What is causing that rotten egg smell in my water?

Cathal Walsh
Environmental Blogger
Friday, 12th April 2019
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Image of an egg submerged within a glass of water

This smell is rather easy to notice when it is coming from your taps and appliances. Nearly everyone knows what a rotten egg smells like. If you happen to get the odour of a rotten egg coming from the water in your home, you’d be very surprised as to how many people share the same problem.  There are two main reasons why you may be getting this smell from your water.  The first is as a result of iron bacteria in your water and the second is a result of hydrogen sulphide in your water.

Iron Bacteria:

Iron bacteria are bacteria that derive their energy from oxidizing iron commonly found in groundwater. These bacteria present no threat to health, but alongside the rotten egg smell they can cause severe staining and damage to fixtures and water treatment devices. They are also an aesthetic nuisance and often result in taste issues. 

The best treatment for this is often regular repeated chlorination treatments of the water or constant chlorination of the water.

Hydrogen sulphide:

The rotten egg smell in these instances is as a result of this volatile gas. The intensity of the odour here depends on the levels of hydrogen sulphide in the water.  Due to the volatile nature of the gas it is often quite difficult to test for it. A sample may be taken and in the time between sampling and lab analysis the hydrogen sulphide would have left the sample. As a result when testing for hydrogen sulphide it is often necessary to test for it at the site of sampling.

Fortunately however hydrogen sulphide is not harmful in the levels it is found at in water. If hydrogen sulphide were to be found in water at harmful levels then the odour and taste coming from the water would be so putrid that drinking it would be greatly challenging. However due to its slightly corrosive nature, water with hydrogen sulphide may cause corrosion and cause silver to tarnish.

H2S gas is found in the ground normally and makes its way into a water source from there. Due to its volatile nature it gets released during the pressure change when a tap is turned on.  This is most noticeable when getting the smell from a direct tap like the cold kitchen tap, IE any tap that comes directly from the pump.

If there is a noticeable smell from the direct tap, but no smell from the indirect taps in the house then this is a fairly distinctive indicator of H2S gas.  It also confirms that the level of the gas is fairly low. At moderate levels you would get a bad smell from the direct taps and a less foul smell from the indirect taps. At high levels one would get a foul smell from all the taps in the house.

For homes with only pressurised water systems, i.e. no water tank, the way to test if the rotten egg odour is being caused by H2S gas is to fill 2/3 of a bottle of water with the tap water and leave it for a few hours. If the smell is gone afterwards, that is a positive indicator of H2S gas.

The best way to get rid of H2S gas is by a filtering system or using reverse osmosis. There are chemical and biological processes to remove it too however. The specific treatment that is best really depends on the context and level of the H2S gas.

If you can confirm H2S in your water supply, the most important think to keep in mind is that although it needs to be treated, it is not harmful and one should not panic.

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