Filtered Is Not Automatically Better

Running tap water through a filter is the standard first step for anyone improving their coffee. Whether it helps depends entirely on what the filter removes and what was wrong with the water to begin with.

Carbon filters target taste and odour

Activated carbon works by adsorption, holding organic molecules and chlorine compounds on its enormous internal surface area as water passes through.

Chlorine in particular is worth removing, since it is added to supplies for disinfection and contributes a distinctly medicinal note to hot drinks.

What carbon does not do is remove dissolved minerals, so hardness and alkalinity pass through essentially unchanged.

Ion exchange changes the mineral balance

Jug filters and plumbed cartridges frequently combine carbon with an ion exchange resin, which swaps some dissolved ions for others.

Softening resins replace calcium and magnesium with sodium, which reduces scale formation but also removes the minerals extraction relies on.

Resins designed for coffee instead target carbonate, lowering alkalinity while leaving calcium and magnesium in place, which is usually what a brewer actually wants.

Capacity is finite and performance declines

Both carbon and resin have a limited capacity, and once exhausted the cartridge passes water through with little effect while looking exactly the same.

Exhausted carbon can also release accumulated material back into the water, so an overdue filter is worse than none at all.

Manufacturers state capacity in litres, and that figure assumes typical water, so a hard supply exhausts a cartridge considerably faster than the rating suggests.

The right filter depends on the water

A supply that is soft but heavily chlorinated needs carbon and nothing else, and adding softening would strip minerals that are already scarce.

A supply high in carbonate needs alkalinity reduction, and a plain carbon filter will improve the smell while leaving the flat cup unchanged.

Testing first, rather than buying a filter and hoping, is what separates an effective intervention from an expensive habit.

Machines have their own requirements

Espresso machines and kettles suffer from scale, and protecting them can require treatment that is not ideal for flavour.

Some setups use different water for brewing and for the boiler, or accept a compromise water and descale on a schedule.

Recognising that machine protection and cup quality are separate goals prevents a good deal of frustration with filters that were only ever solving one of them.