Fog harvesting could assist in dealing with water shortages in rural areas where there were no major surface- or groundwater resources and where it is not economically viable to install a water reticulation network, the Water Research Com-mission (WRC) has found.

Speaking at the Water Research and Development Technology Symposium in Pretoria, Science and Technology Minister Derek Hanekom stated that, while the South African government had achieved a great deal in terms of water service delivery, millions of people in rural communities still had to walk long distances to collect water, mostly from polluted streams.

“The lack of safe water has a profound impact on the health of our poor and vulnerable communities. Providing universal access to safe drinking water is a huge challenge, which requires innovative solutions,” the Minister said.

As fog occurred frequently along the west coast of South Africa, as well as in the mountains forming the southern and eastern escarpment, the WRC had funded research projects to determine the feasibility of fog water harvesting and to optimise the structure and design of fog water harvesting systems.

Fog water harvesting technologies had been piloted in Lepelfontein, in the Northern Cape, as well as at Tshanowa Primary School, in Limpopo, where there were no naturally occurring water sources.

“Water was previously trucked to Lepelfontein from the nearest town about 70 km away, while, at Tshanowa, women and children had to collect water on a daily basis from a dam located 5 km from the school. The terrain is exceptionally steep and rugged in this area. The amount of water available to each family was limited to the amount of water that could be carried per trip,” the WRC said.

Each of the fog harvesting systems installed comprised a water collection screen and one or more storage tanks. The screen consisted of three 6 m wooden poles mounted 9 m apart. Steel cables stretched horizontally between the poles, anchoring the structure, the WRC explained.

Two sections of 9 m × 4 m shade cloth netting were draped over the top cable and secured to the middle and lower cables and to the poles on either side. This formed a fog collection screen of around 70 m2. A gutter was attached to the lower ends of the screen.

“During foggy and wet conditions, droplets are blown against the screen and are deposited on it. As the drops become larger, they trickle downwards and drip into the gutter. From there, the water is channelled through a sand filter to a pipe that leads to 10 000 ℓ tanks located down-slope. When the top tank is full, the overflow is channelled to the next tank further down the hill,” the WRC said.

The design of the fog collector was based on that used in Chile, with significant modifications being made for South African conditions. The structures were specifically designed to be used in rural areas, to be as cost effective as possible, to use material that is readily available in the area and to be suitable for use in areas without electricity.

“The systems at Lepelfontein and Tshanowa were our first designs that were based on the Chile model. We have further developed new modifications and innovations. One modification is that, instead of having one flat vertical panel, we now put three panels of 30 m2 each in the form of a triangle. This provides stability to the system during storms,” WRC project leader University of Pretoria’s Professor Jana Olivier explained.

The results obtained at the two sites were similar; the mean daily water yields were about 4 ℓ/m2 a day, with maximum yields exceeding 3 800 ℓ a day. Although these yields were not very high, they did provide clean water for people who previously did not have easy access to water, Olivier said.

Meanwhile, two WRC-funded fog harvesting innovations were also being showcased at the Water Research and Development Technology Symposium.

The first innovation was a measuring instrument for low and intermittent water flow, which was needed as the volume of fog varied from day to day, while water collection was also intermittent, with a lot of water collected during wet conditions to none during sunny periods.

University of Pretoria’s Professor Johan van Heerden had built a low-flow meter (LFM) that could measure such flows. It was based on a tipping bucket principle that could measure 1 ℓ to 1.5 ℓ a tip. The LFM was made from perspex to enable easy identification of blockages. The system had been tested over the past two years under field conditions and was found to be robust and accurate.

The second innovation to be showcased – the Whirly – was an apparatus that could be used to harvest fog water during near-windless conditions.

“The previous systems – and those used worldwide – are static nets that rely on fog-bearing winds to blow through the system, depositing tiny fog droplets of water on the screen.

“The new invention consists of a vertical shaft with three nets attached. An electronic system has been attached to turn a rotor, thereby rotating the nets when the relative humidity reaches 98%, as it does during foggy conditions. The system switches off when the fog ceases. The batteries that drive the system are charged by means of solar power. This system allows the capture of water during near-windless conditions, a situation that often occurs on the west coast,” the WRC explained.