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Adsorptive Media


Although granular activated carbon (GAC) and powdered activated carbon are adsorptive media commonly used in drinking water treatment, other types of adsorptive media are being utilized, principally for control of arsenic. These other types of adsorptive media are similar in mechanism to activated carbon, that is, they accumulate contaminants on their specially-designed surfaces, and when their capacities are reached or when breakthrough of the contaminant occurs, they must be reactivated or replaced. The design and operational parameters and water quality parameters important to GAC (particle size, uniformity coefficient, empty bed contact time, flow rate, surface loading rate, contaminant concentration, pH, temperature) are important to other adsorptive media.

Most adsorptive media operate in a down flow mode and oftentimes in pressurized vessels. Like GAC, they may serve as filter adsorbers or as post-filter adsorbers. Water quality and pretreatment are key to this choice to avoid fouling and backwashing of the adsorptive media.

For the control of arsenic, pretreatment may also be required to convert As(III) to As(V). As(V) is generally better suited to control by adsorptive media. Oxidants like chlorine, permanganate and ozone can make this conversion. All adsorptive media employed for arsenic control are subject to competition from other ions. Thus, pretreatment may be required to keep adsorptive media effective for the control of arsenic or other target contaminants. Treatment by adsorptive media is therefore oftentimes concurrent with other treatment processes.

There are numerous adsorptive media products. Principally for the control of arsenic are included these types of media: activated alumina, iron modified activated alumina, iron based media and iron modified resin, titanium based media and zirconium based media. In the 2000 to 2010 decade, significant research to evaluate these media for arsenic control took place.

Activated alumina (AA) is pH dependent, generally operating below pH 6.5 and generally most effective at pH 5.5, therefore typically requiring pH adjustment. As(V) has an affinity for iron. This is why iron based media work well for arsenic control. Iron-modified activated alumina may be employed for this purpose.

As(V) also has an affinity for other elements. Thus, zirconium and titanium based adsorptive media may be employed. Compared to AA, iron based media and iron modified resin, titanium based media and zirconium based media offer several potential advantages. They have a stronger affinity for arsenic than AA. They are affected by pH, but not to the extent of AA, and may operate at relatively higher pHs than AA. They have faster kinetics. These factors suggest more cost-effective control of arsenic.

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