FOG CATCHMENT

Information from the 2008 UNISA Research Report:

ANYONE FOR A GLASS OF FRESH FOG? ALTERNATIVE WATER SOURCES FOR SOUTH AFRICA
Clean water is at a premium throughout southern Africa. With approximately 1 700 m3 water available per person per year, South Africa is classified as a waters-tressed country. Should the present trends of use and abuse of water continue, the country will move into the water-scarce category by 2025. There is thus an urgent need not only to conserve the water that is available, but also to identify alternative sources of potable water. Fog is one such a source of water and UNISA is engaged with a large scale research and development project on fog harvesting.

ACCORDING TO CLIMATOLOGIST Prof. Jana Olivier, associate professor in the School of Environmental Sciences in Unisa’s College of Agriculture and Environmental Sciences, and project leader of the collaborative research project on fog harvesting, “Fog is an often underestimated source of water, even though it is the major source of water for many desert plants and animals, for example a beetle in the namib that collects fog on its hind legs by doing a ‘hand-stand’ to lift its hind legs into the air. The water droplets collected on the beetle’s legs and back then run downward to its mouth.” This description can be adequately used to describe the fog water collection system designed by the project team, consisting of Prof. Olivier and Prof. Johan van Heerden (emeritus professor at the University of Pretoria).

Fog water collection: a natural process. Fog contributes significantly to the hydrological balance in forested areas by collecting on vegetation and dripping to the ground. By imitating this natural process, water can be collected for human applications, ranging from drinking water to the establishment of woodlots, vegetable gardens and livestock management.

Fog and cloud are composed of minute droplets suspended in air. These droplets are initially very small ranging from a few microns to 100 microns in diameter depending upon the type of nuclei on which they are condensed. They are so small that they can easily be kept in suspension by air currents. However, when they collide with a solid surface, the droplets are deposited on the surface. As more and more accumulate, they grow until a larger drop is formed. Fog harvesting (or cloud water collection) is concerned with the collection of these drops.

The first significant experiment to collect drinking water from fog was conducted in 1995, when the Water Research Commission funded a project to determine the feasibility of using fog to supplement water sources in rural areas. Based on the results of this project, two projects were initiated to implement operational fog water systems in rural areas and to supply communities with water.

Early experimental systems: The first of these fog water collection systems was set up at the Tshanowa Junior Primary School in the Soutpansberg in Venda, and the second at a small mission village called lepelfontein on the West Coast. yields of around 5 l per m2 per day were obtained at both these experimental sites, with maximum daily yields reaching 3 800 l per day at each.

Additional systems were subsequently set up near Tshanowa, at lepelfontein, as well as in the Transkei and near langebaan in the Western Cape. The latest fog harvesting system was set up at Cabazana in the Eastern Cape in April 2008.

The cloud water collection systems have been designed to allow the small fog droplets to pass through a mesh. During this passage the flow is disturbed and a small percentage of the droplets collide with the mesh material and are collected. The water quality of fog is very high and conforms to World Health Organisation and South African water quality standards for potable water.

Advantages of fog harvesting: Some of the advantages of fog harvesting are:

• Both rainfall and fog droplets are collected, which makes it especially effective during a light drizzle. • Foggy conditions occur throughout the year and not only during the rainy season.

• Fog is a sustainable source of water, even in times of drought.

• Fog harvesting is ideal in mountainous areas where there are no other sources of water, where there are logistical difficulties, or where the costs are too high.

• The system can be applied in sparsely populated areas.

• Environmentally speaking, the impact of the systems is minimal during construction, the system does not influence the amount of water available to the surrounding vegetation, and it does not require electricity.

• Fog/mist is exceptionally clean and does not contain harmful micro-organisms.

Fog water harvesting can thus be an important source of water in those areas where other, more conventional systems, cannot be used – specifically in the mountainous areas of the country. Ideally an integrated water management scheme should be implemented in these areas, using a combination of fog water collection as well as rainfall harvesting from roofs and other structures.

Structure of the fog harvesting system: The original systems comprised of a 70 m2 collecting surface consisting of a double layer of 30% shade cloth net. This was attached to wooden poles, anchored by means of steel cables with a gutter attached to the lower end of the net. Water droplets that collect on the screen run downwards and drip into the gutter from where they are channelled via pipes to storage tanks. The system was specially designed to be used in rural areas, to be as cost effective as possible, to use material that was readily available in the area, and to be suitable for use in areas with no electricity.

Extensive damage was caused to the Transkei systems, primarily due to poor maintenance, vandalism and freak storms that ravaged the area during 2006 to 2007. It was also found that the shade cloth screens frayed and tore due to continuous rubbing against the steel cables during windy conditions. This necessitated the redesigning of the materials and structure.

The following two modifications were therefore introduced:

• Instead of a single flat collector, a system consisting of three 40 m2 panels arranged in the shape of an equilateral triangle is now being used. A number of these systems can be linked together to expand the system to the required size. This design lends stability to the system so as to withstand the onslaught of gale-force winds.

• The shade cloth has also been replaced with a stronger, stainless steel-based mesh. Numerous experiments were conducted to compare the efficacy of the stainless steel mesh with that of shade cloth. It was found that the water collection efficacy of the steel mesh is satisfactory.