**4. Distillation**

#### **4.1 Solar distillation**

Distillation is a physical procedure in which water is converted to steam, and then the steam is condensed back into liquid water. The dissolved salts remain in the brackish water that is left behind. Solar distillation takes advantage of abundant sun rays and is the most applicable technique to circumvent the high costs of electricity. The method is simple, clean, and effective but the resulting wastewater must be removed to prevent encrustation of the vessels and be disposed of with care due to their high salinity. Solar distillation can be used from household scale to large industrial scale. Otherwise, it is inexpensive to run but the initial installation costs are big.

Solar distillation units have been used world-over to treat brackish water [69]. A pilot project of Solar driven membrane distillation has been operated on a small village level at Robanda in Tanzania [70]. Also, a similar solar water defluoridation unit has been built using local materials and successfully operated in Bongo District, Ghana, [71].

#### **4.2 Membrane distillation**

Membrane distillation uses a hydrophobic membrane with air-filled pores. The surface tension of the feed water and distillate prevents the water from entering the membrane pores keeping it out of the membrane. Water vapor pressure difference is then generated by applying sufficient temperature difference across the membrane. This is accomplished by heating the feed water and cooling the distillate at the other side of the membrane to cause a flow of water vapor through the membrane and result in distillate condensation [70].

Naidu et al. [71] evaluated the applicability of a modified design vacuum enhanced-multi-effect membrane distillation for drinking water and projected a 70% recovery ratio for a scaled-up unit. The feasibility of a direct contact membrane distillation (DCMD) process to recover F contaminated waters was also tested and up to 99% rejection of F was reported [72]. Boubakri et al. [73], using a similar DCMD process based on polyvinylidene F membrane, observed high thermal efficiency and high permeate flux favored by elevated temperatures.
