A rotten egg smell in household water is hydrogen sulfide gas, produced either by sulfur bacteria in the well or aquifer or by a reaction inside the water heater. The first job is not buying equipment, it is working out which of the three usual sources is responsible, because the fix for each is completely different.
Run the three-tap test before you buy anything
Fill a glass at an outside spigot or the closest fixture to where the water enters the house, then smell cold and hot separately at a bathroom sink, then smell the drain itself with the water off. If only the hot water stinks, the water heater is the source. If cold and hot both smell at every fixture, the problem is in the supply. If the smell disappears once you carry the glass to another room, you were smelling the drain, not the water.
When it is the water heater
The magnesium sacrificial anode rod in a heater reacts with sulfate in the water and with bacteria in the tank, and the byproduct is hydrogen sulfide. The tell is that the smell is strongest on hot water after the tank has sat overnight. Options are flushing and disinfecting the tank, raising the thermostat temporarily to kill the bacteria, or swapping the magnesium anode for an aluminum or powered anode. None of that helps if the cold water smells too.
When it is the well or the supply
Private wells are not regulated under the federal Safe Drinking Water Act, so nobody is testing your water but you. Sulfur bacteria live in the well casing and the plumbing, while dissolved hydrogen sulfide comes up with the groundwater itself. The distinction matters: bacteria respond to shock chlorination and a continuous disinfection setup, whereas dissolved gas needs oxidation and filtration, typically air injection or a chemical feed followed by a media tank that catches what the oxidation drops out.
Test first, then match the treatment
Hydrogen sulfide is volatile and escapes from a sample bottle, so a lab test needs a proper preserved sample or a field kit used at the tap. Pair it with iron, manganese, pH, hardness and a bacteria screen, because sulfur rarely arrives alone and the presence of iron changes which media will work. Our well water testing guide covers how to collect a sample that the lab will actually accept. Choosing a treatment train from a guess is how homeowners end up owning the wrong tank.
What each fix actually costs you in maintenance
An air injection oxidizing filter has no chemicals to buy but backwashes to drain and needs the air draw checked. A chlorine injection system removes odor reliably and disinfects, but you are refilling a solution tank and replacing a pump head on a schedule. Carbon alone will strip a light smell for a while and then break through without warning, which is why it belongs downstream of an oxidizer rather than on its own on a sulfur well.
Questions people ask about why does my water smell like rotten eggs
Is water that smells like rotten eggs dangerous to drink?
Hydrogen sulfide is objectionable long before it reaches harmful levels, so the smell itself is usually an aesthetic problem. The real concern is what else the same well conditions allow, which is why a bacteria and chemistry panel should accompany any sulfur complaint.
Why does only my hot water smell?
That points at the water heater, where the magnesium anode rod and tank bacteria generate the gas. Replacing the anode with aluminum or a powered unit, plus flushing and disinfecting the tank, usually clears it.
Will a water softener remove the rotten egg smell?
No. A softener exchanges hardness minerals and is not designed for dissolved gas, and sulfur bacteria can foul the resin bed. Odor removal needs oxidation and filtration ahead of the softener.
Does the smell come back after shock chlorinating the well?
Often it does, because shock chlorination kills bacteria present at that moment but does not stop the aquifer from delivering more, or reach every crevice in the formation. Recurring odor after shocking usually means continuous treatment is needed.