Coagulation and Filtration Methods
Metal salts such as alum and ferric chloride or ferric sulfate are widely used coagulants to remove arsenic from drinking water. Alum and ferric salts dissolve upon addition to water, forming amorphous hydrous aluminum and ferric oxides which are relatively insoluble in circumneutral pH ranges. These metal hydroxides form gelatinous flocs that bind to other flocs and settle out of solution, scavenging many dissolved and particulate materials in the process. Vigorous stirring is usually required to ensure uniform mixing for optimal coagulation. If water is soft and of low alkalinity it may be necessary to add alkalinity to ensure floc formation.
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Aluminium Salts Alum is effective for As(V) only, so pre-oxidation is often necessary. Alum has a narrow effective range, from pH 6-7. If pH is above 7, removal may be improved by adding acid to lower pH. Typical doses are 10 to 50 mg/L alum. Iron salts Coagulation with ferric salts works best at pH below 8. Typical doses are 5 to 30 mg/L ferric salts. Ferric salts can remove both As(III) and As(V), but As(V) is more strongly retained so pre-oxidation is often carried out.
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Shawdesh Aqua
Filter using iron salts for arsenic removal |
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Naturally occurring iron
High
dissolved iron concentrations in groundwater pumped from anoxic aquifers can be
utilised to remove arsenic. Tested sand filters comprise two superimposed
compartments, the upper filled with locally available sand while the lower serves
to store the filtered water. Groundwater pumped from a tubewell trickles
through the sand filter into the underlying storage tank. Arsenic removal is
governed by the precipitation of iron (hydr)oxides which form a coating on the sand
surfaces. Arsenic then absorbs to the iron (hydr)oxides and remains immobilised
under oxic conditions. For more details on this method, see Eawag Technical
Report, “Household Sand Filters for Arsenic Removal”
Household sand filter for arsenic removal in Vietnam