What's inside a shower filter? KDF, vitamin C and more

What is in a shower filter? Usually a cartridge packed with filter media that act on free chlorine as the water flows through. Four are common: calcium sulfite, vitamin C (ascorbic acid), KDF copper-zinc granules and activated carbon. Each works differently, and every cartridge eventually needs replacing.

Below, we take each medium in turn: what it is, what its source says about how it reacts with free chlorine, why cartridges combine them and what none of them does. If you are still weighing up the whole idea, our complete guide to filter shower heads in the UK starts from the basics.

We sell a filter shower head whose cartridge includes all four, so each source is linked in the text and listed at the end.

What is in a shower filter? The short answer

A shower filter cartridge usually holds one or more filter media chosen to act on free chlorine, one form of the disinfectant left in UK tap water. Four names come up again and again: calcium sulfite, vitamin C, KDF and activated carbon.

Free chlorine is the total of chlorine, hypochlorous acid and hypochlorite ions in water, as the Drinking Water Inspectorate (DWI) defines it in its treatment guide on chlorine disinfection. It is what a filter shower head is designed to reduce.

The amounts involved are small. The DWI says water companies typically keep the residual disinfectant at 0.5 mg/l or less, and that chlorine at these levels is safe to drink.

Where the cartridge sits varies. In a filter shower head, sometimes called a shower head filter, it is built into the head or the handle. An inline shower filter is a separate unit screwed into the water's path before the head; either way, the media inside do the work.

Each medium on a label tells you how a cartridge is meant to act on free chlorine. It does not tell you how well a given product performs; that needs test evidence.

Calcium sulfite: what it is and why shower filters use it

Calcium sulfite (also spelt sulphite) is a calcium salt that reacts with chlorine in water. In shower filters it comes as granules or beads, and a 2000 US patent sets out why.

US patent 6,056,875, granted in 2000 for shower filter media, starts from a problem. It says powdered calcium sulfite is highly soluble. It also says plain calcium sulfite was not used in shower filters, because of the heat and flow of a typical 5 to 10 minute hot shower.

The inventor's answer was to bind calcium sulfite into ceramic-type beads and pair them with copper-zinc media, to treat hot shower water at high flow rates.

A patent is its inventor's own rationale, not independent proof that a product works. It explains why calcium sulfite is chosen for showers, but not how much free chlorine a given cartridge reduces. On a label, calcium sulfite points to a medium meant for hot, fast-flowing water; for performance, look for the maker's test evidence.

Vitamin C (ascorbic acid): how it neutralises chlorine

Vitamin C neutralises chlorine in water, in either of two forms: ascorbic acid or sodium ascorbate. In April 2005, the USDA Forest Service reported that ascorbic acid's reaction with chlorine is very fast.

The USDA note was written for operators of seasonal water systems, who sanitise tanks and pipes with a strong chlorine solution. They must neutralise that water before it reaches lakes or streams. The note is not about showers, so it shows the chemistry rather than how any shower filter performs.

The note sets out the reaction: ascorbic acid reacts with hypochlorous acid, one form of free chlorine, and becomes dehydroascorbic acid. About 2.5 parts of ascorbic acid neutralise 1 part of chlorine, the note says, so vitamin C is used up as it works.

For a shower, speed can matter. Water moves through a cartridge quickly, so a medium that reacts fast can make use of that short contact. On a label, vitamin C means a fast-reacting medium that is consumed as it neutralises free chlorine.

What is KDF? Copper-zinc granules explained

KDF is a granular filter medium made from copper and zinc, sold under the KDF name by Kymera International. The maker says it works by redox and can convert chlorine into chloride.

Redox is short for reduction-oxidation: electrons pass from one substance to another. In KDF's case, the maker says free chlorine is changed into chloride, which it describes as less reactive. The letters KDF stand for Kinetic Degradation Fluxion.

The maker also says KDF media can operate in high-temperature settings and suit hot water uses, including showerhead filters. It adds that KDF is often used in combination with other filters, such as carbon.

These are the maker's own statements, and its page covers other uses we leave out, because this guide sticks to free chlorine. On a label, KDF means copper-zinc granules that react with free chlorine and, by the maker's account, cope with hot water.

Does activated carbon reduce chlorine in a shower?

Activated carbon can reduce chlorine taste in drinking water, and the DWI points to it for that job. For hot shower water, a 2000 US patent and the maker of KDF both say carbon copes less well, which may explain why shower cartridges tend to combine it with other media.

Activated carbon is a form of carbon used in filters that works by adsorption: substances cling to its surface. The DWI's treatment guide names adsorption on activated carbon as one of the methods used in water treatment. In cartridges it often comes as granules, known as granular activated carbon or GAC.

For chlorine taste at home, the DWI's advice is simple: a jug of tap water in the fridge reduces it, or you could try a jug filter with an activated carbon cartridge. On home water filters, it says that if taste is your reason for buying one, it should contain activated carbon. It adds that the cartridge must be changed as the manufacturer instructs, or it will stop working.

Showers are hotter and faster. The patent says carbon works well on drinking water at room temperature (25 °C) and low flow, but that high flow and temperature limit its efficiency with shower water. Kymera says KDF can operate at high temperatures where carbon may degrade.

Both statements come from people offering an alternative to carbon, so read them as rationale rather than proof. If a shower filter lists carbon, check what else is in the cartridge.

Why do shower filters combine several media?

Shower filters combine media because each one acts on free chlorine differently and, by its own source's account, copes differently with hot, fast-flowing water. The 2000 patent pairs calcium sulfite with copper-zinc, and the KDF maker says its media are often used alongside carbon. The table sets out what each source says.

Each source speaks for its own medium; none compares the four side by side in a shower.

Every cartridge also has a limit. The USDA note shows vitamin C being consumed as it reacts, and the DWI says a home filter cartridge that isn't changed as the manufacturer instructs will stop working.

Test standards put a number on that limit. In NSF/ANSI 177, a US standard for shower filters described by Plumbing & Mechanical in 2005, water with 2.0 mg/l of free chlorine at 40 °C runs through the filter. Its capacity is set by the last of at least five consecutive passing samples.

Capacity is why every cartridge needs a replacement schedule. Our guide to how often to change a shower filter cartridge explains how to set one.

What shower filter media don't do

Shower filter media do not soften water or remove limescale. Water hardness is dissolved calcium and magnesium, the DWI explains, and a shower filter is not a water softener.

The DWI says water softeners work by ion exchange, replacing calcium and magnesium with sodium. That is a different process from the reactions above, and it happens in a softener fitted to your supply, not in a shower head.

The DWI's bands class water of 100 mg/l or less, measured as calcium carbonate, as soft, and say water of 200 mg/l or more will produce scale. In a hard water area, a filter shower head can collect limescale like any other head. If limescale is your concern, our comparison of a shower filter and a water softener sets out which does what.

What the sources don't tell you

The sources in this guide explain chemistry, not the performance of any one cartridge. The USDA note was about dechlorinating water before discharge, the KDF statements come from its maker, and the patent is its inventor's rationale.

None of them tells you how much free chlorine a given shower filter reduces at your temperature and flow, or how long its cartridge lasts in your home. For that you need test evidence and its conditions; our article on whether shower filters work and what the evidence shows explains what to ask for.

The same limit applies to us: the one claim we make for our own head is that it reduces free chlorine.

Inside the Waterhush cartridge

The cartridge in the Waterhush Shower Kit includes four filter media: calcium sulfite, vitamin C, KDF (copper-zinc granules) and carbon. We don't state their order, amounts or proportions, and the one claim we make is that Waterhush reduces free chlorine.

The cartridge comes already fitted in the handle, so the water passes through it on its way to the 10 jets, numbered 01–10. Our page on how the head, cartridge and handle fit together shows the layout.

Refills are the same cartridge, each sealed in its own airtight sachet; see the refill cartridge and its plan options. We suggest a new cartridge every 3 months, or every 2 months for households of two or more: a suggestion, not a promise of how long a cartridge lasts.

The kit also includes a 1.5 m flexible hose, a washer, PTFE tape and an instruction leaflet, and the head weighs about 400 g.

The bottom line

A shower filter cartridge holds one or more media, such as calcium sulfite, vitamin C, KDF and activated carbon, each acting on free chlorine in its own way. Its cartridge runs out over time, the media do not soften water, and their sources explain chemistry rather than any one product's results. Read the label, ask for the evidence behind any claim and change the cartridge on schedule.