Study on Techniques of arsenic removal from drinking water

Abstract

Arsenic is an element with metalloid properties. It may be organic or inorganic. Arsenic occurs naturally in all environmental media and is usually present in thc fOlll1 of compounds with sulfur and with many metals like iron, copper, cobalt, lead, zinc and so on. About 97% of the people of Bangladesh get their drinking water Ii'om 3-4 million tubewells that were installed for the anti diarrhoea campaign to reduce water borne diseases. Now, about one million shallow tubewells are dispensing arsenic laced water with the result that many people are now showing signs of arsenic related diseas;;s. This situation has accelerated in the past two years to a degree such that cUITently almost all the districts in Bangladesh arc more or less affected by arsenic, and an estimated 20 million people are at risk of arsenic poisoning. A variety of treatment processes has been used for arsenic removal from water. The most commonly used technologies include co-precipitation and adsorption onto coagulated floc, lime softening, sulfide precipitation, adsorption onto activated carbon, activated alumina, ion exchange and membrane processes such as reverse osmosis. This work concentrates in coagulation - flocculation process of arsenic removal by readily available common coagulants and natural iron flocs. The co-existence of arsenic with other elements / compounds, removal by plain sedimentation, factors inOuencing arsenic removal by coagulation from nat,lral water, and removal of color and iron along with arsenic from natural waters have been investigated. The study reveals that although several elements / compounds co-exist with arsemc III natural water, no strong correlation exists among them. Ferric Chloride was found most efficient in arsenic removal and effective over a wider range of pH. In case of arsenic removal by naturally occurring iron Flocs, a linear relationship between arsenic content of water and percentage removal was found. The optimum doses of Aluminum Sulfate and Ferric Chloride have been detem1ined for arsenic removal from natural water (tube well water) and verified by adsorption isotherm.