Arsenic Removal Residential Decision Tree

Why this matters

Arsenic is a Class A human carcinogen. EPA set the federal Maximum Contaminant Level for public water systems at 0.010 mg/L (10 ppb) in 40 CFR 141.62. Private wells are unregulated at the federal level, so the homeowner is the only line of defense. Roughly 13 percent of private wells in the United States exceed 10 ppb according to USGS National Water Quality Assessment data. Picking the wrong removal technology either fails the lab post-install or costs the homeowner three to five times what they needed to spend. A clean decision tree built on a quality lab result is the difference between a one-time visit and a callback every six months.

Step 1: get a defensible lab result

Before picking technology, the well needs a state-certified lab analysis. A field strip test is screening only and does not distinguish between the two arsenic species.

Required tests:

• Total arsenic (EPA Method 200.8 ICP-MS, reporting limit 1 ppb or lower)
• Arsenic speciation (As(III) trivalent vs As(V) pentavalent) by EPA Method 1632 or equivalent
• pH (As(V) capture varies sharply with pH)
• Iron and manganese (foul most arsenic media)
• Silica (competes for sorption sites on activated alumina)
• Sulfate and chloride (competitors on anion exchange resin)
• Phosphate (competes on iron-based media and alumina)
• Total dissolved solids and hardness

Without speciation the design is a guess. As(III) is uncharged at typical groundwater pH (6.5 to 8.5) and passes through every sorptive media that depends on anion charge. The pre-treatment plan must convert As(III) to As(V).

Step 2: pre-oxidation if As(III) is present

Reduced groundwater commonly carries arsenic as As(III). Convert to As(V) before the removal stage:

• Free chlorine at 1 to 4 mg/L residual with 1 to 2 minutes contact time (fast and reliable; install a small contact tank)
• Solid-block chlorine pellet feeder or liquid hypochlorite injection at the pressure tank
• Potassium permanganate at 0.5 to 2 mg/L if iron and manganese are also present
• Avoid relying on dissolved oxygen alone; conversion is slow and incomplete

Confirm conversion with a post-oxidation speciation test on the next service visit.

Step 3: pick the technology

The three viable residential technologies each fit a defined raw-water window.

Reverse osmosis (point-of-use)

• Removes both As(III) and As(V) at 90 to 99 percent rejection on a sound NSF/ANSI 58 certified unit
• Best when arsenic is the only major concern AND drinking and cooking water is the only protected use
• Wastes 3 to 5 gallons per gallon produced on standard units, 1:1 on permeate-pump or tankless designs
• Requires pre-sediment, pre-carbon, post-carbon stages and annual membrane sanitization
• Avoid when bathing exposure or whole-house irrigation contact is a stated concern

Activated alumina (point-of-entry or point-of-use)

• Effective on As(V) only; mandatory pre-oxidation if As(III) is detected
• Best at pH 5.5 to 6.0; capacity drops sharply above pH 7.5 and approaches zero by pH 8.5
• A pH-adjustment stage (CO2 injection or acid feed) is often required ahead of the alumina bed
• Capacity 2,000 to 10,000 bed volumes depending on raw water; expect annual to biennial media replacement
• Silica above 30 mg/L and phosphate above 0.5 mg/L sharply reduce run length
• NSF/ANSI 53 certification for arsenic reduction is the buying spec

Anion exchange (point-of-entry)

• Effective on As(V) only; mandatory pre-oxidation
• Works at the natural well pH (no acid feed needed)
• Sulfate is the primary competitor; sulfate above 25 mg/L collapses arsenic capacity
• Risk of chromatographic peaking (arsenic discharges above feed concentration as the bed exhausts); use a duplex lead/lag configuration with weekly post-resin sampling, or replace resin on a conservative time schedule
• NSF/ANSI 44 certified resin
• Brine waste disposal must comply with local septic and stormwater rules

Iron-based adsorbents (granular ferric hydroxide, iron-modified media)

• Effective on both As(III) and As(V) at pH 6.5 to 8.0
• Useful when sulfate or silica rule out alumina or anion exchange
• Capacity 5,000 to 30,000 bed volumes; single-use media (no regeneration)
• NSF/ANSI 61 certification for component material

Step 4: the decision tree

1. Arsenic is the only concern AND only drinking and cooking water need protection: install NSF/ANSI 58 RO at the kitchen sink. Done.
2. Whole-house protection required AND pH is below 7.5 AND silica below 30 mg/L AND phosphate below 0.5 mg/L: activated alumina with pre-oxidation, optional pH adjustment.
3. Whole-house required AND sulfate below 25 mg/L AND no other arsenic competitors: anion exchange duplex with pre-oxidation and lead/lag sampling.
4. Whole-house required AND high sulfate or high silica: iron-based adsorbent with pre-oxidation.
5. Iron above 0.3 mg/L OR manganese above 0.05 mg/L: install iron and manganese pretreatment first; arsenic media will foul within weeks otherwise.

Step 5: post-install verification

• Sample post-treatment within 30 days of startup; require less than 5 ppb as the operational target (half the MCL)
• Sample again at 6 months and annually thereafter
• For anion exchange: weekly post-resin field sampling during the first six months to catch chromatographic peaking
• Track bed volumes treated against the design capacity; schedule media change at 75 percent of design capacity

References

• EPA 40 CFR 141.62, Maximum Contaminant Levels for Inorganic Contaminants (arsenic 0.010 mg/L)
• EPA 815-R-05-006, Treatment Technologies for Arsenic Removal
• EPA Method 200.8, Determination of Trace Elements in Waters and Wastes by Inductively Coupled Plasma Mass Spectrometry
• NSF/ANSI 58, Reverse Osmosis Drinking Water Treatment Systems (arsenic reduction claim)
• NSF/ANSI 53, Drinking Water Treatment Units, Health Effects (arsenic reduction claim)
• USGS Circular 1332, Arsenic in Groundwater of the United States
• WQA Technical Application Bulletin, Arsenic