Glossary of water filtration terms
The words that turn up in quotes, on packaging and in manufacturer literature, explained plainly.
- Activated carbon
- Carbon processed to have an enormous internal surface area, used to adsorb chlorine, organic compounds and the substances that make water taste or smell wrong. The most widely used medium in domestic filtration. Does nothing for hardness. Carbon filters
- Air gap
- A physical vertical gap between a waste outlet and the drain it discharges into, so that waste water can never be siphoned back into the supply. Required on softener and reverse osmosis drain connections, and one of the main reasons those are not DIY jobs. Softeners · Reverse osmosis
- Alkalinity
- A measure of water's capacity to neutralise acid, distinct from hardness though often correlated with it. Matters a great deal in coffee, where it buffers acidity and shapes how the cup tastes. Water filtration for cafés
- Backflow
- Water flowing the wrong way through a plumbing system, potentially carrying contamination into the drinking supply. Preventing it is the central concern of the Water Supply (Water Fittings) Regulations 1999. Do I need a plumber to install a water filter?
- Bypass
- A valve arrangement that lets water route around a softener or filter so the property keeps its supply while the unit is serviced or removed. Not optional on a softener. Bypass kits
- Calcium carbonate (CaCO₃)
- The compound hardness is measured against, expressed in mg/l or ppm. Note that water companies sometimes publish calcium (Ca) instead, which is roughly 40% of the CaCO₃ figure — confusing the two inverts the advice completely. What is hard water?
- Carbon block
- Activated carbon compressed into a solid cartridge. Gives better contact time and finer filtration than loose granular carbon, at some cost to flow rate. Generally the better choice for drinking water. Carbon filters
- Cartridge
- The replaceable element inside a filter housing that does the actual work. Over a system's life the cartridges cost considerably more than the system did. Cartridges
- Degrees Clark
- An older British unit of hardness. One degree Clark is roughly 14.3 mg/l CaCO₃. Still occasionally quoted, particularly on older equipment. What is hard water?
- Drinking Water Inspectorate (DWI)
- The regulator for public drinking water quality in England and Wales. Sets and enforces the standards water companies must meet, and publishes compliance data.
- Fluid category
- A classification from 1 to 5 of how much risk a particular water use poses to the mains supply, which determines what backflow protection is required. Relevant on commercial premises, where the category can be higher than in a house. Commercial systems
- Hardness
- Dissolved calcium and magnesium in water, picked up from chalk and limestone geology. Measured in mg/l CaCO₃. It is what causes limescale. What is hard water?
- Ion exchange
- The process a water softener uses: hardness ions (calcium and magnesium) are swapped for sodium ions on a resin bed. The resin is periodically regenerated with brine, which is why softeners need salt and a drain. Softeners
- Isolation valve
- A small in-line valve letting you shut off one appliance without turning off the whole property. The cheapest insurance in plumbing, and the difference between a fifteen-minute cartridge change and a half-hour one. Isolation valves
- KDF
- A copper-zinc alloy medium used in some filters, particularly shower filters, effective against chlorine and some heavy metals. Often paired with carbon. Shower filters
- Limescale
- The hard deposit left when hard water is heated or evaporates. An excellent thermal insulator, which is why it is so damaging inside boilers and heating elements. Is hard water bad for your boiler?
- Micron rating
- The particle size a sediment filter is rated to stop, in micrometres. Lower is finer. Below 1 micron, flow restriction becomes noticeable. Sediment filters
- NAV
- New Appointment and Variation — a licensed water supplier serving specific developments rather than a whole region. NAVs buy water in bulk from the incumbent, so hardness follows whoever actually supplies the area.
- Plumbosolvency
- Water's tendency to dissolve metals from pipework. Soft, slightly acidic water is more plumbosolvent than hard water, which is why lead pipework is a sharper concern in soft-water areas. Water filtration for landlords
- Point of entry (POE)
- Treatment fitted where water enters the property, so everything downstream is treated. A whole-house filter or a softener. Whole-house filters
- Point of use (POU)
- Treatment fitted at one outlet, typically the kitchen tap. An under-sink filter or a reverse osmosis system. Under-sink filters
- Regeneration
- The cycle in which a softener flushes its resin bed with brine to recharge it, discharging the waste to drain. Metered softeners regenerate on volume used; timed ones on a schedule regardless. How to choose a water softener
- Reverse osmosis (RO)
- Forcing water through a semi-permeable membrane to remove dissolved substances. The most thorough domestic filtration available, and the only one that rejects water to drain. Reverse osmosis
- Rising main
- The pipe bringing mains water into the property, from the stopcock onwards. Work on it cannot be isolated except at the stopcock, which is why cutting into it is professional work. Do I need a plumber to install a water filter?
- Scale inhibitor
- A device that changes how hardness minerals crystallise so they do not bond as scale, without removing them. Protects equipment downstream of it. Not a softener. Scale inhibitors
- Supply zone
- The area served by a particular treatment works or blend of sources. Water companies publish quality data by supply zone, which is why the hardness figure they give you describes a zone rather than your specific tap. How to test water hardness at home
- TDS
- Total dissolved solids, in parts per million, estimated from electrical conductivity. Useful for monitoring reverse osmosis performance over time. Not a safety measure and not a hardness measure. TDS meters
- TrustMark
- A government-endorsed quality scheme covering trades generally. Not water-specific, but a genuine check with a free public register.
- WaterSafe
- The UK-wide accreditation scheme for plumbers approved to work on drinking water systems. Approved plumbers can self-certify work that would otherwise need notifying to your water company. Free public register.
- Water Supply (Water Fittings) Regulations 1999
- The regulations governing plumbing connected to the public supply in England and Wales, aimed at preventing waste, misuse and contamination. They apply to the work regardless of who does it — there is no DIY exemption. Scotland and Northern Ireland have equivalent frameworks. Do I need a plumber to install a water filter?
- WIAPS
- The Water Industry Approved Plumber Scheme, operated by WRAS. Overlaps substantially with WaterSafe and serves a similar purpose.
- WRAS
- Water Regulations Advisory Scheme. Approves plumbing products as compliant with the water fittings regulations, and operates WIAPS. A WRAS-approved fitting is one that will not contaminate the supply.