The system, consisting of a co-knit stainless steel wire mesh provided by Mesh Concepts, started on May 12 and was completed on May 18. It was constructed at an elevation of 1 600 m near Brooksnek Pass.
Fog harvesting company Cloud Water Concepts installed the system. Its members include, professor Johan van Heerden, professor Jana Olivier, Howard Muller and Duncan Evans.
Fog harvesting literally entails harvesting water from fog or clouds. Van Heerden says that fog or cloud consists of small water droplets with diameters ranging from 10 micron to 100 micron. At sizes below 100 micron, the drops can be kept in suspension by the lightest of vertical air currents, and have difficulty in falling from the cloud or fog to reach the surface of the earth as drizzle.
Van Heerden says that fog harvesting occurs when fog or cloud moves through a man made device like a net or a mesh, and the small droplets collide with the mesh, flow down the mesh and are collected by a gutter device and stored or used. For the system to work, the cloud or fog has to move with the wind through the device, and the collection must take place at sites where fog or cloud occurs frequently.
Van Heerden says that fog harvesting provides sustainable water at a low cost. He adds that areas in South Africa suitable for fog harvesting are the entire eastern escarpment as well as parts of the West Coast regions, with local research indicating that the best sites are at an elevation of 1 000 m, unobstructed towards the Indian Ocean. Currently, he says, fog harvesting systems are being erected all over the world, with the Chileans the most prominent.
The present system consists of four triangles with sides of 36m2 each. The nine mesh sections are supported by a system of cables supported by six poles, each 6 m in length. Both the structure and net supporting cables are anchored in the ground. The bottom cable also supports a gutter that feeds the collected water through water filters to a 10kl tank situated some 100 m away, and below the structure.
Van Heerden says that the system requires no external power source, except the presence of cloud or fog, and wind. Typically, he says, the liquid water content of these clouds are well below 3 grams/ m3 and therefore require wind to move the cloud droplets through the system.
"The technology is simple, and, with certain criteria in mind, the clouds can be an alternative source of water that can, in certain areas, provide more water than annual rain provides," he notes.
He says that the system provides the benefit of potable water, which is gravity-fed to communities or to complement existing water supplies.
Van Heerden says that, as fog-harvesting systems operate at high elevation, and are exposed to strong wind, rain and snow, the structures in place must be strong, and require low maintenance. He says that the stainless steel component of a knitted wire mesh provides the system with a measure of strength and durability, allowing it to not be greatly affected by climatic conditions. Further, he adds, various combinations of material at the manufacturing stage have been added, and are able to improve the surface area substantially, which, in turn, increases the water yield.
Van Heerden says that the municipality heard about the systems, and contacted Olivier and Van Heerden during 2007. The parties met in June and July 2007, and were briefed on the potential. At this stage, he says, negotiations started, and the need to provide water for communities was highlighted. A new co-knit mesh was constructed and various others were tested at various sites and, in March this year, a proposal was made to ANDM.
The product locally manufactured by Knitwre SA, (and backed by Knitwire UK), is approved by the South African Bureau of Standards.