Cloudy tap water is usually harmless, but not always. The most common cause, especially on mains water, is simply tiny air bubbles. Water that stays cloudy, turns brown or leaves grit in the glass points to something else, from limescale to sediment and iron. This guide helps you work out which you have and what to do about it.
At a glance
- UK limit: turbidity 4 NTU at the tap
- Usually harmless: milky water that clears from the bottom up (air)
- Worth checking: cloudiness that does not clear, brown water, grit or sand
- Fix: depends on the cause; often sediment filtration
The glass test
Fill a clear glass from the cold kitchen tap and leave it on the side for a couple of minutes. What you see tells you most of what you need to know:
| What happens | Likely cause | What to do |
| Milky white, clears from the bottom upwards within a minute or two | Air bubbles dissolved under pressure | Nothing; it is harmless |
| Stays cloudy or leaves a white film or flakes | Hardness minerals (limescale), often more noticeable in hot water | Check your hardness; consider a water softener |
| Yellow, orange or brown, settles to a deposit | Iron, from old mains or from groundwater | See iron and manganese |
| Black specks | Manganese, or perished rubber from a tap washer or hose | See iron and manganese; check flexible hoses |
| Grit, sand or silt settles at the bottom | Sediment, common on private supplies after rain | Fit sediment filtration |
Why air makes water cloudy
Water in the mains is under pressure, which lets it hold more dissolved air. When it comes out of the tap, the pressure drops and the air forms millions of tiny bubbles, giving a milky look. It is most common in cold weather, because cold water holds more air, and after work on the mains. The bubbles rise and the water clears from the bottom up. The Drinking Water Inspectorate confirms this is harmless.
Cloudy or dirty water on a private supply
On a borehole, well or spring, cloudiness that does not clear is called turbidity. It is caused by fine particles of clay, silt or organic matter, and it often gets worse after heavy rain as surface water reaches the source. Turbidity matters for two reasons:
- It often travels with bacteria, because the same rainwater that carries silt can carry contamination.
- Particles shield bacteria from UV light, so UV disinfection cannot work properly on turbid water. A sediment filter always goes before a UV unit.
Turbidity is measured in a private water supply test; the limit at the tap is 4 NTU.
Dirty water on a mains supply
Sudden brown, yellow or dirty water on mains water is usually caused by sediment in the water main being disturbed by a burst, repair or change in flow, such as firefighting. Run the cold kitchen tap for a few minutes. If it does not clear, contact your water company. Avoid washing laundry until it runs clear. If only the hot water is affected, the cause is more likely to be your cylinder or boiler.
How to fix cloudy or dirty water
- Sediment: a sediment filter or pleated filter cartridge, sized to the particles and flow rate. For whole-house protection see whole house filtration.
- Iron and manganese: iron and manganese reduction filters.
- Limescale: a water softener or scale conditioner. Find out how hard your water is.
- Not sure? A laboratory water test identifies the cause.
Related contaminants
Why Wrekin Water
Wrekin Water is a family run company based in Bettisfield, Shropshire, serving more than 50,000 customers over 15 years and rated Excellent on Trustpilot. Send us a photo of your glass test and a description, and our team will help you identify the cause.