Why removing Arsenic?
Arsenic is a naturally occurring element which can be found in water as arsenate (oxidation degree +V) or arsenite (oxidation degree +III). Arsenic belongs to the most toxic elements in nature, and must therefore be removed if concentration is higher than 10 µg/L (WHO Guideline for drinking water quality, 4th edition). It used to be higher (40 µg/L), and in some countries the threshold is even lower (around 6 µg/L). Effects of Arsenic on human are irritation of stomach and intestines, decreased production of red and white blood cells, skin damages and lung irritation. Exposure can also cause infertility and cancer, as well as leading to lifeloss.
Arsenic removal systems:
There are many different methods to remove arsenic from the water. However all these methods indicate that only the arsenate (+V) can be removed from water effectively. If arsenite (+III) is present in source water, oxidation of the same is essential. The methods that can be used are:
- Reverse Osmosis and nano-filtration may deliver satisfactory removal efficiencies, however water rejection is very high in these technologies
- Ion exchange can remove As (+V) from water. However, high levels of TDS, selenium, sulfate, fluoride and nitrate contained in water can affect the life of resin. Disposal of highly concentrated spent regenerant is a serious problem. As (III) is hardly removable by ion exchange method
- Precipitation and filtration, arsenic can be removed by coagulation with ferric chloride, followed by filtration. Iron coagulation/filtration and iron addition with direct filtration methods can significantly reduce arsenic while removing the iron and manganese from the source water. The coagulation technique is not appropriate for small water facilities, because of high cost.
- Adsorption of arsenic with ferric hydroxide. This technology combines the high arsenic removal efficiency of the coagulation–precipitation process with the advantage of simple operation of the fixed bed adsorption technology.
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