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<Sourcebook of Alternative Technologies for Freshwater Augumentation in Africa> 2.1.3 Fog Harvesting Technical Description Fog droplets are much smaller than both raindrops and drizzle drops, with diameters varying from 1 to 40 microns, and fall at velocities ranging from less than 1 cm/s to about 5 cm/s. These low velocities result in fog droplets being influenced even by light winds that can cause the droplets to travel almost horizontally. An appropriate fog droplets-collector, therefore, is a vertical or near vertical surface. Such surfaces can be constructed as vertical mesh panels on which fog droplets are intercepted and condensed. Extent of Use Fog harvesting is a rarely used technique in Africa. In Namibia, for example, the technology is still in the experimental stage. However, the actual application of this technology on a limited basis did begin in late 1995, as a project of the Ministry of Agriculture, Water and Rural Development. Operation and Maintenance Operational requirements include the measurement of the volume collected and the recording of meteorological data, either manually or by automatic weather station, since changes in weather conditions may change the operational design of the harvesters. Problems encountered include dust, rodents, game, and irregular fog formation. However, virtually all of the input materials to construct, operate and maintain the system are available locally. Costs The input costs are not yet known. However, an automatic weather station costs about $50 000 to purchase and install. Effectiveness of the Technology A surface area of about 50 m2 can harvest a significant amount of fog and convert it into water. However, experimental data from the system in Namibia are still forthcoming. Suitability Fog harvesting is suitable in regions which have hills or mountains close to potential users, on a coastline with a cold current offshore. The quantity of derived water is a function of the scale of the project and the fog available. In Namibia, the resulting water is slightly salty as a result of the inclusion of some marine aerosols, and contains some dust. Environmental Benefits No direct environmental benefits have been reported or are foreseen. Advantages No extensive, permanent structures are necessary to implement this technology, and the derived water is normally potable. The technology can easily cater to the water needs of coastal, or desert, settlements or camps currently relying on a saline water source or some other expensive option such as tanker delivery. The water is available within the demand area and therefore requires little, if any, pumping. The water source is also sustainable over many years. Clouds normally bring a large amount of water and extend over a wide front. Therefore, the amount of water collected can be varied by varying the number of collectors installed. The collectors are simple and require no energy other than the wind. Disadvantages Deforestation can lead to reduced fog water inputs. Further, it may result in dust and other pollutants entering the harvested water. Information Sources Contacts P. Heyns, Department of Water Affairs, Ministry of Agriculture, Water and Rural Development, Private Bag 13193, Windhoek, Namibia, Tel. 061-263141, Fax 061-263222. D. Lucks, Department of Water Affairs, Ministry of Agriculture, Water and Rural Development, Private Bag 13193, Windhoek, Namibia. Bibliography Heyns, P. 1995. Desalination Development in Namibia, New World Water. Schemenauer, R.S. and P. Cereced 1994. Fog Collection's Role in Water Planning for Developing Countries. Natural Resources Forum, 18 (2): 91-100.
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