**Abstract**

Overexposure to fluoride (F) through drinking water is the most widespread water problem in the world, but it has now exacerbated due to rapid population growth rates, adverse climatic changes, and increasing levels of water scarcity. Thus, despite the large amounts of data, which has accrued on mitigation methods of high F is still the primary impediment to drinking water programs among many developing nations. The current review chapter on F mitigation techniques applied world-over is aimed at providing a succinct overview of water defluoridation techniques and strategies being used to combat the impact of human F overexposure. It represents a starting point to understand the prospects of reducing the global F impact. It is anticipated that this work will lay a strong foundation for this and also inform strategies for safeguarding public health and the environment from F pollution.

**Keywords:** defluoridation technologies, drinking water, fluoride, fluorosis, literature review

### **1. Introduction**

The beneficial and detrimental effects of fluoride (F) were established in the early 1940s. Low levels of drinking water F (< 0.1 mg/L) were linked to the occurrence of dental caries, whereas elevated levels of F in water were associated with incidences of dental fluorosis among the communities [1]. Then some countries began artificial fluoridation of drinking water to control teeth decay [2]. Soon the widespread use of F in drinking water and oral products to control teeth decay resulted in a drastic decline in incidences of dental caries with a concomitant rise in dental fluorosis among the communities [3]. The severity of fluorosis increases with increasing F concentrations greater than 1.5 mg/L in drinking water [4] and data over the last few decades indicate trends toward more fluorosis around the world [5].

The new surge in the prevalence of fluorosis around the world has been attributed to, among other the rise in water fluoridation programs [5]; indiscriminate use of fluoridated products [6]; inadequate F legislation in the affected countries [7]; lack of technology and capacity for sustainable F surveillance [2] and widespread water

security problems [8]. The WHO's "guidelines" of 1.5 mg/L as the allowable standards of drinking water F have also come under scrutiny in regard to its success in controlling the adverse F effects among the communities [9]. Severe fluorosis has been recorded among communities using household waters with F levels well within these guidelines [10]. So, there have been efforts to control the adverse F effects in the communities. The strategies that are employed include instituting community health risk assessments and management programs [11]; high water F surveillance [12]; prospecting for safer water [13]; development of water defluoridation strategies; F awareness creation and behavior change campaigns [14]; developing water policies and legislation for F mitigation [9].

It is apparent, however, that there is an urgent need to reconsider the current approaches. The desired approaches for effective control of F impact among the communities should not only be effective but also be affordable, holistic, and comprehensive. In the current work, a review of the previous strategies and methodologies that have been applied in water defluoridation is presented [15]. The chapter aims to provide an update on F mitigation technologies being deployed worldwide and it is expected that this will enhance scientific understanding of the available technologies and the prospect of reducing the global F impact.

Defluoridation of existing waters is the main option where alternate safe water in high F areas is not available. However, the available water defluoridation approaches differ in scale, efficacy, sustainability, affordability, and acceptability. Therefore, the security of supply is heterogeneous. In general, the available options for water defluoridation can be classified as chemical, membrane-based, physical, or adsorption-based.

### **2. Chemical methods for water defluoridation**

Chemical methods of water defluoridation involve the addition of reactive species, which can react with and facilitate water F removal through phase separation steps [16]. Here, five methods of which two are based on precipitation, two on coagulation, and one on electrocoagulation processes will be explained in further detail.

#### **2.1 Precipitation processes**

They basically include lime-softening and contact precipitation. The former is a precipitation chemical method of water defluoridation used to remove calcium and magnesium ions from hard water and it is also used to reduce F levels in potable waters [17]. In the traditional "lime-softening" technique, lime (Ca(OH)2) reacts with soluble Ca(HCO3)2 to precipitate insoluble CaCO3 [18] according to Eq. (1) as follows:

$$\text{Ca}(HCO\_3)\_2(aq) + \text{Ca}(OH)\_2(aq) \to 2\text{CaCO}\_3(s) + 2\text{H}\_2\text{O}(l)\tag{1}$$

In presence of water F, however, part of Ca(OH)2 precipitates and removes insoluble fluorite, CaF2, according to Eq. (2) as:

$$\text{Ca(OH)}\_{2}\text{(aq)} + 2\text{F}^{-}\text{(aq)} \rightarrow \text{CaF}\_{2}\text{(s)} + \text{OH}^{-}\text{(aq)}\tag{2}$$

The removal of F by lime-softening is enhanced in presence of dissolved magnesium salts, which precipitate as Mg(OH)2 according to Eq. (3) below [19].

*Water Defluoridation Methods Applied in Rural Areas over the World DOI: http://dx.doi.org/10.5772/intechopen.105102*

$$\text{Mg}^{2+}\left(aq\text{)} + 2\text{OH}^-\left(aq\right) \rightarrow \text{Mg}\left(\text{OH}\right)\_2\text{(s)}\tag{3}$$

This also helps to eliminate excess alkalinity in treated water [20]. Defluoridation by lime is achieved by surface precipitation of fluoride onto the Mg(OH)2 formed but the process is temperature dependent and the F adsorption onto Mg(OH)2 increases at high temperatures.

Lime softening is the most common water defluoridation method in developing countries [21]. Based on this technique, the Indian Institute of Science in Bangalore, developed a simple defluoridation technique, which uses magnesium oxide, lime, and sodium bisulphate [22]. Due to the presence of MgO, the pH of treated water has to be adjusted to desirable levels (6.5 to 8.5) by adding 0.15 to 0.2 g per liter of sodium bisulphate. Even so, water defluoridation based on lime softening is inefficient and the technique requires large amounts of reagents leading to high volumes of F-laden sludge.

#### **2.2 Contact precipitation**

Contact precipitation employs the simultaneous addition of soluble calcium and phosphate compounds to brackish water. These react with F ions to precipitate CaF2 and fluorapatite (Ca5(PO4)3F2) [23] catalyzed by a saturated bone char medium. The process has been applied in Tanzania on raw water with 13 mg/L F resulting in F removal efficiency of 97.9% [24]. Contact precipitation has been floated as being more efficient and reliable than lime-softening, but it also generates large volumes of F-enriched sludge [25] and it can impart bad taste and smell to the treated water compromising its palatability.

#### **2.3 Coagulation techniques**

Coagulation is a procedure in which soluble metal cations [26] or commercial polyelectrolytes [27] with a large charge-to-volume ratio are added to water to attract and react with organics and other insoluble aggregates to flocculate and sediment and phase them out of the water. The flocculates formed to provide high sorbent surfaces for F ions. Alum (Al2(SO4)3.18H2O) is the usual flocculant in cases where F removal is also desired [26]. The salt reacts with OH<sup>−</sup> ions to form Al(OH)3 flocs according to Eq. (4) as:

$$\rm Al\_2\left(SO\_4\right)\_3\cdot14H\_2O + \rm 3Ca(HCO\_3)\_2 \rightarrow 2Al(OH)\_3 + \rm 3CaSO\_4 + 14H\_2O + 6CO\_2 \tag{4}$$

The flocs sorb F from the water. A little lime is added controllably to replenish the OH− ions. The amount of alum is controlled to prevent the initiation of complexation of Al3+ with F. Also, coagulation is not efficient and complete F removal is not achieved. Large amounts of coagulants are used leading to large volumes of sludge [25] and the residual coagulants in the water must be monitored to meet the drinking water standards.

Nonetheless, several authors have reported the application of this method with varying degrees of success [28, 29]. On the other hand, some authors have reported investigations aimed at improving upon the technique. Atia et al. [30], for example, compared the coagulants and demonstrated that the use of Al2(SO4)3.18H2O as

flocculants was superior to Fe2(SO4)3.H2O. However, F removal by coagulation using poly-aluminum chloride (PAC) has also been reported [31] and compared with other polyelectrolyte coagulants [32]. Most recently some workers have utilized inorganic polymeric coagulant and indicated that 80% defluoridation of 6 mg/L F polluted water [33]. An alternate precipitation technique based on induced crystallization under extreme pH levels and carbonate/bicarbonate equilibriums has recently been applied with high F removal efficiencies [30]. Also, a facile approach to calcium co-precipitation has been reported [34].
