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The majority of established arsenic removal technologies can be divided into three main categories, depending on the specific mechanism that is responsible for removing arsenic. These categories are usually referred as coagulation–precipitation, membrane, and sorption processes. Three processes in this group have been developed for arsenic removal; aluminum or iron coagulation, lime softening, as well as a combination of iron and manganese removal with arsenic. Two types of membrane processes remove arsenic efficiently from water sources: reverse osmosis and electrodialysis. Adsorption processes are another category of arsenic removal technologies; they are based on the adsorption of arsenic onto suitable adsorbing media. Two different technologies have been applied to remove arsenic, activated alumina and ion exchange. Keywords: arsenic; removal methods; treatment; contaminated water; coagulation; membrane; sorption; biological oxidation; chemical oxidation; speciation

Granular media filtration is a water treatment process that uses a porous medium through which water passes to remove particulates or suspended solids. Particulates that are removed by this process include microorganisms (bacteria, viruses, and protozoan cysts), clay and silt particles, colloidal and precipitated humic substances and other organic particulates from natural decay of vegetation, precipitates of an aluminum or iron coagulation process, calcium carbonate and magnesium hydroxide precipitates from lime softening, and iron and manganese precipitates. For granular media filtration to be effective, source water must be pretreated. Chemical destabilization is an essential prerequisite for effective filtration. Chemicals used for particle destabilization are limited primarily to metal salts or cationic polymers as primary coagulants. Pretreatment may also include aeration or introducing an oxidant if water treatment aims to remove iron or manganese. Sometimes a filter aid polymer is added in the influent to the filter to improve particle capture efficiency.

As in other chemical water softening technologies, the objective of the lime–soda ash process is to precipitate polyvalent cations from solution by pH adjustment. In water treatment applications, Ca2+ and Mg2+ are the two major cations of concern, though Fe2+, Fe3+, Al3+, and Mn2+ may also be present at problematic concentrations, depending on the properties of soil or geological material encountered by the surface or groundwater. Typically, a total hardness ranging from 75–120 mg/L as CaCO3, is sought in domestic water softening. In this process, lime and soda ash are added to solution, Ca2+ precipitates as CaCO3(), and Mg precipitates as Mg(OH)2(). To precipitate CaCO3, the pH of the water must be raised to approximately 10.3. Similarly, Mg(OH)2() forms when the pH is raised above 11. When there is not enough naturally occurring bicarbonate alkalinity (HCO3−) in water to form the CaCO3 precipitate, alkalinity must be added (typically as Na2CO3). Keywords: water softening; lime–soda ash process; solubility; cation removal; precipitation–dissolution; carbonate hardness; noncarbonate hardness