8. Arsenic removal systems

These arsenic removal technologies can be employed in centralised treatment systems or in household point-of-use (POU) systems.

Most arsenic removal in centralised systems in both developed and developing countries is based on conventional coagulation-separation and adsorption.

Conventional coagulation involves adding a coagulant (aluminium or iron based salts) followed by flocculation and floc separation, usually through sedimentation and rapid sand filtration. Treatment can in certain cases be simplified by direct filtration without sedimentation.

An aeration step normally precedes coagulation to introduce oxygen into anoxic groundwater and to remove impurities such as methane. Oxygen oxidises dissolved iron and will result in the removal of some arsenic. Chlorination may also be required if a significant proportion of the arsenic is present in As(III) form. This increases treatment costs and the need for skilled personnel.

One serious drawback associated with coagulation based arsenic removal is the generation of a large volume of toxic liquid waste, which must be dealt with.

Hybrid arsenic removal treatment systems using a combination of coagulation and micro- or ultrafiltration have been extensively tested in pilot projects but are probably too expensive for applications in developing countries.

Adsorption based arsenic removal systems often use activated alumina (AA) as an adsorbent. AA is widely available, cheaper than other adsorbents, and effective at removing As(V), but less effective for As(III).

Iron based adsorbents are increasingly being introduced, due to the growing availability of commercial adsorbents with high adsorption capacity and simplicity of operation. A procedure for rapid small-scale assessment of arsenic adsorbents was recently proposed by AWWA Research Foundation and USEPA

An innovative alternative to commercially available and relatively expensive arsenic adsorbents is the use of iron oxide coated sand (IOCS) that originates from groundwater treatment plants. This by-product has a high adsorption capacity and is highly effective, although it requires longer contact time and lower filtration rates.

In addition, aeration and rapid sand filtration is usually required to make water fit for drinking.

Household level point-of-use arsenic removal systems can make an important contribution, especially for rural populations in developing countries. Since the health hazards from arsenic in water are confined to drinking and food preparation, only 2%-3% of household consumption needs to pass through the household system.

Several POU arsenic removal technologies based on adsorptive filtration have been tried by various agencies but long-term success has proved elusive. Some household systems also include oxidation through the addition of chlorine tablets or potassium permanganate. The Government of Bangladesh has approved four household systems for commercial sale.