A greensand filter is a backwashing tank of manganese-coated media that oxidizes dissolved iron, manganese and hydrogen sulfide into solid particles and then traps them in the bed. It is the workhorse for metals in well water, and it is a system rather than a cartridge: a mineral tank, a control valve, an oxidant supply and a drain.
What it removes, and why it needs an oxidant
Iron and manganese come out of the ground dissolved and invisible. Penn State Extension notes that both carry secondary, aesthetic standards, with recommended maximums of 0.3 mg/L for iron and 0.05 mg/L for manganese, which is why the complaint is usually orange or black staining rather than a health worry. Greensand media catalyzes the conversion of that dissolved metal into a solid, and then filters the solid out. The catalytic coating is consumed as it works, so the system needs an oxidant to regenerate it, most commonly potassium permanganate or a continuous chlorine feed.
The two regeneration schemes
Intermittent regeneration draws a measured dose of potassium permanganate solution from a tank during backwash, recharging the media between service runs. It handles higher metal loads and is the traditional configuration, but it means keeping a permanganate solution tank in the utility room, mixing it, and handling a chemical that stains anything it touches a vivid purple. Continuous regeneration feeds a small amount of chlorine or permanganate ahead of the filter at all times, so the media never depletes. That is less handling for the owner but adds a chemical feed pump and a contact tank to the train, and any carbon downstream will consume the residual.
The water chemistry that makes or breaks it
Greensand is fussy about conditions. It needs adequate pH to oxidize iron and considerably more to handle manganese, so acidic well water often needs a neutralizer ahead of the filter. It has a temperature limit, it is intolerant of some oxidizers on the classic glauconite form, and dissolved organics or tannins bound to the iron can lock it in a form the bed will not catch. Iron bacteria, which produce slime rather than simple particles, will blind a bed that was correctly specified for chemistry alone. This is why a full water test, not just an iron number, comes before choosing a greensand system.
Living with one
A greensand tank backwashes on a timer or meter, and that backwash needs a real drain with an air gap and enough flow to lift and scour the bed. Undersized backwash flow is the usual reason a bed cements up and stops working. Expect to top up or replace media over time, since some is carried out with each backwash, and expect to check the oxidant supply the way you check softener salt. Placed correctly, ahead of the softener, it protects the resin from the oxidized iron that would otherwise foul it, and our whole house water filter page shows where it sits in the wider train.
Questions people ask about greensand filter
Does a greensand filter remove hardness?
No. It targets iron, manganese and hydrogen sulfide. Calcium and magnesium pass straight through, so a softener is still needed for hardness, installed after the greensand filter.
Do I need potassium permanganate?
With intermittent regeneration, yes. Continuous chlorine feed is the alternative and avoids handling permanganate, at the cost of adding a chemical feed pump and contact tank.
Why has my greensand filter stopped working?
The common causes are a depleted oxidant supply, insufficient backwash flow letting the bed cement, low pH, or iron bacteria slime coating the media.
Should the greensand filter go before or after the softener?
Before. Oxidized iron fouls softener resin, so the greensand filter must capture it upstream of anything containing ion exchange resin.