Because anything the filtered water touches afterwards can undo the filtration. A shared spout carries residues from the mixed supply, and water sitting in it between uses is untreated.

A dedicated tap solves it by having its own outlet entirely. A tri-flow tap solves it internally, with a separate tube through the spout so filtered water only contacts that channel.

A three-way tap without genuine internal separation is not the same product, however similar it looks. Where the filtered supply simply joins the mixer, the filter is treating water that is then blended with what it was protecting against.

Bench space and plumbing against replaceability. A tri-flow tap keeps one outlet on the sink and one hole in the worktop, which suits a small or minimal kitchen.

A separate faucet needs a second hole and its own supply, but it isolates the filtered system completely from the mixer, so either can be changed without touching the other.

Tri-flow bodies are more complex castings with more internal seals, and a fault in one channel usually means replacing the whole tap. That is the trade for the tidier installation.

It depends entirely on the cartridge. Activated carbon addresses chlorine taste and odour and some organic compounds; finer media and membranes address particles, certain metals and microbiological contaminants.

No single cartridge does everything, and specifications are written against particular contaminants at particular reduction rates rather than as general purification.

Matching the cartridge to the local supply is the practical step. Filtering for taste in an area whose issue is hardness, or for hardness where the issue is particulate, produces a system that works and does not solve the problem.

By volume passed rather than by the calendar, which is why better systems count litres rather than months. A household drawing five litres a day exhausts a cartridge far more slowly than an office drawing fifty.

Manufacturers publish a capacity and a maximum service life, and both apply: a lightly used cartridge is still replaced eventually because the medium degrades and can harbour growth.

A spent carbon cartridge does not stop working abruptly. It gradually stops adsorbing, and the first sign is usually the return of the taste it was fitted to remove.

It needs a cold feed taken ahead of the filter, and the filter housing needs a position under the sink with room to change the cartridge, which is often the binding constraint in a cabinet already full of waste pipe and a bin.

An isolating valve on the feed is important, because the cartridge change involves opening the housing and nobody wants that to depend on the main stopcock.

Some systems fit a leak detection tray or shut-off under the housing. In a cabinet that is rarely opened, that is a reasonable precaution against a slow weep going unnoticed.

It changes the product from a tap into an appliance. Chilling and carbonation require a unit under the counter with a compressor or a gas cylinder, power, ventilation and its own servicing.

In an office or hospitality setting that is usually justified, because it replaces bottled water entirely and the usage supports the equipment.

In a domestic kitchen the decision is about how much cabinet space and maintenance appetite exists. A plain filtered tap is close to maintenance-free by comparison, needing only a cartridge.

More easily than most tap changes, provided there is a spare hole in the worktop or sink, or a position where one can be cut. Many sinks are supplied with a blanked second hole for exactly this.

Cutting a fresh hole depends on the worktop material. Stainless and composite are routine, engineered stone and granite need a specialist, and a solid timber top needs the cut edge sealed properly.

The under-sink work is straightforward by comparison: a tee off the cold supply, an isolating valve and a mounting position for the housing.