What Brita filters remove -- and what they do not
Brita Standard and Longlast filters use activated carbon and ion exchange resin. They are certified to reduce: chlorine taste and odor (NSF 42), lead (NSF 53, Longlast filter only), copper, mercury, cadmium, zinc, and particulates. Brita filters are effective for aesthetic improvement (taste, odor) and select health-effect contaminants (lead).
Brita filters do NOT significantly reduce: total dissolved solids (TDS), hardness minerals (calcium and magnesium), sodium, nitrates, or fluoride. A Brita pitcher does not soften hard water. If your supply water is 20 GPG hard and you run it through a Brita pitcher, the filtered output is still 20 GPG hard -- no scale reduction. Brita carbon media has no ion exchange capacity for divalent cations (Ca2+, Mg2+) at the flow rates achievable in a gravity pitcher.
The Brita Maxtra Pro for limescale -- what it actually does
In European markets, Brita sells the Maxtra Pro filter with an "anti-limescale" claim. This filter includes an additional media that uses template-assisted crystallization (TAC) to partially convert dissolved calcium bicarbonate to aragonite crystals that do not adhere to surfaces. This is NOT ion exchange softening -- the calcium remains in the water (TDS is not reduced) but in a form that causes less scale adhesion in kettles and coffee makers. The effect is meaningful for appliance scale in European-style hard water (300-500 mg/L), but it does not deliver soft water for plumbing, laundry, or bathing purposes.
When to use Brita alongside a water softener
Many households benefit from both: a whole-home ion exchange softener to protect plumbing, appliances, and fixtures from hardness scale, and a Brita pitcher or under-sink filter for drinking water to remove any residual taste from the sodium added during softening. Ion exchange softeners add 20-40 mg/L of sodium to softened water (the amount depends on source hardness). Brita carbon filtration does not remove sodium -- if sodium-in-softened-water is a concern, an under-sink reverse osmosis system (not a Brita filter) is required to reduce sodium below 20 mg/L.