Begin with the reason for treatment
Domestic systems include jugs, countertop units, gravity systems, plumbed cartridges and reverse osmosis. DWI lists these as distinct domestic treatment types and warns that marketing claims can create concern about tap-water quality.[3]
Choose a format after defining the outcome. A stage count or “purifier” label is not an evidence standard.
Compare the physical format
| Format | Questions that change the decision |
|---|---|
| Jug or pitcher | volume, contact time, fridge space, cartridge identity and hygiene |
| Countertop | connection, footprint, output, reservoir cleaning and replacement availability |
| Gravity | exact element, throughput, cleaning and claim scope |
| Under-sink cartridge | dedicated tap, flow, housing space, isolation and cartridge schedule |
| Reverse osmosis | pressure, drainage, product-water output, storage or tankless design, pre/post-filters and membrane |
The table organises questions. It does not rank a product.
Read claims at model level
“Carbon,” “multi-stage” and “RO” describe a process or arrangement. They do not establish what an exact model reduces. Look for the parameter, claim type, test/certification basis, capacity, flow and conditions.
Do not treat Regulation 31 approval as a domestic product-performance badge. DWI states that products used within a property boundary do not require approval under Regulation 31.[7]
Reverse osmosis needs a complete brief
DWI’s domestic-treatment page describes RO as changing water properties, reducing pressure and producing a waste stream.[3] Keep that statement attributed to DWI and verify current exact-model figures from the manufacturer before any comparison.
Record:
- feed-water evidence and any pre-treatment need;
- feed pressure and temperature range;
- rated and realistic product-water output;
- storage volume or tankless behaviour;
- drain connection and reject-water conditions;
- pre-filter, post-filter and membrane part numbers;
- sanitisation and service process;
- electricity use where a pump is present.
A household comparison should never turn one manufacturer’s rated condition into a universal RO result.
Maintenance changes the product
DWI advises following manufacturer cleaning, maintenance and cartridge-change instructions.[3] A filter used beyond its specified trigger is not the same operating condition as the product described in the evidence.
For each candidate, build a replacement table with exact part, time/volume rule, current price checkpoint, availability and disposal route. Use “whichever comes first” only when the manufacturer actually specifies it.
Installation questions
Check isolation, bypass, space, drainage, electrical supply, tap compatibility, pressure and who carries out the work. Confirm the applicable water-fittings requirements with the water company or a suitably qualified installer rather than treating a retailer badge as permission.
A fair comparison
Compare products only when they address the same documented objective. Keep these separate:
- evidence fit;
- installation fit;
- capacity/output;
- maintenance burden;
- five-year cost;
- evidence quality;
- unknowns.
An unknown specification stays unknown. It is not scored as a zero or replaced by a plausible value.
Next steps
- Confirm the evidence →
/water-testing/ - Compare whole-house alternatives →
/whole-house-treatment/ - Separate hardness treatment →
/hard-water-softening/ - Review taps and installation →
/taps-showers-leaks/ - Build ownership cost →
/costs-maintenance/