ppm for the tank-fed KW-02 — is a reading from the unit's sensor at the moment of dispensing. It is a ceiling, not a floor, and it is a measurement rather than a promise.

How to check your own kitchen

Two numbers tell you most of what a filter will change in your water:

• Total dissolved solids (TDS) — the overall dissolved content. In a hard London area it is usually high; after a reverse-osmosis membrane it is dramatically lower. Comparing before and after is the clearest proof the membrane is working.
• Hardness — calcium and magnesium specifically, usually reported in mg/L as calcium carbonate.

A handheld TDS meter answers the first question in ten seconds. The 6-in-1 and 8-in-1 testers in this store answer it along with pH, conductivity and temperature, if you want the wider picture.

Your water supplier publishes its own analysis for your postcode every year, and that report is the authoritative starting point — not a blog, including this one.

Frequently assumed, rarely true

• "Hard water is unsafe." It is not; it is water with minerals in it. It is hard on kettles and boilers, not on people.
• "A filter removes everything." A carbon cartridge removes chlorine and taste. A reverse-osmosis membrane removes dissolved material. They do different jobs, which is why the machine uses both.
• "Filtered water needs no maintenance." Cartridges are consumables with rated volumes — 1,000 litres for the PAC and CF-Strontium, 2,000 for the RO membrane — and they stop working when they are spent.

Sources

1. Thames Water, published water quality reports for London supply zones (thameswater.co.uk) — hardness and the blend of groundwater and river sources by area.
2. Drinking Water Inspectorate (dwi.gov.uk) — what is regulated in public supply, and why a chlorine residual is maintained to the tap.
3. This store's own product data (read 11 November 2026): the four-stage order, the 0.0001 µm membrane rating, the 1,000 L / 2,000 L cartridge volumes, and the dissolved-hydrogen figures for the KW-01 and KW-02.

If you want to know what the machine would change in your specific kitchen, the honest answer starts with your postcode's water quality report and a TDS reading — not with a page like this one.