[Image: Fog Harvesting / LinkedIn]
The City of Cape Town has shared an update on what is happening with its fog harvesting plans, which were first initiated amid the panic of the City and province’s water crisis.
The project aims to investigate fog harvesting as a means to augment drinking water supply.
The principle of fog harvesting is based on the fact that air contains a certain amount of water vapour originating from ocean evaporation. At higher temperatures, air contains a larger volume of water vapour than at lower temperatures. As air is transported up a gradient by wind, it interacts with colder air and is condensed into fog.
Various studies have been conducted on fog harvesting for domestic water supply purposes elsewhere. Most of the studies involved the use of a polymer mesh or shade cloth, kept in an upright position by stainless steel poles, that collects droplets as the wind blows the fog through the mesh. Droplets then trickle into a collection chamber.
“The technique has been used for decades in dry and isolated regions, such as in Chile, Peru, and Spain, though mostly managed by research/academic institutions as opposed to a municipal scale.”
According to Campbell Scientific, the first official studies concerning moisture density on Table Mountain were conducted over a century ago by German-born botanist Rudolf Marloth. Studies conducted since then have measured yearly fog precipitation at approximately 3,294 mm (129.7 in.), which is triple the average yearly rainfall recorded in Cape Town.
The mountain’s famous “tablecloth”—scientifically referred to as an orographic cloud formation—is responsible for much of the high-altitude moisture and optimism surrounding the fog harvesting program.
Mayco Member for Water and Sanitation, Zahid Badroodien, said two sites were identified for the project.
Site 1 was located at 714 metres above sea level near the Woodhead Reservoir Conference Room, which commenced in 2018. Site 2 was situated 1,000 meters above sea level near the Table Mountain cable car station, which began in 2020.
The City of Cape Town tested various available material options for the screens. One screen is made from 40% shade cloth, while the other uses 316 stainless steel. As fog condensates on the surface of the screen, moisture accumulates on the surface and gathers in the gutter below. The droplets are fed through a tipping-bucket rain gauge. Each drop is equivalent to 0.649 mL (0.022 oz) of water.
Site 1 was decommissioned in 2023 due to low water yields and logistical communication challenges.
“The continuation of Site 2 will depend on the outcomes of this review, including the consistency of water yields and the practical feasibility of expanding the system.”
“The City is actively investing in its New Water Programme to diversify its water sources and build a resilient supply so that we can reduce our dependence on unpredictable rain-fed dams and navigate future climate change shocks.”
Badroodien concluded that the City is committed to investing annually in its water and sanitation infrastructure to ensure that residents continue to benefit from reliable services.
[Source: City of Cape Town & IOL]