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 Coagulation/filtration is a process that removes contaminants from water by precipitation. Aluminum salts and ferric (iron) salts are the two most commonly used coagulants in the water treatment industry. These salts work by hydrolyzing to form aluminum hydroxide and iron hydroxide particulates, which can then be effectively filtered out of the water supply.

Because of iron’s strong affinity to arsenic (As), iron-based coagulants such as ferric sulfate and ferric chloride are more effective than aluminum coagulants when it comes to removing arsenic (As) from water. In addition, iron hydroxides are less prone than aluminum hydroxides to go into solution in environments ranging from 5.5 to 8.5 in pH.

Coagulation with ferric salts is most effective in a pH range between 5 and 8. For aluminum salts, the optimal range is 5 to 7; at pH values above 7, the efficiency of aluminum-based coagulants falls significantly. To increase coagulant effectiveness, the pH level of feed water should be brought into the target pH range before the coagulant is added. However, the pH level may need adjustment after filtration both to prevent corrosion and to bring the water output into compliance with regulations.

Filtration after coagulation can be accomplished using either media filters or micro-filters. The hydroxides adsorb arsenic and the resulting 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 which reduce the treatment plant size and treatment capital costs.

Several studies have shown that the level of arsenic (As) removal is directly related to the amount of coagulant used, but the coagulant dose required to meet specific arsenic (As) removal objectives depends on the initial quality and pH of the source water. Effective doses of ferric chloride range from 5 to 25 mg/L. For alum the effective dose can run as high as 40 mg/L.

The coagulation/filtration process produces spent filter backwash water as a liquid waste. Treating this waste in a solids thickener will produce iron or aluminum solids. Liquid residuals may then be disposed of through indirect discharge, provided that all TBLLs are met both for arsenic and for TDS. Solids can be dewatered by a sequence of gravity thickening followed by other mechanical or non-mechanical dewatering techniques. Resulting solids that pass the paint filter liquids test (no free liquid) and the TCLP can be disposed of in a municipal solid waste landfill. Studies have demonstrated that typical solids resulting from the coagulation/filtration process will not exceed RCRA toxicity characteristic limits.

Filtronics uses coagulation filtration to remove arsenic, iron, manganese and sulfides with our permanent media Electromedia® and ferric chloride as the coagulation agent. There are several advantages Electromedia® brings to the coagulation/filtration process including:

View the different types of Filtronics Electromedia® media

Take the first step, simply complete our online General Mineral Analysis (GMA) form which gives us the water quality and site profile. From that, we will be able to custom design a solution to meet your needs, prepare a budget proposal and arrange a pilot test.

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