Run the cold tap, then the hot. If only the hot water smells, the cause is the magnesium anode rod in your water heater, and swapping it for an aluminium rod fixes it for the price of a part. If both smell, the hydrogen sulfide is in the groundwater and treatment is sized by concentration.
The test that decides everything
Two minutes, no cost.
Run the cold tap and smell it. Then run the hot tap and smell that.
Result
Cause
Fix
Hot water only
The water heater’s magnesium anode rod
Replace it with an aluminium rod
Both hot and cold
Hydrogen sulfide in the groundwater
Treatment, sized by concentration
If it is hot-water only, you do not need a water treatment system. Bryan
Swistock at Penn State Extension explains the mechanism: the magnesium rod
“can chemically reduce sulfates to form hydrogen sulfide.” Swapping it for an
aluminium rod “should eliminate the rotten egg odor while maintaining
corrosion protection for the heater.”
That is a part and an afternoon, against several thousand dollars of
filtration. It is also the single most common reason someone buys a sulfur
filter they did not need.
If both taps smell
Then the hydrogen sulfide is in the water coming out of the ground. It occurs
naturally “as a consequence of the activities of sulfur-reducing bacteria,”
and is particularly common in wells drilled into acidic bedrock such as shale
and sandstone.
Treatment by concentration
The ranges barely overlap, so guessing is expensive.
Concentration
Method
Below 1.0 mg/L
Activated carbon
Below 2.0 mg/L
Aeration
Up to 2–3 mg/L
Oxidising filter
Above 6.0 mg/L
Chlorination or potassium permanganate
Note the gap between 3 and 6 mg/L. Penn State’s guidance does not name a
single method there, which in practice means an oxidising system sized at the
upper end or continuous chlorination — and it is the range where a water
treatment professional earns their fee.
Why there is no legal limit
There is no official drinking water standard for hydrogen sulfide. Penn State
gives the reason plainly: the gas makes water “aesthetically undrinkable long
before it reaches harmful concentrations.”
Most people can detect it “well below 0.5 mg/L” — far under any level that
would harm them. The sulfur-reducing bacteria producing it “do not cause human
health concerns” either.
So the smell is a nuisance problem, not a health emergency. Anyone selling you
a sulfur filter on health grounds is selling on fear.
Test before you buy
Because treatment is banded by concentration, a nose test tells you the cause
but not the size. A well panel that includes hydrogen sulfide gives you the
number the table above needs.
Once you have it, the sizing method places sulfur treatment
in the correct position in the treatment train — it usually shares a vessel
with iron removal, ahead of the softener.
The bottom line
Run the cold tap, then the hot, before you price a single system. If only the hot smells, the cause is the water heater's magnesium anode rod and an aluminium replacement fixes it for the price of a part. If both smell, the hydrogen sulfide is in the groundwater and treatment is banded by concentration — carbon below 1.0 mg/L, aeration below 2.0, oxidising filters to 3, chlorination above 6. Test for the number before buying, because those bands barely overlap. Nobody should be selling you a sulfur filter on health grounds.
Who this is not for
This will not help if the smell is musty, earthy or chemical rather than rotten eggs — those point at different causes and different treatment. It also does not address sulfate as a dissolved mineral, which is a separate problem measured on a water test rather than diagnosed by nose.
Questions
Why does only my hot water smell like rotten eggs?
The magnesium anode rod inside the water heater. Penn State Extension notes it can chemically reduce sulfates to form hydrogen sulfide. Replacing it with an aluminium rod should eliminate the odour while keeping corrosion protection for the heater. No water treatment system is needed for this case.
Is the sulfur smell in well water dangerous?
The sulfur-reducing bacteria that produce it do not cause human health concerns. Hydrogen sulfide gas is poisonous and flammable, but there is no official drinking water standard for it because the gas makes water aesthetically undrinkable long before it reaches harmful concentrations. Your nose is the limiting factor, not toxicity.
What removes hydrogen sulfide from well water?
It depends on concentration. Penn State Extension matches activated carbon to levels below 1.0 mg/L, aeration below 2.0 mg/L, oxidising filters up to 2 to 3 mg/L, and chlorination or potassium permanganate above 6.0 mg/L. Test before buying — the ranges barely overlap.
