Arsenic removal in water treatment: Possible processes

One possible arsenic removal method in water treatment is the adsorption process. In this procedure, the arsenic is bound to the surface of a solid adsorbent. Before the development of iron- based adsorbents for arsenic removal in water treatment, granulated aluminium oxide ("activated alumina") was used. However, since activated alumina has a lower capacity than iron-based adsorbents and dissolved aluminium poses a health threat, it is no longer used in practice for arsenic removal from drinking water.
Instead, iron hydroxide-based adsorbents are the state of the art in arsenic removal in water treatment. The safe and simple operation of adsorption filters is only one of the major advantages that result. The arsenic removal from drinking water to below the detection limit is also one of the big plus points. Moreover, no wastewater streams or contaminated sludge are generated – neither during operation, nor during the arsenic removal from drinking water. In addition, the arsenic removal in water treatment with granulated iron hydroxide is very selective. In this way, the natural composition of the water stays unaffected. Nevertheless, both arsenate and arsenite are reliably removed in this procedure.

Another possibility for arsenic removal in water treatment is the flocculation process. Iron or aluminium salts are usually used for this. For this process, they are precipitated in the raw water and filtered off. The dissolved arsenic binds to the surface of the filterable flocs and can be removed from the raw water in this way. The amount of flocculant required depends on the arsenic concentration.
It is regularly determined by analysis for the removal of arsenic from drinking water. The filtered sludge is disposed of either as residual or special waste, depending on local regulations and arsenic concentration. As this arsenic in water treatment is technically rather complex, it is mainly used for very large treatment plants for the removal of arsenic from drinking water.

Alternatively, arsenic can also be removed from drinking water with ion exchange resins. In this process, ion exchangers remove anions or cations from the raw water, depending on the type of material. At the same time, they release ions of the same charge back into the water. However, this arsenic removal in water treatment can only be used if arsenic is present as arsenate in the groundwater or drinking water. This is because it is then charged and can be removed by the ion exchanger. The removal of arsenite is not possible with this method. Since the selectivity is also low for arsenate, the capacities are low in practice. In order to achieve a practicable residence time, the ion exchange resins have to be regenerated. However, this results in toxic regenerate solutions that must be further processed. Overall, the effort is too high for this method to be used often in practice.

Water treatment processes with the use of membranes can also pose an alternative for arsenic removal in water treatment. For this purpose, high pressure is used, which in the membrane process ensures that water constituents are removed through size closures and interactions with the membrane material. To ensure that arsenic can be safely removed from drinking water, nanofiltration or reverse osmosis membranes are used because of their small pore size. Arsenic and
other water constituents cannot pass through these membranes and are retained accordingly. They are then discharged in a wastewater stream. For this method of arsenic removal in water treatment, the raw water is completely or partially desalinated. The wastewater streams have a high arsenic content and must be further treated. As this arsenic removal in water treatment is not selective, not only the arsenic content but also the rest of the water composition is changed. Depending on the further water use, this can be beneficial or detrimental.

Now that you have gained more insight into the various arsenic removal technologies in water treatment with GEH Wasserchemie, we will be happy to accompany you further on your way to clean drinking water. Learn more about us and our international efforts for clean drinking water, such as our performance check in cooperation with Aqua Dällikon in Switzerland.