for Fluoride Contamination
2.5.1 Remediation of Fluoride
As we all know, clean water is one of our basic needs. The increase of fluoride content in drinking water due to various sources ultimately increases the above‐mentioned detrimental effects, so a temporary solution may be that it's better to avoid that particular source. The methods used for the treatment of fluoride‐contaminated groundwater are in‐situ and ex‐situ methods that reduce fluoride content and bring it to a usable form. Several types of remediation techniques have been shown in Figure 2.2.
Figure 2.2 Remediation techniques for fluoride removal.
2.5.2 In‐situ Treatment Methods for Fluoride Removal
These methods are focused on straight dilution of fluoride concentration of groundwater in aquifer only, which can be accomplished by artificial recharge. In order to evaluate the efficacy of managed aquifer recharge (MAR), measures like check dams, percolation tanks, and recharge pots came to light as part of the in‐situ treatment methods.
2.5.2.1 Check Dams
Check dam construction reduces fluoride concentrations and also improves the quality of groundwater in accordance with domestic and agricultural use. The fact that groundwater quality has been improved by recharge using the dams has also been confirmed by spatial variation in the saturation index of minerals. The content of fluoride in wells of groundwater reduces near the check dam whereas areas away from the dam have higher fluoride content. Higher groundwater level and reduced fluoride content near the check dam represent the major advantage of recharging from the dam. The fluoride concentration becomes lower than the permissible limit of BIS (Bureau of Indian Standards) around the area of 4 km2 near a dam present in Krishnagiri District of Tamil Nadu (Gowrisankar et al. 2017). One of the known examples of in‐situ method in India is the Anantapur district check dam construction in Andhra Pradesh, which aided in reduction of fluoride content of groundwater (Bhagavan and Raghu 2005).
2.5.2.2 Percolation Tank and Recharge Pits
Artificial recharging structures built at suitable places decrease fluoride concentration. The method of rainwater harvesting also known as rainwater recharge could be adopted using percolation tanks and recharge pits that may prove to be very helpful. Utilizing existing wells to recharge rainwater after filtration is also an important and useful method to improve groundwater quality. Artificial recharging structures can be built in suitable places which will decrease concentration. See Figure 2.3 for in‐situ remediation techniques.
Figure 2.3 In‐situ remediation techniques: (A and B) Check dams; (C) Recharge pits; (D and E) Percolation tanks.
2.5.3 Ex‐situ Treatment Methods for Fluoride Removal
Several ex‐situ treatment methods are available for defluoridation of water at both domestic and community levels.
2.5.3.1 Adsorption
Adsorption is one of the most feasible and cost‐effective methods for removal of fluoride (Figure 2.4). In this process, water is passed via a contact bed adsorbing fluoride on the matrix. The effective removal depends upon the concentration of fluoride, time of contact, pH, size, and type of adsorbent. After a period of time, the saturated column or bed should be refilled or regenerated. The first defluoridation was done around 1930 using activated alumina. It has a large surface area and porous aluminum oxide; the continuous aluminum lattice provides a localized positive charge area, creating a good adsorbent for many anionic adsorbates. Since it has a greater preference for fluoride ion in comparison to other ions, it is thus used widely in defluoridation (George et al. 2010). Activated charcoal, activated aluminum, fly ash, serpentine, brick, charfines, bone char, waste mud, red mud, kaolinite, rice husk, ceramic, bentonite, bioadsorbents, etc. are several capable adsorbents used for removal of fluoride‐contaminated water.
2.5.3.2 Ion Exchange
The ion exchange method involves the passage of water through a column containing ion exchange resin, and calcium ions are replaced by fluoride ions in resin. Once the saturation is obtained, the resin is backwashed with a chloride‐containing solution like sodium chloride, in which chloride replaces back the fluoride ions in resin so that it can be reused. The backwashing should be done with proper care.
Figure 2.4 Adsorption technique for fluoride removal using various adsorbents.
2.5.3.3 Coagulation‐Precipitation
The precipitation method involves certain chemical additions like calcium resulting in precipitation of fluorite; in the place of calcium, one can also use aluminum. One of the widely used Nalgonda techniques uses alum, lime, and bleaching powder with vigorous mixing, flocculation, sedimentation, and filtration. The method was initiated and developed by NEERI (National Environmental Engineering Research Institute) for community and household levels under Rajiv Gandhi Drinking Water Mission (Figure 2.5). The sludge obtained by this process has high aluminum and fluoride, disposal of which is also problematic.
2.5.3.4 Membrane Process
The membrane process is one of the ex‐situ techniques also known as reverse osmosis and electrodialysis. This method requires financial input as it uses a semipermeable membrane to remove dissolved solutes from water (Brindha and Elango 2011). This process is utilized as advanced water treatment technology mainly in the treatment of pure and ultrapure water.
The research of Somnath Rudra (2012) pointed out some important holistic alternatives to mitigate the problem of fluorosis:
Utilization of fluoride removal filters for treatment of excess fluoride at home.
River and stream water based piped water supply.
Harvesting rainwater.
Utilizing traditional sources of water (bunds/ponds) after treatment.Figure 2.5 Coagulation and precipitation method.
Substantial water quality monitoring program.
Diagnosis
