# Arsenic in Water Treatment: Media, RO, and Oxidation

> Arsenic treatment depends on the species: As(V) responds to adsorptive media and RO, while As(III) needs oxidation first. Compare costs and limits.

Canonical: https://watertreatmentpro.com/arsenic-in-water-treatment/

Last updated: 2026-11-09

Arsenic in water is treated with iron-based adsorptive media, anion exchange, or reverse osmosis. The federal limit is 0.010 mg/L. The species matters: As(V) is captured readily, while As(III) needs oxidation first, and treatment chosen without speciation often underperforms.

## Two Species, Two Behaviors

Arsenic occurs in water as arsenate, As(V), or arsenite, As(III). The ratio depends on pH and oxygen levels, and deeper anoxic wells tend toward As(III).

As(V) is charged and binds to iron oxide media and anion exchange resin. As(III) is uncharged and passes through many of those media with little removal.

That is why a treatment system can work in one house and fail next door. The species, not just the concentration, decides the design.

## Adsorptive Media

Iron-based media, including granular ferric hydroxide and iron-modified activated alumina, adsorb arsenic as water passes through the bed.

Whole-house systems cost $1,200 - $3,500 installed. The media lasts months to a few years depending on the arsenic load and the water chemistry, and replacement is the main running cost.

Phosphate and silica compete for adsorption sites, so a water with high phosphate reduces media life. A lab panel that includes those parameters helps predict it.

## Anion Exchange

Anion exchange resin swaps arsenic for chloride, with regeneration by salt. It is effective for As(V) and is often used when sulfate is low.

Installed cost is $1,000 - $3,000. The system needs a brine tank and a drain, similar to a softener.

High sulfate reduces capacity, and the resin must be selected for arsenic rather than a standard softener resin.

## Reverse Osmosis

RO removes both As(V) and, to a lesser degree, As(III). An under-sink unit at $300 - $900 installed treats drinking water at one tap.

RO is the practical choice for a household that wants a certified barrier at the kitchen tap without a whole-house system. It also removes other dissolved contaminants.

The limitation is scale: it treats one tap, and the rest of the house still uses untreated water.

## Oxidation First

If the water contains As(III), oxidation converts it to As(V) before the media. Chlorine, ozone, or air injection can do the job.

Many whole-house systems include an oxidant feed or an aeration step for that reason. Without it, the media may remove only a fraction of the total arsenic.

A lab speciation test tells you whether oxidation is needed. It is worth the extra cost before buying a system.

## Comparison

| Method | Arsenic form | Installed cost | Whole house |
|---|---|---|---|
| Adsorptive media | As(V), with oxidation for As(III) | $1,200 - $3,500 | Yes |
| Anion exchange | As(V) | $1,000 - $3,000 | Yes |
| Reverse osmosis | Both, better for As(V) | $300 - $900 | No |
| Distillation | Both | $150 - $500 | No |
| Softener | Neither | $1,200 - $4,500 | No |

The softener row is worth stating plainly. A softener does not remove arsenic, and a household that installs one for hardness and assumes the arsenic is handled is mistaken.

## Sizing and Installation

1. Test the water for total arsenic and speciation.
2. Test pH, iron, manganese, sulfate, phosphate, and silica.
3. Choose the method from the species and the competing ions.
4. Size the system to the flow, typically 8 - 15 GPM.
5. Provide a drain and power for backwashing or regeneration.
6. Install a sample tap after the treatment.
7. Retest treated water at 30 days.

A point-of-use RO at $600 installed plus $100 a year in filters costs $1,600 over ten years.

An adsorptive media system at $2,200 installed plus $400 every two years in media costs $4,200 over ten years.

An anion exchange system at $1,800 installed plus $250 a year in salt and service costs $4,300.

## What Treatment Cannot Fix

Treatment does not remove the source. Arsenic occurs naturally in some aquifers, and a well that draws from that layer will keep producing it.

A deeper well or a different aquifer may be lower in arsenic, and that is a drilling decision rather than a treatment one.

Blending with a low-arsenic source is possible where a second supply exists. Most households do not have that option and treat the water instead.

Those numbers assume the system is sized correctly and maintained. A media tank that is too small exhausts early, and the annual cost rises without the sticker price changing.

A household that treats only drinking water can use RO at a fraction of the whole-house cost, provided the rest of the water is not used for cooking or formula. That is a household-by-household decision, and it depends on where the water is consumed.

One more cost to plan for is testing. A treated system should be retested annually, and a speciation test every few years if the source changes. Those tests cost $40 - $150 each and are the only way to know the media is still working between service visits.

One more point on media: adsorptive tanks are usually rated for a total volume of water, not a calendar interval. A household using 300 gallons a day will exhaust a bed faster than one using 150, so the replacement schedule should be calculated from the treated volume rather than copied from a neighbor. A media bed that is backwashed on the wrong schedule can compact and lose contact time, which shows up as a rising treated result long before the rated capacity is reached.

For the testing side, see [arsenic in water test](https://watertreatmentpro.com/arsenic-in-water-test/). For the well problem, see [arsenic in well water](https://watertreatmentpro.com/arsenic-in-well-water/). For RO costs, see [reverse osmosis system cost](https://watertreatmentpro.com/reverse-osmosis-system-cost/).