Harvesting Water from Natural and Industrial Fogs—Opportunities and Challenges

The growing demands of potable water for human civilization and a rapidly changing climate have resulted in progressively grave scarcity of useable water in the last few decades. Besides developing different technologies to reduce freshwater footprints, harnessing several non-conventional water resources is also explored to meet this global challenge. Fog, both of natural and industrial origins, has emerged as one such promising water resource. Depending upon the origin, fog contains finely divided water droplets (1–40 µm in diameter) flowing along the wind. By creating hydrodynamic flow obstruction in the path of fog, e.g., a fog screen, these droplets may be made to deposit on the obstruction and then be collected by gravity. Harvesting fog from atmosphere using large-sale fog nets has long been tried, providing water to satellite communities in arid, semi-arid, costal, and highland regions in over 17 countries all over the world. The daily collection of water from such fog harvesters ranges from 1 to 12 Lm⁻². On the other hand, harvesting industrial fog can remarkably reduce freshwater consumption in those very industries. Recent studies related to capture of power plant cooling tower fog using metal mesh have shown excellent prospect of reducing the circulating water makeup consumption. With a recorded fog water capture of up to 54 Lm⁻² with metal mesh, such arrangement has shown potential of saving nearly 10.5 m³ of water per hour for a 500 MW unit. Albeit, current design of fog nets offers poor overall collection efficiency, which may be attributed to low aerodynamic, deposition and drainage efficiencies. While the first two parameters can be improved by carefully designing the fog harvester to maximize droplet impingement on the mesh fibers with minimal pressure drop across them, drainage efficiency may be improved by appropriately tuning the mesh fiber surface wettability to facilitate adequate, directional drainage of the collected water with minimum re-entrainment into the air stream. This chapter focuses on the potential of natural and industrial fogs as alternative sources of freshwater and discusses several strategies to capture them for human use. Salient features of existing and futuristic designs of fog harvesters are discussed, and their operational characteristics are highlighted.