**2.5 Electrocoagulation**

Electrocoagulation (EC) uses an electrolytic cell (**Figure 1**) [38] to supplies coagulant Al3+ ions in a controlled manner [26].

As the electric current passes through the cell, the Al anode gets oxidized to Al3+ ions, which are transformed into polymeric species and reacted with hydroxyl ions in solution to form the Al(OH)3 flocs, facilitate F removal from water as in the other coagulation techniques [39].

#### **Figure 1.** *Schematic representation of an electrocoagulation cell (adapted from Ref. [38]).*

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

Even though the EC technique has been associated with demerits including high running costs, need for reliable electric power, and for specialized personnel to operate [25], the technique is efficient, reliable, and produces high-quality water. The sludge generation is low, and the method is used in different water conditions. In Algeria [40], for example, EC was applied as an efficient affordable water defluoridation technique, and, elsewhere, it has been used to reduce borehole F level from 3.5–4.8 to 0.8–1.0 mg/L [41]. Also, Khatibikamala et al. [42] reported that F concentration was reduced, based on an EC cell, from 4.0–6.0 mg/L in raw water to lower than 0.5 mg/L. Further, EC was applied to treat groundwater from Shivdaspura (Rajasthan): Sinha et al. [43], reported initial F levels of 5.0 mg/L were reduced to 0.2 mg/L plus. Then, Emamjomeh and Sivakumar [40] confirmed the technique as an effective electro protocol for domestic and industrial water defluoridation.

On their part, Vasudevan et al. [44] compared the performance of different electrodes in an EC protocol and found that F removal efficiency reached 96% with a magnesium alloy anode and a stainless steel cathode at a current density of 0.2 A/dm2 and pH of 7.0 more recently Khan et al. [45], confirmed earlier findings by Takdastan et al. [46] that aluminum electrodes were more efficient and economical in F removal than iron electrodes. In related analyzes, Hu et al. [47] showed that the efficacy of an EC system in water defluoridation was controlled by the molar ratio of hydroxide and F to Al(III) and related that optimum activity coefficients for defluoridation in coagulation and electrocoagulation are both close to 3.

### **3. Membrane methods**

Membrane methods are those that employ the use of a casing that selectively separates a component in water. They include electrodialysis, reverse osmosis, and nanofiltration [48].

**Figure 2.** *Schematic representation of an electrodialysis unit.*

## **3.1 Electrodialysis**

Electrodialysis (ED) technique uses an electric field to separate ions of one charge from the counter ions [49]. A typical ED unit (**Figure 2**) consists of about 400 alternating cation- and anion-exchange membranes, which are 0.5–2.0 mm wide, sandwiched between an anode and a cathode in a cell [25].

The membranes have charged groups bound into polymeric substrates which attract and adsorb mobile counter ions. The anionic-exchange membranes permeate cations only, while the cation-exchange membranes permeate anions but trap the cations. Under an electric field, cations and anions move in opposite directions and the membranes capture respective ions resulting in alternating cells of ion-concentrated solutions called concentrates and ion-depleted solutions referred to as dilutes [50]. ED is an efficient defluoridation protocol and the sludge volume generated is low. However, the overall protocol is costly, complex, and requires reliable source power and specialized personnel to operate. The process is also non-selective and removes essential ions required for quality drinking water [51].

The technique has been applied to defluoridation of saline water with 3000 mg/L total dissolved salts (TDS) and 3.0 mg/L F [52]. Elazhar et al. [53] compared the performance of ED and nanofiltration (NF). Kabay et al. [54], on the other hand, was able to optimize a water defluoridation process and evaluated its mass transfer and energy use efficiency. ED was applied in Brazil with 97% defluoridation efficiency [49]. In India, ED was applied to saline water with high TDS of 5000 mg/L and 10 mg/L F levels [51] and it has been reported that ED was used to treat brackish water with 2.9 mg/L to just 0.4 mg/L [F] [55].

### **3.2 Reverse osmosis**

Reverse osmosis (RO) is a membrane process in which dissolved pollutants are removed by applying pressure on raw water to force it through a semi-permeable membrane against the osmotic pressure (**Figure 3**) [56].

The level and rate of contaminants removal depend on the sizes and electrical charge of the polluting ions [57]. It is found that RO is efficient and generates little sludge, but it is expensive to install and to run—it requires specialized personnel and reliable electric power to generate necessary pressures [48]. Some of these limitations of RO can, however, be bypassed. A study in Tanzania [58], for example, applied nanofiltration (NF) and reverse osmosis (RO), with an autonomous membrane system, which was powered by solar energy and the tested membranes could achieve the WHO drinking water standards [59]. The process reached 1000–2500 L daily total permeate volume of portable water with an additional 3500–5000 L of nonpotable water fit irrigation and washing. However, the integration of such advanced

**Figure 3.** *Schematic representation of reverse osmosis.*

technologies has not proven successful in many rural areas of developing nations where the necessary power is always available.

#### **3.3 Nanofiltration**

Nanofiltration (NF) operates on the same principle as reverse osmosis but the membranes have larger pores [48] offering less resistance to the flow of solvent and solute particles. The procedure is, therefore, able to operate at much lower pressures reducing energy costs. The retention of solutes is ascribed to steric and charge effects and the procedure is selective and considered to be suited to defluoridation of brackish waters. RO/NF has been applied to the treatment of various groundwater contaminants in India [60] and in the efficient removal of F and salinity from high-F brackish water at a village scale in Senegal [61]. In Finland, Kymenlaakso Water Limited, which is a public company, has operated a 6000 m3 /day water RO plant with a permeate [F] of <0.03 mg/L since 2003 [62]. Richards et al. [63] evaluated the effect of speciation on the retention of F by NF and RO and found that F retention was independent of pH. In a study realized in Tunisia, F removal from water and from wastewater by NF was found to be controlled by trans-membrane pressure, feed water concentration, ionic strength, type of counter-ions, and pH and higher retentions were linked to pH values and vice versa [64]. Some researchers have reported F retention efficiencies of an NF process of 70% [65] mark above pH 7 but another team of researchers in France reported an RO F rejection efficiency greater than 98% [66].

Nonetheless, the protocol continues to attract the interest of researchers from around the globe [67]. Furthermore, hybrid treatments with sequential use of two or more simple techniques have become common in the recent past. For instance, filtration and ultrafiltration as subsequent treatment of coagulation have been recently tested for water defluoridation by a team of workers in India [68].
