Arsenic Filtration Systems for US Water Supplies

Arsenic contamination in drinking water is a documented public health concern across the United States, particularly in regions with high natural geologic arsenic deposits and in communities relying on private wells. This page covers the classification of arsenic filtration technologies, how each mechanism removes arsenic from water, the scenarios in which filtration is most commonly required, and the decision factors that guide system selection. Understanding these elements is essential for property owners, plumbers, and water treatment professionals navigating both regulatory requirements and treatment options.

Definition and scope

Arsenic in drinking water exists in two primary oxidation states: arsenite (As III) and arsenate (As V). The distinction matters because filtration technologies differ substantially in their ability to remove each form. Arsenate is negatively charged and more easily captured by adsorption media, while arsenite is uncharged and typically requires oxidation as a pretreatment step before removal.

The U.S. Environmental Protection Agency (EPA) established the Maximum Contaminant Level (MCL) for arsenic in public water systems at 10 micrograms per liter (µg/L), or 10 parts per billion (ppb), under the Arsenic Rule (40 CFR Part 141), which took effect in January 2006 (EPA Arsenic Rule). Private wells are not subject to this federal MCL, meaning millions of well-water users may have no mandatory monitoring requirement, even in states with elevated geologic arsenic concentrations.

Geologically elevated arsenic is most common in the western United States, parts of New England, and portions of the Midwest. The U.S. Geological Survey (USGS) has mapped arsenic occurrence across aquifer systems nationally. Arsenic exposure above the 10 µg/L threshold is associated with bladder, lung, and skin cancers, according to the World Health Organization (WHO).

For a broader orientation to contaminant-specific filtration topics, the contaminants filtered by type resource provides comparative context across major regulated substances.

How it works

Arsenic filtration systems employ four primary mechanisms, each with distinct performance boundaries:

1. Adsorptive media filtration — Granular ferric hydroxide (GFH), granular ferric oxide (GFO), and iron-based titanium oxide media attract and bind arsenate (As V) to their surface. These are the most widely deployed point-of-entry and point-of-use arsenic-specific systems. Effective arsenic removal rates exceed 95% when influent is predominantly As V.

2. Reverse osmosis (RO) — Semipermeable membranes reject dissolved ions, including both arsenite and arsenate, by pressure-driven filtration. RO systems typically achieve rejection rates of 85–95% for arsenic, depending on membrane quality, water pressure (generally 40–80 psi), and influent pH. For more on RO mechanics, see the reverse osmosis systems reference page.

3. Ion exchange (IX) — Anion exchange resins, particularly strong-base resins, exchange arsenate ions for chloride or hydroxide. This method is more effective at higher pH levels and where competing anions such as sulfate and phosphate are low. Ion exchange is more commonly deployed in larger residential or light commercial applications.

4. Coagulation/filtration — Alum or ferric coagulants are dosed upstream of a filter bed, causing arsenic to precipitate and be captured. This approach is predominantly used in municipal treatment plants rather than in-home systems.

Oxidation pretreatment is frequently required when water contains significant As III. Chlorination, aeration, or potassium permanganate dosing converts As III to As V before media contact. Well water with no residual disinfectant often requires this step, particularly when oxidation-reduction potential (ORP) is low.

NSF International and ANSI jointly maintain NSF/ANSI Standard 58 (for RO systems) and NSF/ANSI Standard 53 (for point-of-use reduction claims), which govern certified arsenic reduction performance. The NSF/ANSI certification standards page covers how these certifications apply to system selection.

Common scenarios

Private well water with geologic arsenic is the most prevalent scenario in the US. Wells completed in granite-bearing aquifers, glacial deposits, or volcanic rock zones frequently exceed 10 µg/L without treatment. The USGS National Water Information System documents arsenic detections exceeding 50 µg/L in portions of Nevada, Montana, and Michigan. Whole-house adsorptive media systems or point-of-entry RO systems are typically deployed here. See the well water filtration page for additional context on well-specific system requirements.

Legacy municipal supplies near the MCL represent a second scenario. Public systems with source water between 8–15 µg/L may use central coagulation/filtration or large-scale IX systems. Households on municipal supplies near this threshold sometimes install point-of-use RO as supplemental protection.

Agricultural and industrial legacy sites create localized arsenic plumes where groundwater has been affected by pesticide application (particularly orchards with historic lead arsenate use) or mining drainage. These scenarios may involve higher influent concentrations — above 100 µg/L in some documented cases — requiring multi-stage treatment rather than single-media systems. Multi-stage filtration systems addresses these layered configurations.

Decision boundaries

Selecting the appropriate arsenic filtration technology depends on several intersecting variables:

• Arsenic species ratio — A water quality test identifying the As III to As V ratio determines whether oxidation pretreatment is necessary before adsorptive or RO filtration.
• Influent concentration — Systems certified to NSF/ANSI 58 or 53 are tested at specific challenge concentrations (typically 0.3 mg/L for NSF/ANSI 58). Systems treating influent above certified challenge levels require engineering review.
• Flow rate and system sizing — Adsorptive media beds are rated by service flow rate, typically in gallons per minute (gpm). Undersized media beds reduce contact time and drop removal efficiency below certified levels. The filter sizing and flow rate page details sizing methodology.
• Competing ions — High phosphate, silica, or sulfate concentrations reduce the binding capacity of GFH and GFO media, shortening media life.
• Point-of-entry vs. point-of-use — RO at the point of use protects drinking water but not water used for bathing, cooking prep at other fixtures, or irrigation. Whole-house adsorptive systems protect all end points but require larger media volumes and more frequent regeneration or replacement. The inline vs. standalone filters page contrasts these deployment architectures.
• Permitting — Many states require permits or inspections for well modifications and treatment system installations. State drinking water programs, operating under EPA primacy authority (Safe Drinking Water Act, 42 U.S.C. §300f et seq.), set specific requirements. Plumbers or water treatment specialists installing arsenic systems in regulated states must verify local licensing requirements; the plumber vs. water treatment specialist page outlines how professional scope differs by jurisdiction.

Media replacement schedules are not fixed — they depend on influent arsenic loading and system throughput — and ongoing effluent testing is the only reliable verification that a system remains effective after initial installation.

References

• U.S. Environmental Protection Agency — Arsenic in Drinking Water
• EPA Drinking Water Regulations — 40 CFR Part 141
• U.S. Geological Survey — Arsenic in Groundwater of the United States
• NSF International — NSF/ANSI 58 Reverse Osmosis Drinking Water Treatment Systems
• NSF International — NSF/ANSI 53 Drinking Water Treatment Units
• World Health Organization — Arsenic Fact Sheet
• Safe Drinking Water Act, 42 U.S.C. §300f et seq. — EPA SDWA Overview