Arsenic Removal From Hungerford & Terry

Hungerford & Terry, Inc. has conducted numerous pilot studies within the United States demonstrating the effectiveness of using filtration for arsenic removal, the most recent of which include municipalities in Michigan, Nevada, California, and Arizona. The principle of the treatment is to coprecipitate the arsenic with iron and filter the resulting floc. Many water sources have iron naturally occurring in the water and require treatment before use. In such instances, the water can be treated for both iron and arsenic using the same equipment. Applications that do not have iron, or have insufficient iron to adequately reduce the arsenic, can use ferric chloride to supplement the treatment process.

H&T has treated water sources with various arsenic levels. The highest arsenic levels have been encountered in the more arid regions of the United States, i.e. Southwest United States. Regardless of the arsenic concentration, it was readily treated by complexing the arsenic with iron and directly filtering the constituents together. The arsenic ranged from as low as 12 ppb to as high as over 80 ppb. The arsenic was typically reduced to 3-4 ppb, often reaching non-detectable levels.

The species state of arsenic can impact the removal efficiency. Arsenite (As⁺³) must first be converted to Arsenate (As⁺⁵) to best utilize this technology. The conversion to As⁺⁵ is known to be easily accomplished with chlorine. The conversion, if any, represents a minimal chlorine demand given the fact that arsenic concentrations are low compared to any iron that is typically present. Additionally, the amount of chlorine required for maintaining a disinfected system is magnitudes of order greater than the typical arsenic concentration.

The Origin Of Arsenic
Arsenic is an element that occurs in the earth's crust. Accordingly, there are natural sources of exposure. These include weathering of rocks and erosion depositing arsenic in water bodies and uptake of the metal by animals and plants. Consumption of food and water are the major sources of arsenic exposure for the majority of the population. People may also be exposed from industrial sources, since arsenic is used in semi-conductor manufacturing, petroleum refining, wood preservatives, animal feed additives, and herbicides.

Maximum Contaminant Level
The EPA established a maximum contaminant level for arsenic, 50 micrograms per liter (50 uq/L) or parts per billion (ppb) in 1976. In 2002, the MCL was lowered to 10 ppb.

Public Health Concerns
Arsenic can combine with other elements to form inorganic and organic arsenicals. In general, inorganic derivatives are regarded as more toxic than the organic forms. While food contains both inorganic and organic arsenicals, mainly inorganic forms are present in water. Exposure to arsenic at high levels poses serious health effects, since it is a known human carcinogen. In addition, it has been reported to affect the vascular system in humans and has been associated with the development of diabetes. Arsenic is poisonous in doses significantly larger than 65 mg (1 grain), and the poisoning can arise from a single large dose, or from repeated small doses, as, for example, inhalation of arsenical gases or dust.

Treatment Technology
In water, the most common valence states of arsenic are As+5 (or arsenate) which is more prevalent in aerobic surface waters, and As⁺³ (or arsenite) which is more likely to occur in anaerobic ground waters. As⁺³ may be converted through pre-oxidation to As⁺⁵. In the arsenate state, arsenic tends to adhere to ferric hydroxide, a common precipitate produced in the CR Ferrosand Filtration process.