It’s estimated that 4,000 (approximately 5 percent) of the nation’s community water systems, serving a total of 13 million people, could require additional treatment to meet the new US Environmental Protection Agency (EPA) arsenic standard of 10 parts per billion (ppb).

The new 10 ppb limit is designed to reduce the negative health effects of arsenic ingestion. Some states, such as California and New Jersey, have legislated more stringent standards, allowing a maximum arsenic level of only 4 ppb and 5 ppb, respectively.

Arsenic levels across the US can vary greatly. Although there are some geographic hot spots, the highest arsenic levels tend to be found in states west of the Continental Divide. Several New England states have public water systems with arsenic levels above the new limits. In New Hampshire, up to 30 percent of the systems exceed the limit, and in Maine, up to 14 percent.

Cost-effective technologies
The EPA has identified a number of existing technologies for removing arsenic from drinking water: sorption processes (including ion exchange, activated alumina, and iron-based sorbents), membrane processes (including reverse osmosis), and precipitative processes (including enhanced lime softening, enhanced coagulation/filtration, coagulation-assisted microfiltration, coagulation-assisted direct filtration, and oxidation/filtration).

Some new technologies make cost-effective arsenic removal available to both large and small system operators and private well owners.

In one, a membrane process that uses the surface complexing properties of ferrous hydroxide with a 0.1-micron hollow fiber microfiltration system has been shown to reduce arsenic in drinking water to below detectable limits.

The backwash water generated during the process, usually less than 5 percent of the water treated, is not considered a hazardous waste based on arsenic concentrations, which fall far below the limit of 5 milligrams per liter (mg/L) set by the Resource Conservation and Recovery Act (RCRA).

This type of system has been tested on several high-arsenic waters, and a number are being placed in service this year.

Many communities also are using microfiltration membranes to comply with the provisions of the EPA’s Surface Water Treatment Rule and remove Cryptosporidia, Giardia lamblia, oocysts, and bacteria.

The microfiltration membrane system works by the addition of an iron-based coagulant, such as ferric chloride, to the water. The offending arsenic is adsorbed onto positively charged ferric hydroxide particles, which are then removed by microfiltration. A flow diagram for this type of system is shown in Figure 1.

Filtration follows coagulation
Ferric chloride hydrolyzes in water to form ferric hydroxide particles, which have a net positive surface charge at pH values less than 8. Arsenate (As[V]) ions are negatively charged and will absorb onto the positively charged ferric hydroxide particles. The particles, with arsenic attached, are removed with microfiltration.

Pretreatment consists of chemical additives and rapid mixing; flocculation is not required. The amount of ferric chloride required to remove arsenic is strongly influenced by pH and ferric hydroxide speciation. Figure 2 presents the speciation of ferric hydroxide as a function of pH.

Effective removal takes place at pH values below 7.5, where the positive species dominates. Because it is acidic, the addition of ferric chloride alone will depress the pH. The amount of ferric chloride needed can be reduced by introducing acid in the pretreatment process.

Arsenic treatment to comply with the new 10 ppb limit is most commonly needed for small systems. More disadvantages — such as smaller economies of scale and smaller budgets — typically are involved in the selection of a method for a smaller system compared with a larger one.

Ion exchange for POE
To make the selection process easier, EPA has identified three small system compliance technologies (SSCT) for systems serving communities of 25 to 10,000 people: ion exchange, activated alumina, and oxidation/filtration.

Of these, ion exchange seems best suited for small systems. It typically exhibits high arsenic removal efficiencies while allowing a relatively wide range of inlet water quality conditions.

Ion exchange treatment also can be adapted for point-of-entry (POE) applications for the approximately 15 million households that have private well water sources.

In the ion exchange process, unwanted ions in solution are swapped with ions of similar charge that are electrostatically attached to a solid, spherical resin bead. Figure 3 outlines a typical ion exchange process.

Oxidizing As(III) to As(V)?
In the case of arsenic removal through ion exchange, speciation is important for removal efficiency because arsenite (As[III]), which is found mainly in anaerobic ground waters, exists with a neutral charge at natural pH levels, and thus cannot be removed through ion exchange.

As is the case with the microfiltration membrane process described above, an oxidizing pretreatment must be employed to oxidize arsenite (As[III]) to arsenate (As[V]), which maintains a negative charge at natural pH levels.

Chlorine or chloramine at concentrations of 1 mg/L can be used for pretreatment. With a 5-second detention time, a conversion rate of 95 percent can be achieved.

Competing-ion effect
Another key driver of the removal efficiency — and therefore the economics — of ion exchange systems is the exchange affinity of the resin for arsenic relative to other aqueous ions such as nitrates and sulfates.

Most ion exchange resins have high exchange affinities for nitrates and sulfates, as well as for arsenate. This results in competition for the resin, which can greatly reduce the performance of an ion exchange system.

The competing ion effect can cause reduced bed life and chromatographic peaking, resulting in excess arsenate and nitrate in the system effluent if the resin bed is exhausted and sulfates exist in the incoming water.

One solution to the competing ion effect is the use of resin that has a much greater affinity for arsenate than any other ions present in the raw water. Resins are now under development that show promise for enhanced removal efficiency.

EPA’s arsenic pages
The EPA specifies a number of options for arsenic removal and has provided a number of useful tools to help system operators choose among them.

New technologies and variations on existing technologies are continually becoming available. Before selecting an arsenic treatment method, review the EPA’s arsenic pages to learn about recent developments in the field.