7. Arsenic removal technologies

The major method of removing arsenic from water is physical-chemical treatment such as:

• Precipitation processes often involving coagulation and filtration
• Adsorptive processes
• Ion exchange
• Membrane filtration, including nano-filtration, reverse osmosis and electrodialysis reversal
• Alternative treatment processes, especially greensand filtration
• In situ (sub-surface) arsenic removal
• Biological arsenic removal

Some represent improvements to traditional treatments, while others have been used in small and domestic systems. Innovative technologies such as permeable reactive barriers, biological treatment, phytoremediation (using plants), and electrokinetic treatment have been investigated . For some processes, the cost may be prohibitive.

Precipitation and co-precipitation is capable of successfully treating a wide range of arsenic-contaminated water to achieve drinking water standards. It is also capable of treating other water contaminants. However, this technology requires skilled operators and is more cost-effective at large scale. The use of precipitation based technologies also results in the generation of large volumes of toxic sludge.

Adsorption (where arsenic is ‘captured’ on the surface of granular adsorptive media) is effective but can be affected by other contaminants. Small capacity systems using these technologies have lower operating and maintenance costs and require less expertise.

Ion exchange (IX) is used in small-scale and point-of-entry (POE) systems because of ease of handling. However, ion exchange does not remove As(III) and consequently a chemical pre-oxidation is required when treating As(III) containing waters. Treatment costs are higher than for conventional treatment.

Membrane processes can remove arsenic through filtration, electric repulsion, and adsorption. Efficiency in removing As(V) is higher than for As(III) and filtration is sensitive to a variety of untreated water contaminants. It produces a larger volume of residuals and tends to be more expensive.