Quick answer: Rotten-egg smell is hydrogen sulfide (H2S) or sulfur-reducing bacteria. Before buying anything, run the cold/hot split test: if only the hot water smells, your water heater’s magnesium anode rod is the culprit — a $20–40 aluminum/zinc replacement fixes it with zero filtration equipment. If cold and hot both smell, it’s the well/aquifer — then KDF-85, manganese-dioxide media (Katalox-class), or aeration is the honest ladder. The smell threshold (~0.05 ppm) is far below any health concern — this is a comfort problem with a diagnostic order.
Test first — the 60-second split. Run the cold tap hard for 30 seconds into a clean glass, smell it (waft, don’t inhale). Then the hot tap the same way. Cold clean + hot rotten = heater/anode problem (read no further than the anode section — you may be done today for $25). Both smelly = source-water problem (continue to the comparison table). One faucet only = that fixture’s drain or aerator — often the cheapest non-water answer of all.
Where the Sulfur Smell Comes From
1. The water heater anode (hot-only cases). Water heaters ship with a magnesium sacrificial anode; in water with sulfate-reducing bacteria (harmless themselves, ubiquitous), the magnesium feeds a chemistry that produces H2S inside the tank — hot water arrives pre-loaded with the smell while cold stays clean.
2. Sulfur-reducing bacteria in the well and plumbing (both-taps cases). SRB convert natural sulfate to H2S in low-oxygen environments: well columns, pressure tanks, and long pipe runs. The diagnostic companion signs: black slime in toilet tanks and black stains on silverware — see the manganese page for distinguishing those.
3. Naturally occurring H2S in the aquifer. Some formations deliver hydrogen sulfide directly — usually modest concentrations, regionally clustered. Its odor threshold (~0.05 ppm) is dramatically lower than any health-relevant concentration, which is the framing that keeps this page calm: by the time H2S is a health issue, the smell has evacuated the building.
The diagnostic tree (one minute, no equipment)
Observation
Diagnosis
First move
Cost
Only hot water smells
Water heater SRB + magnesium anode
Replace anode with aluminum/zinc; disinfect tank per manual
$20–40 DIY
Both hot and cold smell, everywhere
Source-water H2S / SRB in the system
Sulfide-inclusive lab test; then media selection below
$40 test, then $800–2,500
One faucet smells, others clean
Fixture drain/overflow or aerator gunk
Clean the drain and aerator — the water is fine
$0
Smell appears only after water sits (mornings/vacations)
Stagnation biofilm in pipes
Flush protocol; persistent → chlorination
$0–20
Health Effects — What the Limits Are Protecting
Number
What it is
What it means
~0.05 ppm
Odor threshold
The nose detects H2S far below any health line — smell ≠ danger
No federal MCL
H2S is unregulated as a primary contaminant
It’s classified as an aesthetic problem; the SMCL framework addresses odor generally
$20–40 vs $1,500+
The anode-vs-system gap
The reason the split test runs before any purchase
The honest framing: at household concentrations, H2S is a quality-of-life problem — embarrassing glasses of water, dying appetite for tap water, corroded silverware and copper fixtures over time. The documented health effects of chronic low-level exposure are modest and mostly nuisance-tier (some GI and headache associations at high chronic exposures, which households essentially never reach without knowing). Treat it for comfort and plumbing protection — with the diagnosis first, because the correct treatment ranges over two orders of magnitude in price.
How to Remove the Sulfur Smell — The Honest Comparison
Method
Fit
Honest notes
Anode replacement (Al/Zn)
Hot-only diagnosis
The $25 fix for the most common presentation — a plumber installs in an hour if the rod’s stuck
KDF-85 media (or KDF-85/carbon blend)
Light-to-moderate both-taps H2S
Effective point-of-entry cartridge/tank media for modest levels; finite capacity
Manganese dioxide media (Katalox-class)
Moderate H2S with iron/manganese riding along
The trio-media: one system when all three appear (see iron, manganese)
Aeration (air-stripping) systems
Higher concentrations
Venturi or tank aeration strips H2S to atmosphere; the heavy-artillery option that also handles iron
Chlorine or peroxide injection + contact tank
Stubborn/bacterial cases
Continuous oxidation; the same chemistry as municipal practice — requires maintenance discipline
~~Carbon cartridges alone~~
Weak partial
Carbon adsorbs H2S briefly then exhausts fast — a $40 cartridge bought monthly is a slow-motion system purchase
~~Boiling~~
Partial and pointless
Drives off some H2S while concentrating everything else; nobody boils their way out of a sulfur well
~~New water heater~~
The misdiagnosis purchase
Replacing the heater without changing anode chemistry re-creates the smell within months — the ✗ that pays for this page’s diagnosis section
Matched to the Numbers Above
Diagnostic tier ($0–40): The split test (free), followed — for both-taps cases — by a lab panel that includes sulfide (not all standard panels do; ask specifically) plus iron/manganese, which travel together.
Anode tier ($20–40 + optional plumber hour): For hot-only diagnoses; add tank disinfection if the manual supports it.
System tier ($800–2,500): Media or aeration selected from the lab numbers — with the honest note that “one filter for sulfur” is usually undersized; SRB systems (well chlorination + media) outlast cartridge approaches when bacteria are the engine.
Wait — Is My Refrigerator Filter Enough for the Sulfur Smell?
Partially — and “partially” is doing unpaid work. A fresh carbon cartridge will mask modest H2S at the dispenser briefly, then exhaust early because sulfur loads carbon faster than chlorine ever will. You’ll conclude the fridge “needs new filters constantly” when the actual problem is upstream. Fix the source (anode or system treatment); let the fridge filter do its real job (chlorine and taste).
Related problems worth checking while you’re here:
Smelly water (all odors) — the six-odor router page: musty, chlorine, fishy, fuel — each is a different page
Bacteria in water — the SRB backstory and when to sanitize the system
Sulfur Smell Questions, Answered Straight
Why does my well water smell like rotten eggs? Hydrogen sulfide — from sulfur-reducing bacteria feeding on sulfate in your heater (hot-only), your well/plumbing (both taps), or the aquifer itself. The cold/hot split test on this page identifies which in one minute.
How much sulfur smell is too much? There’s no federal health limit — H2S at household levels is an aesthetic problem. You treat it when it bothers the household or corrodes fixtures; the smell threshold (~0.05 ppm) sits far below any health-relevant concentration.
Does boiling water remove the sulfur smell? It drives off some H2S (the smell is volatile) — while concentrating other dissolved content. It’s a tea-kettle workaround, not treatment.
Is my refrigerator or pitcher filter enough for sulfur smell? Briefly. Carbon adsorbs H2S at modest levels but exhausts quickly — chronic “my fridge filter stopped working” in sulfur homes is the source problem wearing a filter costume. Fix upstream.
Why does only my hot water smell? The water heater: magnesium anode + sulfate-reducing bacteria produce H2S inside the tank. Replace the anode with aluminum/zinc ($20–40) — do not buy a whole-house system for this signature.
Can city tap water smell like sulfur, or is it only wells? Mostly wells — city water runs chlorinated, which suppresses the sulfur-reducing bacteria that make H2S. But municipal sulfur smells do happen: seasonal source switches (lake-to-well rotations), algae events adding taste-and-odor compounds people read as “sulfur,” or a building’s own water heater hosting the same anode+bacteria pair on a smaller scale. The split test works the same: hot-only → your heater’s anode; both taps on chlorinated city water → call the utility and ask what changed with the source before buying anything.
Is hydrogen sulfide in water toxic? At household concentrations, no meaningful toxicity — the odor threshold is orders of magnitude below danger levels. Chronic high-exposure occupational settings are the context for H2S harm, not residential water.
Picking between the treatment tiers — a follow-the-number selector
Once the split test has sent you past the $25 anode fix, the both-taps ladder prices itself by concentration and companions. The sulfide-inclusive lab report (ordered by name — the reminder from the top of this page matters here) hands you three numbers: sulfide level, iron, and manganese.
Your lab picture
The honest tier
Low sulfide (<0.3 ppm), no metals
KDF-85 or KDF/carbon blend — cartridge scale handles it
Moderate sulfide + iron/manganese riding along (the classic trio)
Manganese-dioxide media (Katalox-class) — one tank, three contaminants, the well-country default
High sulfide (1+ ppm)
Aeration — the heavy-artillery tier that strips gas and oxidizes metals in one pass
Sulfide that returns weeks after shock treatment
Bacterial engine — continuous oxidation (chlorine/peroxide injection) addresses the SRB colony, not just its product
The pattern to notice: the trio diagnosis saves the most money in this whole category — buying “a sulfur filter” while ignoring co-occurring iron (or vice versa) is how households end up owning two systems where one media tank was the answer.
