An acid filter, more properly an acid neutralizing filter, is a backwashing tank of calcium carbonate media that acidic water partly dissolves as it passes through, consuming acidity and raising pH towards neutral. It is fitted on private wells and other untreated supplies, and the reason to fit one is usually written on your bathroom fixtures before it is measured.
How acidic water announces itself
Blue green staining in sinks, baths and toilet cisterns is the classic sign, and it is dissolved copper coming out of your own pipework rather than anything arriving in the supply. Pinhole leaks in copper lines, particularly several within a short period, are the same process finishing. A metallic taste, rapid corrosion of brass fittings and premature failure of a water heater all point the same way. The EPA notes private wells are not federally regulated under the Safe Drinking Water Act, so nobody is monitoring pH on your behalf.
The media and how it is chosen
Calcite, which is crushed calcium carbonate, is the standard media and it self limits: it dissolves in acidic water and largely stops once the water approaches neutral, so overshooting is difficult. For very acidic water calcite alone works too slowly, and magnesium oxide media is blended in, which is far more aggressive and can overshoot badly if used alone or over dosed. The blend ratio is set by how low the pH actually is, which is why the water test comes before the media order rather than after it.
Sizing, contact and backwash
The result depends on contact time, so the tank is sized to household peak flow rather than to available space, and an undersized bed lifts pH only slightly no matter what media it holds. The control head backwashes the bed periodically, lifting and reclassifying the media so it does not cement into a channelled mass, and that backwash needs a flow rate and a drain. On a low yield well the backwash requirement is a genuine design constraint. Our whole house filter guide covers positioning it in the treatment train.
The trade, and the upkeep
The media works by dissolving into the water, so it adds calcium, which is hardness. On water that was already hard the result is harder water and more scale, and the usual plan is neutralizer first with a softener after it, sized for the raised figure. The media is consumed and the bed level drops, so it is topped up once or twice a year depending on acidity and use. Letting it run low is the common failure, because pH drifts back down quietly and the pipework starts corroding again with nothing visibly wrong.
Questions people ask about acid filter
How do I know I need an acid filter?
Blue green staining in sinks and baths, repeated pinhole leaks in copper pipe and a metallic taste all point to low pH. A pH test confirms it.
Which media should the tank hold?
Calcite for most supplies, since it self limits near neutral. Very acidic water needs magnesium oxide blended in, at a ratio set by the test result.
Does it make the water harder?
Yes. It works by dissolving calcium carbonate into the water. Expect more scale unless a softener is fitted after it and sized accordingly.
What maintenance does it need?
Topping up the media once or twice a year as it is consumed. Running the bed low lets pH drift back down and corrosion restart unnoticed.