If the water from your taps smells of rotten eggs, the cause is almost always 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 marks on sinks and is hard on pipework. It's one of the easier well water problems to fix once you know where it's coming from, and the first step costs nothing: work out which taps smell.
Quick answer
- Hot taps only: the hot water cylinder is making the gas, usually because the magnesium anode inside it is reacting with the water. A plumber can swap the anode and flush the cylinder.
- Hot and cold taps: the gas is in the well water. It's treated where the supply comes into the house, with an air injection oxidising filter or a catalytic carbon filter.
- Bacteria in the well: if sulphate-reducing bacteria are making the gas, shock chlorinating the well with household bleach clears them, though a filter is usually still needed if the smell returns.
- Before buying anything: get the water analysed by an accredited lab, including iron, manganese, pH and bacteria.
First, work out which taps smell
Run a glass of water and smell it away from the sink. A smell coming up from the plughole is a drain or trap problem, not the water.
| What you notice | Likely cause | What to do |
|---|---|---|
| Hot taps only | The cylinder anode reacting with the water, or bacteria in a cylinder kept too cool | A plumber replaces the anode and flushes the cylinder |
| Hot and cold taps all over the house | Hydrogen sulphide in the well water, or sulphate-reducing bacteria in the well | Lab analysis, then treatment on the incoming supply |
| One tap only | Water lying in a little-used pipe, an old flexible hose or a dirty trap | Flush the line, replace the hoses, clean the trap |
| Clears after running the tap for a minute | Water standing in the well or pipework between uses | Lab analysis to see how much gas is in the water |
Check the outside tap. On most houses it's fed straight off the incoming supply, before the cylinder and any filters. If the outside tap smells too, the gas is in the well water. If it doesn't, look at the cylinder and the plumbing inside first.
What causes the rotten egg smell
- Hydrogen sulphide in the groundwater. Some wells pick it up naturally from the rock or from rotting organic matter underground, mostly where the water carries very little oxygen.
- Sulphate-reducing bacteria. These live in low-oxygen water in the well, pipework or cylinder and give off hydrogen sulphide. The EPA names them as the source of a rotten egg odour in private wells.
- The anode in the hot water cylinder. Some cylinders have a magnesium rod called an anode fitted to protect the tank from corrosion. In some waters it reacts and produces hydrogen sulphide, which is why the smell can be far worse from the hot tap.
- Stagnant water in little-used pipe runs, dead legs and old flexible hoses.
Is it harmful?
The World Health Organization doesn't set a health-based limit for hydrogen sulphide, because at the levels found in drinking water it isn't a health concern. You'll smell it long before it could do any harm.
Don't ignore it, though. The EPA advises that some odours in well water can point to a risk to health, and that you should get the well tested and tell the lab about the smell. A smell that appeared recently, after heavy rain or work on the well, is a good reason to test for bacteria such as E. coli too, and to boil drinking water until the results come back. Our guide to bacteria in well water explains what a failed result means.
Hot taps only: sort out the cylinder
If only the hot side smells, a water filter won't help. The gas is being made inside the cylinder, and the fix is a plumbing job:
- Replace the anode. Swapping the magnesium anode for an aluminium-zinc one, or a powered anode, usually stops the reaction. Don't just remove it, as it protects the cylinder from corrosion.
- Flush the cylinder to clear the smelly water and any sludge in the bottom.
- Keep the stored water properly hot. A cylinder run lukewarm to save money gives bacteria an easy home.
Ring us anyway: the plumbing side of the business, CPS Plumbing & Electrical, can sort the cylinder.
Hot and cold taps: treating the well supply
When every tap smells, the treatment goes on the incoming supply, after the pump and pressure vessel and before the water reaches the rest of the house:
| Treatment | How it works | Best for |
|---|---|---|
| Catalytic carbon filter | A vessel of specially treated carbon turns the gas into a solid and holds on to it, then backwashes itself clean through an automatic valve such as a Clack | Low to moderate levels, with little iron |
| Air injection oxidising filter | Air drawn into the vessel turns hydrogen sulphide, iron and manganese into solid particles, and the filter bed traps them. It backwashes and takes in fresh air automatically | Moderate levels, especially with iron or manganese too |
| Dosing plus filtration | A small pump adds a measured dose of chlorine or hydrogen peroxide, a contact tank gives it time to react, and carbon polishes out what's left | High levels, or where the other two can't keep up |
| Carbon filter at the kitchen tap | Improves taste and smell at one tap | A final polish for drinking water, not a fix for the house |
Which one suits your house depends on the lab results, mainly the hydrogen sulphide level, iron, manganese and pH. A plain carbon cartridge on a well with real hydrogen sulphide is used up quickly and the smell comes straight back. If your water also stains orange or black, read our guide to iron in well water, as one filter can often deal with both.
When bacteria are the cause: shock chlorination with bleach
If sulphate-reducing bacteria are living in the well, disinfecting the well and pipework with chlorine can clear the smell. This is called shock chlorination, and it's done with plain household bleach with no added scent. The EPA sets out a method, which runs like this:
- Bypass any filters, softener, UV unit and drinking water system first, as a strong chlorine dose can damage them.
- Mix the bleach with water and pour half of it into the well.
- Run each tap in turn, starting with the kitchen, until you can smell chlorine. Add the rest of the mix, switch off the pump and cover the well.
- Leave it overnight, at least 8 hours.
- Turn the pump back on, run every tap until the chlorine smell is gone, then send a fresh sample to the lab.
How much bleach you need depends on its strength and the size of the well, so follow the EPA method or talk to us first.
Before you start: never mix bleach with other cleaning products, keep children and animals away from the open well, and don't drink the water until it has been flushed and retested. The EPA is clear that a one-off shock dose can't replace a treatment system if the problem keeps coming back. If the smell returns within weeks, a filter is the long-term answer.
Get the water analysed first
We don't test water ourselves. An independent, INAB-accredited laboratory does the analysis, and we specify the treatment from the lab report. When you book, ask for:
- Hydrogen sulphide. Mention the smell, as the gas escapes from a sample quickly and the lab may want it taken a particular way.
- Iron, manganese and pH, which decide between catalytic carbon and an oxidising filter.
- E. coli and coliforms. The EPA says these are the most important tests for any private well, and recommends testing for bacteria at least once a year.
Then send us the report through our well water treatment page, and tell us which taps smell and when.
Well water smells of rotten eggs?
Send us your lab report and tell us which taps smell. We'll specify the right treatment from your results and our own plumbers will fit it, nationwide except Donegal, Kerry, Leitrim and Sligo. Hot side only? We can sort the cylinder too.
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Common questions
Will a carbon filter on its own remove the smell?
At low levels, a catalytic carbon vessel that backwashes itself can. A standard carbon cartridge only manages it briefly before it's used up. Higher levels need the gas oxidised first.
Why is the smell worse from the hot tap?
Hot water holds less dissolved gas, so more is released when you run it, and the cylinder anode can make extra. If only the hot side smells, it's a cylinder job, not a filter job.
Will a UV system get rid of the smell?
No. UV kills bacteria but doesn't remove hydrogen sulphide. On a private well, UV is often fitted after the filter that deals with the smell.
Will a water softener remove the smell?
No. A softener only removes hardness. Hydrogen sulphide needs its own treatment, fitted ahead of the softener so it doesn't foul the resin.
Is there a grant for well water treatment?
The EPA says local authorities give grants for improvement works on private wells where they're needed. Ask yours what's available before you go ahead.
