If your water smells of rotten eggs, the cause is almost always hydrogen sulphide gas. The nose picks it up at tiny concentrations, so even a trace makes a shower or a glass of water unpleasant. It is common in borehole water, and it can also be created inside your own hot water cylinder. Working out which one you have is the first step to fixing it.
At a glance
- Odour standard: water must have an odour that is acceptable to consumers with no abnormal change
- Most at risk: boreholes and wells in low-oxygen groundwater; hot water cylinders
- Signs: rotten egg or sulphur smell, black tarnish on silver and copper, sometimes black slime
- Fix: hydrogen sulphide treatment or catalytic carbon filtration
Hot water only, or hot and cold?
This simple check tells you where the smell is coming from:
- Only the hot water smells. The problem is almost certainly inside your hot water cylinder. Bacteria that feed on sulphate in the water react with the cylinder's magnesium anode rod, which produces hydrogen sulphide, especially if the cylinder is kept at a low temperature or the hot water is rarely used. A heating engineer can check the anode and cylinder temperature. Treating the incoming water may not be needed.
- Hot and cold water both smell. The gas is coming in with your water supply. This is typical of borehole and well water drawn from low-oxygen groundwater, where natural bacteria break down sulphur compounds in the rock.
- Only one tap smells. The cause may be a drain or a little-used pipe rather than the water. Fill a glass, step away from the sink and smell it again.
On a mains supply, a sudden rotten egg smell from the cold tap is unusual; report it to your water company.
Is hydrogen sulphide harmful?
At the low levels that cause a smell in drinking water, hydrogen sulphide is mainly a nuisance rather than a health risk, because the smell makes water unpleasant long before it reaches harmful levels. It is corrosive, though: it tarnishes silverware and copper, can blacken plumbing fittings and is often found alongside iron and manganese, which react with it to form black deposits. If your supply suddenly develops a smell, test for bacteria too, as it can be a sign the source has changed.
How to test
Hydrogen sulphide escapes from water quickly, so it is hard to measure accurately in a posted sample. Your nose is a good first indicator. A private water supply test covering iron, manganese, pH and bacteria tells you what else is going on, which decides the right treatment.
How to remove the rotten egg smell
- Dedicated hydrogen sulphide systems oxidise the gas into solid sulphur and filter it out, often removing iron and manganese at the same time. Browse our hydrogen sulphide (H2S) range.
- Catalytic carbon handles low levels and is a good option for a drinking water point or modest whole-house problems. See carbon filters.
- Aeration strips the gas out by exposing water to air in a tank, suiting higher levels on larger supplies.
- Hot water only? Ask a heating engineer about the cylinder anode and storage temperature before buying treatment.
On a private supply, treatment goes before UV disinfection.
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. Tell us whether the smell is hot, cold or both, plus any test results, and our team will point you to the right fix. In North Shropshire we can install it for you.