In addition to iron and manganese, arsenic is a further trace substance occurring especially in reduced groundwater (groundwater in deep aquifers).
Removal through oxidation:
When using ozone or other oxidation agents the arsenic removal can occur parallel to iron removal and demanganisation in one single filter stage. The arsenic is oxidised in this case to arsenate which is held back in the filter. The arsenate is removed from the filter bed during filter reverse-flushing with iron slurry and manganese.
Removal through adsorption:
Alternatively, filtration via adsorbing filter materials such as granulated iron hydroxide (GIH) is possible. In this process, arsenic agglomerates at the GIH material. Before adsorption iron and manganese have to be removed completely from the water. Adsorption filters cannot be rinsed. When exhausted the material has to be replaced completely. Adsorption filters are as a rule designed for two to three years service life.
Removal through precipitation:
A further alternative is the precipitation by dosing iron salts before a filter stage. Arsenic then accumulates at the iron sludge similarly to adsorption. A disadvantage of this process is the high sludge occurrence that as a rule requires extensive slurry treatment with dewatering and pressing.