How often should a carbon filter be changed?

Follow the manufacturer's rated capacity in gallons rather than waiting for a sign, because there isn't one — carbon saturates silently with no pressure drop and no visual change. For whole-house cartridges that is typically every six to twelve months.

Will a carbon filter remove the rotten egg smell?

For light hydrogen sulfide, below about 0.5 mg/L, yes — and catalytic carbon does better than standard. Beyond that the carbon saturates very quickly and you need an oxidising system instead.

Does a carbon filter remove bacteria?

No, and a neglected one can harbour and release them. Carbon should be followed by UV if disinfection matters, not relied on for it.

Do I need carbon on well water if there's no chlorine?

Often not. Carbon's headline job is removing chlorine, which well water does not contain unless you are injecting it. It earns its place for taste and odour, for organic chemicals where there is a known risk, and after any chlorination stage you have added.

Carbon filters

The short answer

Activated carbon removes tastes, odours, chlorine and a range of organic chemicals by adsorption — contaminants stick to the enormous internal surface area of the carbon. It saturates rather than clogging, which means it stops working without any visible sign, so it must be changed on schedule.

Where it goes: After the softener; before UV if both are fitted.
Typical cost: $300–$1,500 installed depending on type and size

What it removes — and what it doesn't

This is the part sales pages are vaguest about, so it is worth being blunt. No single stage treats everything, and buying the wrong one for your water is the most common way people end up with expensive equipment that changes nothing.

Removes

Chlorine and chloramine
Tastes and odours, including mild hydrogen sulfide
Many volatile organic compounds and pesticides
Some disinfection byproducts

Does not remove

Hardness
Nitrate or arsenic
Most dissolved metals
Bacteria — and a saturated bed can actually harbour them

Adsorption, and why it ends without warning

Activated carbon works by adsorption rather than filtration. The carbon is processed to have an extraordinary internal surface area — a single gram has hundreds of square metres of it — and contaminant molecules adhere to that surface as water passes.

The critical property is that adsorption capacity is finite. Every site that gets occupied is one that cannot be used again. When the sites run out, the carbon stops removing anything, and water passes through unchanged.

This is the important difference from a sediment filter, and the reason carbon gets neglected. A sediment filter announces its exhaustion by restricting flow — you notice, because the pressure drops. A saturated carbon filter passes water exactly as freely as a fresh one. There is no pressure drop, no taste change at first, and no visual cue. It simply stops working, silently, and continues to look fine.

Granular carbon versus carbon block

Granular activated carbon is loose media, either in a cartridge or in a backwashing tank. Water flows around the granules. It handles high flow with little pressure drop, which makes it the practical choice for whole-house duty, and a backwashing tank version lasts years rather than months.

Carbon block is carbon compressed into a solid element. Water is forced through the material itself, so contact is far more thorough and the block also acts as a mechanical filter down to a few microns. That makes it much more effective per unit of carbon, at the cost of a real pressure drop. It is the usual choice at a single tap or as an RO pre-filter.

Catalytic carbon is a modified form worth knowing about, because it handles chloramine and hydrogen sulfide considerably better than standard carbon does.

The bacteria caveat

A carbon bed is a large, damp surface with organic material adsorbed onto it, which makes it a reasonable place for bacteria to grow. In a well-maintained system on treated water this is not a practical concern.

It becomes one when a carbon filter is left in place far beyond its life, particularly on water that has any biological activity. The bed can then shed bacteria into the water downstream — a filter making the water worse rather than better.

This is the specific reason carbon goes before UV rather than after it. If both are fitted, the UV lamp should be the last thing the water passes, so anything the carbon releases is dealt with.

Why UV always goes last

Cost figures are planning ranges for typical residential equipment, gathered from published pricing. They are not quotes, and installed cost varies widely with your plumbing, your water and your region. Get written quotes before committing.

Frequently asked questions

How often should a carbon filter be changed?: Follow the manufacturer's rated capacity in gallons rather than waiting for a sign, because there isn't one — carbon saturates silently with no pressure drop and no visual change. For whole-house cartridges that is typically every six to twelve months.
Will a carbon filter remove the rotten egg smell?: For light hydrogen sulfide, below about 0.5 mg/L, yes — and catalytic carbon does better than standard. Beyond that the carbon saturates very quickly and you need an oxidising system instead.
Does a carbon filter remove bacteria?: No, and a neglected one can harbour and release them. Carbon should be followed by UV if disinfection matters, not relied on for it.
Do I need carbon on well water if there's no chlorine?: Often not. Carbon's headline job is removing chlorine, which well water does not contain unless you are injecting it. It earns its place for taste and odour, for organic chemicals where there is a known risk, and after any chlorination stage you have added.

Keep going

Reverse osmosisUses carbon as a pre-filter and a post-filter.Rotten egg smellWhere carbon helps, and where it saturates too fast.UV disinfectionWhat should follow carbon in the train.The correct treatment orderCarbon's place in the sequence.