Sulphur smell or rotten egg smell in tap water is typically caused by the formation of hydrogen sulphide gas. The chemical breakdown of bacteria living in the water causes this gas. In most cases, the bacteria are harmless. Conduct a water test and submit the water sample to your local health department for signs of possible contamination.
Locate the Source
The sulphur smell can be located in one of two areas--the well itself or the hot-water heater. Flush the cold water lines, and fill a container to identify whether the rotten-egg smell is coming from the cold-water source. Run hot water from the water heater, and check for odour. In some instances the bacteria may be affected by the warm growing environment of the heater. Hot water alone that emits the smell can be treated in the heater itself.
If the sulphur smell comes from both the hot- and cold-water taps, chances are the bacteria are growing inside the well.
Attach a hose to the drain of the hot-water heater. Run the hose outdoors and open the drain. Flush the hot-water heater until a full volume of the tank has been run through the heater's tank. In most cases, flushing the water through the tank for a period of a half hour will be sufficient.
Close the drain on the water heater. Turn the thermostat on the heater to 71.1 degrees C for a period of three to four hours. The high temperature will kill the bacteria growing inside the vessel. Return the thermostat to the normal temperature range, and flush the water tank again. This process may have to be performed again if the smell is noticed in the future.
Treat the Well
If the smell comes from both the hot-water and the cold-water taps, the well will have to be treated with common household bleach. Most wells will respond overnight to such a treatment. In most instances, adding up to one gallon of household bleach into a private well will kill harmful bacteria, along with the sulphur-smell-producing bacteria.
Pour the bleach into the well and run the water from the faucets until you notice the heavy bleach smell. Run both the hot-water and cold-water taps. Allow the bleach to stay in the plumbing pipes for at least six to eight hours. The best results are obtained from overnight treatments of 12 hours or more.
Flush the water system until the bleach smell is has dissipated from the piping. This may entail draining the hot-water heater separately as described above. Attempt to flush the chlorinated water from the household system by using a hose attached to an outdoor spigot. Allow the water to run into an area where the chlorine bleach will not harm plant or aquatic life.
Excessive bleach running into a septic system may kill good bacteria that digest the solids in a properly operating septic system. If you think the waste system has been compromised, add a bacterial agent designed for septic systems.