Clouds on tap
Fog collection is an innovative solution to the problem of water shortage. Researchers have been quick to latch on to it, but many countries have been surprisingly slow to implement it.
The Namib fog beetle is a feisty little creature. Every morning he makes an arduous journey to the top of a sand dune, where he turns his body into the wind, straightens out his rear legs and lowers his head. The fog rolling in from the sea gradually collects on his back, forming droplets of water, which glide downwards and hang from the insect’s mouthparts. In this way, the fog beetle is always assured of a healthy morning drink, despite being kilometres away from the nearest fresh water.
Humans have been tapping fog water in similar ways since ancient times. The inhabitants of what is now Israel, for instance, used to build small, low, circular honeycombed walls around their vines, so that mist and dew could precipitate in the immediate vicinity of the plants.
Historically, in the Atacama Desert in western Chile, both dew and fog were collected by means of a pile of stones, arranged so that the condensation would drip to the inside of the base of the pile, where it was shielded from the day’s sunshine. The same technique was employed in Egypt, with the collected water stored underground in aqueducts.
The first fog-collection installation in South Africa was at Mariepskop in Mpumalanga, in 1969/70. It was used as an interim measure to supply water to the South African Air Force personnel manning the Mariepskop radar station. Two large fog screens, constructed from plastic mesh and measuring about 28m x 3,5m each, were erected at right angles to each other and to the fog and cloud-bearing winds. These yielded more than 11 litres of water per square metre of collecting surface each day. Unfortunately, the project was terminated once an alternative water source was found.
Fog collection is an innovative solution to the problem of water shortage — one that researchers have been quick to latch on to, but which many countries have been surprisingly slow to implement.
Professor Jana Olivier, of the University of South Africa’s department of anthropology, archaeology, geography and environmental studies, explains that the idea of harnessing fog as a source of drinking water has been studied for decades. “The first experiments were conducted in 1901, on Table Mountain. But it was only in 1987, in the arid coastal desert of northern Chile, that it was implemented on a large scale.”
For years the remote Chilean fishing village of Chungungo relied solely on trucked-in water. In 1987 it was transformed by the installation of a fog-collecting system. With a dependable and affordable water supply, not only did the growing population have domestic water, they were also able to cultivate commercial crops and plant trees.
Although unconventional, the technology behind fog collection is amazingly simple: massive vertical shade nets are erected in high-lying areas close to water-short communities. As fog blows through these structures, tiny water droplets are deposited on to the net. As the droplets become larger, they run down the net into gutters attached at the bottom. From there, water is channelled into reservoirs, and then to individual homes.
In Chungungo, this system saw water flowing from local taps for the first time ever in 1992, providing more than 40 litres of water per person per day.
Like Chile, South Africa is an arid country in which large sections of the population have inadequate water supply. Only 35% of the country gets more than 500mm of annual rain and, with few unpolluted surface-water sources, many contaminated ground-water supplies and water tables that drop out of reach during drought, the advantages of an effective alternative water source are obvious.
Olivier, who has been involved in fog collection research since 1995, says the potential for fog collection in South Africa is clearly shown by what has already been achieved at two fully operational sites — one in Limpopo province and the other on the West Coast.
Water for thought
Tshanowa Junior Primary School in the Soutpansberg, in Limpopo province, is frequently shrouded in dense mist and rain, but the nearest water sources are a non-perennial spring located 2km away, and a dam 5km away. Since most water sources in the province are contaminated, the quality of the dam water is suspect. The 130 school children rely on what water they can carry with them to school each day.
The school is located at the crest of one of the easternmost promontories of the Soutpansberg, at 1 004m above sea level. Despite its relatively low elevation, this region is ideal for fog collection because moist maritime air from the Indian Ocean moves over the escarpment and against the mountains during the night and early morning. This cloudiness sometimes persists throughout the day.
Permission was obtained from the relevant local and tribal leaders to erect a fog water-collection system on vacant land adjacent to the school. Construction started in 1999 and local inhabitants were employed to assist.
Each fog collector consists of three 6m-high wooden poles, mounted 9m apart. Steel cables stretch horizontally between the poles, and from each pole to the ground. A double layer of 30% shade cloth is draped over the cables and fixed to the poles on each side. Water dripping from the net into the gutter runs through a sand filter and is then emptied into a tipping bucket. From there, it flows into a 10kl storage tank further down the slope.
Two additional tanks were erected at the school to collect the overflow from the first tank. An automatic weather station was also installed to record rainfall, wind speed and wind direction. Within four days of completion, school children and members of the local community were drinking water collected by the fog screen. Although weather conditions have made accurate data collection difficult, daily yields of as much as 3 800 litres of rain and fog combined have been recorded. The average collection rate from March 1999 to April 2001 is more than 2,5 litres per square metre of fog screen.
Heavy clouds, no rain
The same system was also set up at Lepelfontein, a small missionary station about 400km from Cape Town and about 5km inland of the West Coast. Although ground water here is abundant, it is of such bad quality that it is considered a health risk. A small solar distillation plant was installed in 1998 to provide limited drinking water, but most water is still transported to the village from elsewhere.
The fog screens were installed in 1999, and the overflow from one of the 10kl tanks is now being used to supplement the water from the desalination plant. At least 80% of the water collected at this site is from fog alone, as the region receives very little rain. Fog conditions are mostly associated with onshore breezes originating either from the South Atlantic anticyclone to the south of the continent, or from north-westerly and westerly winds on the northern perimeter of a coastal low.
Again, daily yields of more than 3 000 litres have been recorded, with a daily average of about 5 litres of water collected per square metre of fog screen.
While Lepelfontein’s water initially showed high levels of sodium — possibly due to the proximity of the ocean and wind-blown spray — Olivier says water quality at both sites is good, with no disease-forming organisms present in samples. “In fact, at Tshanowa, water was rated as Class 0 — ideal quality,” she says. “Since the water is used for drinking purposes, quality is tested regularly.”
Virtually all the materials needed to construct, operate and maintain the system are available locally. Yet fog harvesting is a rarely used technique in Africa. Despite education and information programmes initiated by the fog-collection project team, there seems to be little awareness of it, and even less support in terms of corporate funding or involvement.
But the potential is vast. Olivier points out that experiments conducted at other high-elevation sites around the country have yielded more than 10 litres per square metre of collecting surface per day. “This shows that in terms of quality and magnitude of yield, fog harvesting could go a long way to alleviating water shortage problems in the fog-prone mountainous regions of the country. The costs are low, the technology is simple and the source is sustainable for hundreds, even thousands of years.”