An air injector is the stage in a well water filter that puts a pocket of air into the incoming water so that dissolved iron, manganese and hydrogen sulfide oxidize into solid particles the media bed underneath can trap. It is a treatment component, not a pump accessory, and it only works inside a fairly narrow set of water conditions.
What the air actually does
Dissolved iron is invisible in the glass and passes straight through a sediment cartridge. Air changes it into rust particles that a filter bed can hold, and the same reaction turns rotten-egg sulfide into elemental sulfur. Penn State Extension notes that iron and manganese carry secondary, aesthetic standards, with recommended maximums of 0.3 mg/L iron and 0.05 mg/L manganese, and that oxidized iron fouls softener resin, which is exactly why oxidation belongs ahead of a softener rather than after it.
Venturi, air draw and compressor designs
A venturi injector pulls air in through a narrowing in the pipe while water flows. An air-draw or air-charge head builds a pocket of air in the top of the media tank during regeneration and pushes service water down through it. A compressor-fed system injects metered air and gives the most consistent dose on demanding water. The differences that matter to an owner are the number of moving parts, whether the head needs a drain and how much service the venturi throat will need when it silts up.
When an air injector is the wrong tool
Air oxidation slows sharply at low pH, so acidic well water often needs neutralizing first. Very high iron loads, iron bacteria with their slime, or tannin-bound iron will defeat it, and those cases call for chlorination or peroxide injection with contact time instead. Sizing matters too: an injector that cannot keep up with peak household flow lets untreated water slip through. Send a current water test to whoever is quoting, and ask for the assumptions behind the sizing in writing.
Questions people ask about air injector
Does an air injector need a drain line?
Yes, in almost every design. The media bed backwashes to flush trapped iron and sulfur out, and that flow has to go somewhere legal and adequately sized.
Will an air injection filter remove a rotten egg smell?
Usually, when the smell comes from hydrogen sulfide in the water. If the odor appears only on hot water, the culprit is more often the water heater anode rod.
Can I put an air injector after my softener?
That is the wrong order. Oxidized iron ruins softener resin, so the injector and its media bed belong upstream of the softener, not downstream.