Iron, manganese and sulfur filters

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.

Dissolved iron is invisible and passes straight through a filter, because there is nothing solid to catch. The entire job of an iron filter is to make it solid first.

The filter introduces an oxidant — air, in most modern systems — which converts dissolved ferrous iron into ferric iron particles. Those particles are then trapped in a bed of catalytic media. Periodically the unit backwashes, lifting and scouring the bed to flush the accumulated iron to drain, and drawing in a fresh pocket of air for the next cycle.

Air injection oxidation is the dominant residential design because it consumes nothing but air and water. Older systems used potassium permanganate regeneration — effective, but it means handling a chemical that stains everything it touches and is genuinely toxic to swallow. Chlorine injection is reserved for heavy iron, heavy sulfur or iron bacteria, where air cannot keep up.

This is the thing to understand before spending anything. Oxidation is pH-dependent, and the dependency is steep.

Iron oxidises reasonably above about pH 7. Below that the reaction slows dramatically, and in genuinely acidic water it barely proceeds at all. Manganese needs more still — reliable oxidation generally wants pH above 8, which is higher than a lot of well water reaches naturally.

The practical consequence is that in acidic water, an iron filter is not underperforming because it is too small. It is underperforming because the chemistry it depends on is not happening. Fitting a larger filter changes nothing. Fitting an acid neutraliser ahead of it changes everything.

Acid neutralisers→

Both remove iron, which is why this question comes up constantly and why people are sold the wrong one.

A softener removes iron incidentally, as part of ion exchange, and it does so well at low concentrations. Up to about 3 mg/L it is a perfectly reasonable approach — and if you need a softener for hardness anyway, it is the cheaper path because it is one machine rather than two.

Above roughly 3 mg/L the arithmetic reverses. Iron fouls the resin faster than regeneration can strip it, capacity falls off, the bed eventually needs chemical cleaning or replacement, and most manufacturers explicitly void the warranty for iron above their stated limit. At that point an iron filter ahead of the softener is not an upgrade, it is a requirement.

Where you have both iron and hardness above the thresholds, you need both units — and the iron filter goes first.

See where your iron figure lands→

Iron filters are sized on flow rate rather than on concentration. The media bed needs enough contact time to oxidise and enough backwash flow to clean itself, and both are functions of how fast water moves through the tank.

The mistake is sizing on average use rather than peak flow. A filter that copes with a single tap will be overrun by two showers and a washing machine, and iron breaks through when it is. Work from the house's simultaneous peak demand, not from daily gallons.

The other constraint is backwash. A bed that cannot be lifted properly during backwash compacts, channels and stops working, so the well has to be able to supply the backwash flow rate the unit requires. On a low-yield well this can be the deciding factor in which system you can fit at all.