If the water from your well smells of rotten eggs, the cause is hydrogen sulphide, a gas dissolved in the water. You can smell it at very low levels, so a strong smell doesn't always mean a lot of gas. It also tarnishes silver, leaves black stains and is hard on pipework and fittings.
First, check which taps smell
- Only the hot taps: the smell is being made in the hot water cylinder, not the well. In some cylinders the protective rod inside (the anode) reacts with the water, and bacteria can also grow in a cylinder that is kept too cool. That is a job for a plumber, not a water filter.
- Hot and cold taps: the gas is in the well water itself, and it needs treating where the supply comes into the house.
- Only when the water hasn't been run for a while: it is likely to be building up in the well or the pipework between uses.
Where it comes from
Hydrogen sulphide occurs naturally in some groundwater, from the rock the well is drilled through or from rotting organic matter. It can also be produced by bacteria that live on sulphur in the well. The bacteria themselves are not normally harmful, but a smell that has appeared recently is a good reason to have the water tested for other bacteria at the same time.
Getting it tested
An independent, accredited laboratory does the analysis; we don't test water ourselves. Tell the lab about the smell when you book the test. The gas escapes from a sample quickly, so they may want the sample taken or preserved in a particular way. Ask for iron, manganese and pH as well, because those decide which treatment will work.
What removes it
- Catalytic carbon filter. A vessel of catalytic carbon on the main supply turns the gas into a solid and holds on to it, then backwashes itself clean. It suits low to moderate levels.
- Oxidising filter. Where there is iron or manganese as well, a filter that oxidises all three and traps them in one bed is often the better answer.
- Dosing and filtration. High levels may need a measured dose of an oxidiser ahead of the filter.
If bacteria in the well are producing the gas, disinfecting the well with chlorine (bleach) can clear it for a while, but it tends to come back unless a filter is fitted.
What to do next
Send us the lab report through our well water treatment page, and tell us which taps smell and when. We'll specify the system from your results and our own plumbers will fit it. If the smell is only on the hot side, ring us anyway: the plumbing side of the business can sort the cylinder.
Also on well water: iron in well water and bacteria in well water.