10. Fluoride Removal from Water Using Filtration and Chemical Precipitation

In precipitation, fluoride-rich water is treated with aluminum salts and lime, which is then taken through filtration or sedimentation. Lime reacts with the free fluorides, originating from salts such as NaF and HF (acids released by industries), forming calcium fluoride (CaF₂), an insoluble impurity that can be eliminated. Since aluminum acts as a coagulant, its salts Aluminium Chloride (AlCl₃) or Aluminium sulfate (Al₂(SO₄)₃) are used for the viable removal of fluorides. This leads to the formation of Aluminium Hydroxide (Al(OH)₃) with the introduction of alum. This leads to the accumulation of all fluoride impurities, which makes it easier to eliminate them. This coagulation process can also be carried out using Ferric salts (Fe salts) as an alternative to aluminum salts. An improvement in this is electrocoagulation (EC), which involves electric current passed through an aqueous medium wherein sacrificial electrodes are used. An aluminum anode is used, which breaks the metal into ions that react with the free fluoride ions in the aqueous solution to form an Aluminium Hydroxide-Fluoride complex. It coagulates, thus increasing the speed of impurity elimination. These complexes need to be filtered to further reduce the fluoride content of the water; carbon, or gravel filters can be used. Here, a layer of a sand filter is used along with layers of gravel. Thus, chemical precipitation (coagulation) is utilized to reduce the fluoride concentration from a high bulk level to a few milligrams per L. This is further reduced below the critical limit of minerals/elements with filtration and sedimentation. Thus, this chapter discusses methods of removal of fluoride impurities through chemical precipitation using coagulation, electrocoagulation, and filtration, including membrane filtration. Explores recent technology and methods that have been developed to improve the efficiency and cost-effectiveness of precipitation and filtration methods.