http://siteresources.worldbank.org/INTSAREGTOPWATRES/Resources/ArsenicVolII_PaperIII.pdf
The most commonly used media for arsenic removal in small treatment plants include: • Activated alumina
Arsenic removal by activated alumina is controlled by the pH and arsenic content of water. Arsenic removal is optimum in the narrow pH range from 5.5 to 6.0 when the surface is positively charged. The efficiency drops as the point of zero charge is approached and at pH 8.2, where the surface is negatively charged, the removal capacities are only 2–5% of the capacity at optimal pH (Clifford 1999). The number of bed volumes that can be treated at optimum pH before breakthrough is dependent on the influent arsenic concentration. The bed volume can be estimated using the following equation, where As is the initial arsenic concentration in water in micrograms per liter (Ghurye, Clifford, and Tripp 1999): Bed volume = 210,000 (As)-0.57 The actual bed volume is much lower due to the presence of other competing ions in natural water. Arsenic removal capacities of activated alumina have been reported to vary from 1 mg g-1 to 4 mg g-1 (Fox 1989; Gupta and Chen 1978). Clifford (1999) reported the selectivity of activated alumina as: OH-1>H2 AsO4 -1>Si(OH)3 O-1>HSeO3 -1>F>SO4 -2>CrO4 -2>>HCO3 -1>Cl-1>NO3 -1>Br-1>I-1 Regeneration of saturated alumina is carried out by exposing the medium to 4% caustic soda (NaOH), either in batch or by flow through the column resulting in high-arsenic-contaminated caustic waste water. The residual caustic soda is then washed out and the medium is neutralized with a 2% solution of sulfuric acid rinse. During the process about 5–10% of the alumina is lost and the capacity of the regenerated medium is reduced by 30–40%. The activated alumina needs replacement after 3–4 regenerations. As with the coagulation process, prechlorination improves the column capacity dramatically. The activated alumina-based sorptive media used in Bangladesh and India include: • BUET activated alumina • Alcan enhanced activated alumina • Apyron arsenic treatment unit • Oxide (India) Pvt. Ltd. • RPM Marketing Pvt. Ltd. Arsenic is removed by sorptive filtration through activated alumina. Some units use pretreatment (for example oxidation, sand filtration) to increase efficiency. The Alcan enhanced activated alumina arrangement is shown attached to a tubewell in figure 6 (see page 178). The unit is simple and robust in design. No chemicals are added during treatment and the process wholly relies on the active surface of the media for adsorption of arsenic from water. Other ions present in natural water, such as iron and phosphate, may compete for active sites on alumina and reduce the arsenic removal capacity of the unit. Iron present in shallow tubewell water at elevated levels will eventually accumulate in an activated alumina bed and interfere with flow of water through the bed. The unit can produce more than 3,600 liters of arsenic-safe drinking water per day for 100 families. Apyron Technologies Inc. (United States of America) has developed an
Table 3. Comparison of Costs of Different Arsenic Treatment Technologies in India Technology Treatment process Type Capacity Cost (US$) (manufacturer)
AMAL (Oxide Adsorption by activated Household 7,000–8,000 L 50 India Catalyst alumina Community 1,500,000 L/cycle 1,250; 400/charge Pvt. Ltd., WB) RPM Marketing Activated alumina + Community 200,000/cycle 1,200; 500/charge Pvt. Ltd. AAFS-50 (patented) All India Institute Oxidation followed Household 30 L/d 5 of Hygiene & by coprecipitation- Community 12,000 L/d 1,000 Public Health filtration Public Health Adsorption on red Community 600–1,000 L/h 1,000 Engineering hematite, sand, and Department, India activated alumina Pal Trockner Ltd., Adsorption by ferric Household 20 L/d 8 India hydroxide Community 900,000 L/cycle 2,000; 625/charge Chemicon & Adsorption by ferric Community 2,000,000 L/cycle 4,500; 400/charge Associates oxide Ion Exchange Adsorption by ion Community 30,000 L/cycle 2,000
AMAL Arsenic Filters Adsorption by activated alumina
