Two Free State teenagers have found a way to generate water for agricultural use from cow dung.

Mookgo Mofokeng and Reatlehile Sekaja, both 17, who are grade 12 pupils at Beacon Secondary School in Qwaqwa, came up with the idea while in grade 10 in 2016.

Now they have been awarded a gold medal at this year's Eskom Expo for young scientist international science fair for their innovation.

The expo took place in Boksburg, Ekurhuleni, yesterday.

The pupils' idea was inspired by the scarcity of water in their area which mostly affected farmers.

Sekaja, who wants to study hydrology or mechanical engineering next year, said they discovered that it took about 13-million litres of water to breed one cow.

"Our focus was to come up with ways to get water from the cows which will be used to irrigate crops," said Sekaja. "We had to come up with the means to extract water from cow dung.

"We designed a dewatering [removing water from waste material] machine to extract water from cow dung. We used pipes, a tap, a tin and a blade as a shaft and a wire for [our] pulling system."

The two were among 500 Africa's top future engineers, chemists, physicists, mathematicians and innovators competing in SA's largest school-level science fair for prizes worth more than R8.5m.

Mofokeng said the process of extracting water involved pouring cow dung inside the pipes and stirring for about 30 minutes.

Then water will come out of the tap while the by-product will come out from another pipe.

For 1kg of cow dung they produce about 800ml of water. "This water is also very good for plants as they grow much faster because it acts as a fertiliser. The water can still go for purification."

Percy Shibambu, 21, from Sebitja Secondary School in Zebediela, Limpopo, impressed the judges with his 3D power station that reduces carbon dioxide.

The idea was born out of pain when he lost his father to asthma in 2014.

"I wanted something to decrease carbon dioxide because people are dying; I could not afford to lose any other member of my family," Shibambu said.

"This [3D power station] project will use sources which are less harmful to human health and also save the environment in the reduction of ozone-depletion."

Shibambu explained that he relied on cardboard boxes, water paint, bulbs and solar to invent the power station.

Eskom Expo executive director Parthy Chetty said the expo was a platform for future scientists and engineers to establish a base of their future careers.

"The competition is a great launch pad for motivated youngsters keen to explore these fields and change not only their circumstances but their environments for the better," Chetty said.