A sulfur filter treats hydrogen sulfide, the dissolved gas behind the rotten egg smell in well water. It works by oxidizing that gas into solid elemental sulfur inside a media bed, then flushing the solids to drain on a backwash. The smell disappears because the gas is gone, not because it has been masked.
Confirm it is sulfur before you buy a filter
Not every bad smell is hydrogen sulfide. Run cold and hot separately at several taps. If only the hot side smells, the culprit is usually the water heater anode reacting with sulfate-reducing bacteria in the tank, and the fix is at the heater rather than a whole-house filter. If both smell everywhere, the gas is arriving with the water and a treatment system is the answer. If only one fixture smells, suspect the drain rather than the supply. Get a sulfide test done on a properly collected sample, and take iron, manganese, pH and hardness at the same time, because those readings decide which media will work on your water and which will not.
The common media, and what each one suits
Catalytic carbon adsorbs and catalyzes sulfide and does it quietly, with no chemical feed, which suits light odor and doubles as a chlorine and taste filter on city water. Manganese dioxide style oxidizing beds handle heavier sulfide, and usually iron and manganese along with it, but they backwash more and several want a healthy pH to perform. Air injection systems draw a pocket of air into the top of the tank, oxidize the sulfide against it and backwash the solids away, which handles moderate loads with no consumables. Chemical injection, usually chlorine ahead of a contact tank and a filter, is what heavy sulfide and iron bacteria need, and it disinfects at the same time. Filter brands publish these as distinct product families sized in flow steps.
Order of treatment matters
Sulfur treatment rarely stands alone, because the same wells that make hydrogen sulfide usually carry iron and sometimes manganese. Oxidation is the shared mechanism, so a well designed train handles them together: sediment first to protect what follows, oxidation and filtration next, then softening, then any point of use polishing. Putting a softener in front of an oxidizing filter is a common and expensive mistake, since oxidized iron and sulfur foul resin. If pH is low, a neutralizer usually comes first, because several oxidizing media lose effectiveness on acidic water. And where a chlorinator is used, the contact time before the filter is part of the design, not an optional extra.
Living with it
Backwashing filters need a drain that can take the flow, enough incoming pressure and flow to lift and rinse the bed properly, and a power supply for the control valve. Set the backwash frequency to the load rather than to a default: too seldom and the bed cements and smells, too often and you waste water. Media beds do not last forever, and a bed that has been fouled by iron or exhausted by heavy loading will let odor through even though the valve is cycling normally. If the smell returns, test again before replacing anything, since a returning smell can equally mean the well has changed, the bypass is open, or a heater anode is the real source.
Questions people ask about sulfur filter
Why does only my hot water smell of rotten eggs?
That points at the water heater rather than the well. Sulfate-reducing bacteria reacting with the tank anode produce the gas inside the heater itself.
Will a softener remove the sulfur smell?
No. Ion exchange targets hardness, not dissolved gas. Putting a softener ahead of untreated sulfide also risks fouling the resin with oxidized material.
Do sulfur filters need chemicals?
Not all of them. Catalytic carbon and air injection systems use no added chemical, while heavier sulfide with iron bacteria usually needs chlorine injection.
Is hydrogen sulfide in water dangerous?
At household well levels it is mainly a taste, odor and corrosion nuisance. It is still worth testing, since the same wells often carry iron and bacteria too.