Arsenic Removal

Arsenic can be found in groundwater, surface water supplies and in industrial wastewater. There are a number of treatment approaches that have been found to be effective. Among them are:

Coagulation and Filtration
Adsorptive Media
Greensand

The approach best suited to a particular application depends on the amount of arsenic present, whether the arsenic is present as AsIII (arsenite) or AsV (arsenate), the pH of the water, its alkalinity, and other ions in the water. Other considerations include the volume of water to be treated daily and the willingness or ability to use coagulants or chemicals in the treatment process. A brief overview of arsenic removal strategies is provided below. Arsenic can be removed via chemical coagulation followed by filtration. The coagulants used typically include:

Aluminum Sulfate
Ferric Chloride
Ferric Sulfate

Jar tests using samples of the water to be treated are needed in order to determine which coagulant, dosage level and pH will produce the required arsenic removal. In instances where there is a high concentration of flocs formed, a settling step may be needed prior to filtration. pH adjustment may also be needed. In most cases multi-media filters are used to remove the suspended solids formed through coagulation. However, where the volume of water to be treated is low, consideration can be given to the use of backwashable cartridge filters.

Adsorptive Media

Adsorptive media are granular substances that operate in pressure vessels. Included in this category are synthetic aluminum oxide, activated alumina and granular ferric hydroxide. Synthetic aluminum oxide is a high capacity adsorbent media that is highly selective for arsenic. Used in pressure vessels, this media is most effective if pH is 5.5-6.0 and when TDS is not high. Increased temperature of the raw water increases capacity of the media up to 212oF. AsIII is not removed as well as AsV, so pre-chlorination to convert arsenite to arsenate may be required. When maximum adsorption has been reached, the media is regenerated using sodium hydroxide follow by neutralization with acid.

Activated alumina is considered an adsorptive media, although the chemical reaction is an exchange of arsenic ions with the surface hydroxides on the alumina. When all the surface hydroxides on the alumina have been exchanged, the media must be regenerated. Regeneration consists of backwashing, followed by sodium hydroxide, flushing with water and neutralization with a strong acid. Effective arsenic removal requires sufficient Empty Bed Contact Time. pH can also be a factor, with neutral or slightly acidic conditions being considered optimum. If AsIII is present, it is generally advisable to increase Empty Bed Contact Time, as AsIII is adsorbed more slowly than AsV. Alumina dissolves slowly over time due to contact with the chemicals used for regeneration. As a result, the media bed is likely to become compacted if it is not backwashed periodically.

Granular ferric hydroxide works by adsorption, but when the media is spent it cannot be regenerated and must be replaced. The life of the media depends upon pH of the raw water, the concentrations of arsenic and heavy metals, and the volume of water treated daily. Periodic backwashing is required to prevent the media bed from becoming compacted. pH may need to be adjusted if it is high in order to extend media life. For maximum arsenic removal, filters operate in series. For less stringent removal, filters can operate in parallel.

Greensand

Greensand is available in two forms:

As glauconite with manganese dioxide bound ionically to the granules.
As silica sand with manganese dioxide fused to the granules.

Both forms operate in pressure filters and both are effective. Greensand with the silica sand core operates at higher water temperatures and higher differential pressures than does greensand with the glauconite core. Arsenic removal requires a minimum concentration of iron. If a sufficient concentration of iron is not present in the raw water, ferric chloride is added. Sodium hypochlorite and/or potassium permanganate are added to the raw water prior to the filters. Chemical addition may be done continuously or intermittently, depending on raw water characteristics. These chemicals oxidize the iron in the raw water and also maintain the active properties of the greensand itself. Arsenic removal is via co-precipitation with the iron.

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Everfilt : Arsenic Removal Water Filtration