Filtration, softening and conditioning: three different things

Start with the complaint

Filtration

Sediment filtration

A physical barrier with a defined pore size, removing suspended particles. Rated in microns – a lower number means finer filtration and a faster-blinding cartridge. Appropriate where there is a genuine particulate problem: a private supply, a borehole, or an older property with corroding steel pipework. Not usually necessary on a mains supply.

Activated carbon

Carbon with an enormous internal surface area that adsorbs organic compounds, chlorine and the compounds responsible for taste and odour. This is what makes water taste better, and it is the right answer for almost everybody whose complaint is about taste. Available as a cartridge under the sink, as a combined sediment-and-carbon cartridge, or in a filter jug.

Carbon exhausts. Once the surface is saturated it stops adsorbing and, importantly, can begin to release what it captured. Cartridges must be changed on the manufacturer’s schedule, by date rather than by whether the water still tastes acceptable – taste is a poor indicator of exhaustion.

Reverse osmosis

Water is pushed through a semi-permeable membrane that rejects the great majority of dissolved substances, including minerals. Extremely effective, and the right answer for specific contamination concerns and for applications needing very pure water. The trade-offs are real: it is slow, it sends a proportion of the feed water to drain, it needs a storage tank and space, and it removes beneficial minerals along with everything else.

UV disinfection

Ultraviolet light inactivating bacteria and other microorganisms. The standard treatment for a private supply with a bacteriological issue. It disinfects only – it removes nothing – and it requires the water to be clear, because particles shield organisms from the light. UV is therefore almost always preceded by sediment filtration.

Softening

Covered fully in how an ion-exchange water softener works. The essential point for this comparison: it removes calcium and magnesium and nothing else. It does not filter, does not disinfect, does not improve taste, and does not remove chlorine.

A softener and a drinking water filter are complementary rather than alternatives, and the common arrangement – a softener for the house, with a carbon-filtered hard tap for drinking – gives you both.

Conditioning

Devices that prevent scale adhering without removing hardness. The water remains chemically hard; what changes is how the calcium precipitates. Polyphosphate dosing is well established and genuinely effective downstream; electrolytic and magnetic devices have a more mixed evidence base.

Conditioning protects equipment. It does not change how soap behaves, does not stop the kettle furring much, and does not stop deposits on taps and screens. See scale inhibitors.

Private supplies: test before you treat

A borehole or private well is a different situation entirely, because no water company is monitoring it. Treatment must be specified from a laboratory analysis, not from a guess.

1. Get an accredited laboratory test covering bacteriology, and the chemical parameters relevant to the supply – iron, manganese, nitrate, pH, hardness and others depending on the geology.
2. Talk to your local authority. They have duties in relation to private water supplies, including risk assessment and sampling for supplies serving certain premises, and they are a good first call.
3. Specify treatment against the results, in the right order – typically sediment removal first, then any chemical treatment, then UV last, because UV needs clear water.
4. Plan for maintenance. Every element of a private supply treatment train is a consumable or a serviceable item, and an unmaintained UV lamp or a blinded filter provides nothing.
5. Retest periodically, because a supply can change.

Fitting a carbon filter to a private supply with a bacteriological problem is worse than doing nothing, because it feels like you have acted. See water filtration and agriculture.

Maintenance, which decides whether any of it works

• Change cartridges by date, not by taste or by appearance.
• Fit isolation valves, so a cartridge change is not a drain-down.
• Sanitise housings at cartridge changes, particularly on anything that stands unused for periods.
• Keep filters out of heat and light, which encourage growth inside the housing.
• Do not create a dead leg. Filtered water sitting stagnant in a rarely used branch is a worse proposition than moving unfiltered water.
• Replace UV lamps on schedule. A UV lamp continues to glow long after its output has fallen below the level that disinfects – it gives visual reassurance while doing nothing.