Arsenic in Drinking Water
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Coagulation/filtration is a precipitative process. The most widely used coagulants for water treatment are aluminum and ferric salts, which hydrolyze to form aluminum and iron hydroxide particulates, respectively.
The efficiency and economics of the system are contingent upon several factors including the type and dosage of coagulant, mixing intensity, and pH. This process can be optimized to remove dissolved inorganic As(V) from water. The mechanism involves adsorption of As(V) to an aluminum or ferric hydroxide precipitate and removal by filtration. As(III) is not effectively removed because of its overall neutral charge under natural pH. Because As(III) is more difficult to remove than As(V), pre-oxidation is typically necessary. In general, optimized coagulation/filtration systems are capable of achieving over 90% removal of As(V).
Iron-based coagulants, including ferric sulfate and ferric chloride, are more effective at removing As(V) than their aluminum-based counterparts. This is because iron hydroxides are less likely than aluminum hydroxides to go into solution in the pH range 5.5 to 8.5 and iron has a strong affinity to As(V).
The optimal pH ranges for coagulation with aluminum and ferric salts are 5 to 7 and 5 to 8, respectively. At pH values above 7, the removal performance of aluminum-based coagulants drops markedly. Feed water pH should be adjusted to the appropriate range prior to coagulant addition. Post-filtration pH adjustment may be necessary to optimize corrosion control and comply with other regulatory requirements.
The filtration step can be accomplished using either media filters or micro-filters. The iron hydroxides adsorb the arsenic and the resultant solids are filtered out in the pressure filter vessel. The removal of the chemical solids is easier than removing turbidity solids from surface water, and therefore higher filtration rates can be used. This allows the use of smaller filters reducing the treatment plant size and treatment capital costs.
Several batch studies have demonstrated that As(V) removal is directly related to coagulant dosage. However, specific dose requirements needed to meet As(V) removal objectives were contingent upon the source water quality and pH. Effective coagulant dosage ranges were typically 5-25 mg/L of ferric chloride and as much as 40 mg/L of alum.
The coagulation/filtration process produces spent filter backwash water as a liquid waste. When this waste is treated in a sludge thickener, iron or aluminum sludge will be produced. Liquid residuals may be disposed of through indirect discharge if all TBLLs are met for TDS and arsenic. Dewatering of sludge can be accomplished by gravity thickening, followed by other mechanical or nonmechanical techniques. The resultant sludge can be disposed of in a municipal solid waste landfill if it meets the criteria of the paint filter liquids test (no free liquid) and the TCLP. Previous studies have indicated that typical coagulation/filtration sludge will not exceed RCRA toxicity characteristic limits.