Harvesting water from fog

The provision of potable water being one of the basic social services, the Eritrean government had ever since independence exerted continuous efforts to make available adequate water supply in all parts of the country. As a result of these efforts, many parts of the country, even the most remote and dry ones, previously suffering from water shortage have now easy access to drinking water. The Warsai-Yikeallo campaign has also added momentum to the process, enabling the construction of many water supply centers in different parts of the country. However, despite these ongoing efforts some parts of the country still suffer from inadequate or at least inconsistent water supply systems.
The Eritrean eastern escarpment region receives rain twice annually, however the mountainous topography of the region makes it diffi cult for the land to retain water and thereby provide an adequate water source. Situated at more than 2500 meters above sea level, the eastern escarpment region is thickly covered with fog especially from January through March. Thus to combat the potable water shortage in the area, the Government of Eritrea, together with its partners in development has introduced a project for collecting water from the fog spread.
It is generally accepted that there is an ever-going need to identify new source and methods for collecting fresh water in both the developed and developing countries specially Arid and Semi Arid regions. Trees or other tall vegetation collect the droplets from fog. They can also be collected by appropriately designed man made collectors, to provide large volumes of water for domestic, agriculture or forestry
uses. Water collection from fog has been tried in several countries including Chile, with successful results.
The potential for extracting water from fog is a function of the amount of water it contains, how often it occurs, and wind speed.
The fog water content depends upon its altitude. Its frequency of occurrence depends on regional atmospheric circulation, the temperature of the ocean water and the stability and intensity of the thermal inversion processes. If the climatic phenomenon that produces the fog is stable, fog will form regularly; however, its behavior may vary from one area to another and specific seasonal variations may occur. In Eritrea pilot projects have been introduced in Nefasit and Arberobu with promising results.
According to studies conducted in 2005, in Arberobu, the maximum amount of water that could be collected from a 1meter sq of fog collector is estimated to be 13 liters per day.
Fog is basically defined as a cloud at ground level formed by microscopic droplets of water. The damp air mass mostly originates from water bodies like oceans and seas, and when intercepted by mountains it forms a fog. The water collected from fog can be used for drinking as well as irrigation. The basic process of water collection from fog includes selecting a conducive site, spreading meshes where the fog is trapped and the preparation and installation of gutters or PVC canals through which water is collected into a purifying reservoir. Since fog is carried through wind, prior study of wind direction plays a determinant role in the collection of fog. The fog collectors are fl at rectangular nets supported by a post at either and arranged perpendicular to the direction of the prevailing wind.
They can be simple units that made up of a series of collection panels joined together. The collection surface is a fi ne-mesh net made from a nylon material. As water collects on the net, droplets join to form larger drops that fall under gravity toward a trough or gutter located at the bottom of the panel.
If properly handled, these meshes can be used for more than 10 years. The collector itself is completely passive and water is pulled by gravity through the system to the site where it will be used.
Water in the incoming fog and from the fog collectors can be expected to be of good quality. It will contain some marine salts and soil dust but little contamination from anthropogenic sources given the remote locations of most proposed sites.
Although this type of water collection is limited to areas only where there is marked fog formation, the method and simple and the source reliable. Thus, if harvested at a larger scale, this method could play an important role in alleviating water supply shortages in the eastern escarpment area.
By Rigat Tesfahunegn

Source: Eritrea Profile, Saturday 6th of October, 2007