CS(AR)-2/2007-08 : Technological options for the removal of Arsenic with special reference to south-east Asia-Technology overview document.

Abstract

In the context of prevalence of high concentrations of arsenic in ground water, a wide range of technologies have been developed for the removal of arsenic for drinking water. The most common technologies available for removal of arsenic from water include conventional processes of oxidation and filtration, co-precipitation, adsorption, ion exchange and membrane techniques. However, question remains regarding the efficiency and applicability/appropriateness of the technologies, particularly because of low influent arsenic concentration and differences in source water composition. Besides, the system must be economically viable and socially acceptable. The conventional technologies have been scaled down to meet the requirements of households and communities and suit the rural environment. Some technologies have also used indigenous materials for the removal of arsenic and have shown promising results. This report presents a review of the technological options available for the removal of arsenic from water.

The most commonly used conventional technologies for the removal of arsenic from water include oxidation/filtration, coprecipitation, adsorption, ion exchange and membrane filtration. Precipitation/coprecipitation uses chemicals to transform dissolved contaminants into an insoluble solid or form another insoluble solid onto which dissolved contaminants are adsorbed. The solid is then removed from the liquid phase by clarification or filtration. Adsorption technology concentrates solutes at the surface of a sorbent, thereby reducing their concentration in the bulk liquid phase. The adsorption media is usually packed into a column. As contaminated water is passed through the column, contaminants arc adsorbed. Ion exchange technology exchanges ions held electrostatically on the surface of a solid with ions of similar charge in a solution. The ion exchange media is usually packed into a column. As contaminated water is passed through the column, contaminants are removed. Membrane filtration separates contaminants from water by passing it through a semi- permeable membrane. The membrane allows some constituents to pass, while blocking others.

Among the conventional technologies, precipitation/coprecipitation is the most frequently used technology to treat arsenic-contaminated water. The effectiveness of this technology is less likely to be reduced by characteristics and contaminants other than arsenic, compared to other water treatment technologies. It is also capable of treating water characteristics or contaminants other than arsenic, such as hardness or heavy metals. Systems using this technology generally require skilled operators; therefore, precipitation/coprecipitation is more cost effective at a large scale where labor costs can be spread over a larger amount of treated water produced.

The effectiveness of adsorption and ion exchange for arsenic treatment is more likely than precipitation/coprecipitation to be affected by characteristics and contaminants other than arsenic. Small capacity systems using these technologies tend to have lower operating and maintenance costs, and require less operator expertise. Adsorption and ion exchange tend to be used more often when arsenic is the only contaminant to be treated, for relatively smaller systems. Membrane filtration is used less frequently because it tends to have higher costs and produce a larger volume of residuals than other arsenic treatment technologies.

The report also discusses details on emerging approaches. such as permeable reactive barriers, phytoremediation, biological treatment and electrokinetics. for addressing arsenic in ground water. Permeable Reactive Barriers (PRBs) are walls containing reactive media that are installed across the path of a contaminated ground water plume to intercept the plume. The barrier allows water to pass through while the media remove the contaminants by precipitation, degradation, adsorption, or ion exchange. Phytoremediation involves the use of plants to degrade. extract, contain, or immobilize contaminants in soil, sediment, and groundwater. Biological treatment involves the use of microorganisms that act directly on contaminant species or create ambient conditions that cause the contaminant to leach from soil or precipitate/coprecipitate from water. Electrokinetic treatment is an in situ technology intended to be applicable to water, waste water and soil. This technology is most applicable to fine-grained soils, such as clays.