Drinking Water can be beneficial. Precipitated iron can adsorb arsenic V for removal by
have been shown to effectively oxidize
filtration.
ing free chlorine, hypochlorite, ozone and potassium permanganate, have all
Arsenic can be removed from water
using a variety of processes including iron adsorption, activated alumina adsorption, ion exchange, reverse osmosis, nanofiltration, lime softening, iron and alum coagulation and coagula tion-assisted microfiltration. Because
most of these methods rely on ionic charge, As V will be easier to remove than As III in the normal working pH range of drinking waters, and, there fore, if arsenite is present, pre-oxidation should be the first step. Bench test ing, e.g., jar or column, will take out much of the guesswork as to the appli cability of the methodology and pro vide the practical, decision-making data to treat a specific water chemistry. Oxidation
Arsenic speciation must be done to determine whether all or part of the arsenic present is trivalent. If arsenite is present, it should be oxidized to
As III to As V Chemical feeds, includ
been shown to oxidize arsenic. Solid
oxidizing media, such as those used for iron and manganese removal and sulphide removal, have also performed well, as long as the contact time is ade quate. Some methods that were expect ed to be successful, such as aeration, were not able to oxidize As III to As V
It is also important to note that monochloramine and chlorine dioxide are not effective oxidizers of arsenite.
Adsorptive Processes Adsorbent media are engineered mate rials used in fixed beds for the removal
of arsenic. A fixed-bed adsorbent sys tem usually consists of a tank (or tanks) filled with media. It is typically operated in downflow service with the ability to backwash the vessel. A num ber of media are available; most are based on iron and aluminium. The removal mechanism is a reaction
facilitate removal. Water that contains
between the arsenic and the surface of
only As V does not require further oxi
the media. The tank size is determined
dation for treatment. Several methods
by the media's flow requirements. Fine
mesh media and deep bed depths increase the contact efficiency, but also increase the pressure drop and pumping energy requirements. Low velocity and long contact time will improve media capacity and reduce leakage, but require larger tanks and greater media inventories. Any system will be a compromise of system cost, floor space required, arsenic capacity and leakage and the interval for media regeneration or replacement. a)Activated alumina Activated alumina (AA) is formed by thermal dehydration of an alumini um hydroxide. Its principal character istic is a high surface area and high porosity. It is able to remove a wide range of anions using an ion exchange mechanism with the hydroxylated sur face. Two screen size ranges of gran ules are commonly used for arsenic removal, 28x48 and 14x28. The finer
material has more surface area per unit volume, which gives it a higher arsenic removal capacity. The finer material also has a higher pressure drop, and that must be considered in a process continued overleaf...
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