Arsenic Removal

The EPA MCL for arsenic in drinking water is 10 micrograms per liter, which took effect in 2006 and replaced the previous 50 ppb standard. Meeting this limit reliably requires treatment down to 5 ppb or below to maintain a compliance buffer, because arsenic levels fluctuate seasonally in groundwater sources. AMPAC USA designs arsenic removal systems to consistently achieve finished water below 5 ppb.

Arsenic occurs primarily as arsenate and arsenite in groundwater. Arsenate is negatively charged at typical drinking water pH levels and is readily removed by adsorption media and RO. Arsenite is uncharged at neutral pH and passes through many treatment processes unaffected, which is why oxidation to arsenate using chlorine, potassium permanganate, or ozone is a standard pretreatment step before adsorption or membrane filtration.

Iron-based adsorption media, including granular ferric hydroxide and iron oxide-coated sands, attract arsenate ions through surface complexation. Capacity depends on arsenic influent concentration and competing ions like phosphate and silica, but a well-designed media system typically treats 10,000-50,000 bed volumes before media exhaustion in low-arsenic groundwater. AMPAC USA sizes media beds with safety factors and designs for simple changeout when capacity is reached.

RO membranes reject 90-95% of arsenate when operating at normal recovery rates, and in most cases can bring 50-100 ppb feed water below the 10 ppb MCL. However, RO rejection of arsenite is lower and less consistent, which is why we always recommend pre-oxidation before an RO system treating arsenic-contaminated groundwater. For very high arsenic concentrations above 500 ppb, a combination of media adsorption and RO provides the most reliable barrier.

A basic water analysis for arsenic, pH, iron, manganese, phosphate, silica, and hardness is the minimum needed to select media type, size the system correctly, and predict media life. Iron above 0.3 mg/L and high silica concentrations reduce adsorption capacity and may require pretreatment. We review customer water analysis at no cost and recommend systems sized for verified site conditions rather than worst-case assumptions.