Membrane distillation plants provide water for the world's poorest

Lack of water causes great distress among the population in large parts of Africa and Asia. Small decentralized water treatment plants with an autonomous power supply can help solve the problem: They transform salty seawater or brackish water into pure drinking water.

Large industrial plants for the desalination of seawater deliver 50 million cubic meters of fresh water every day – particularly in the coastal cities of the Middle East. However, the technology is complex and consumes large amounts of energy. It is not suitable for the arid and semiarid regions of Africa and India, though these are the very places where it is becoming increasingly difficult to supply drinking water, particularly in rural areas. “The regions have a very poor infrastructure. Quite often there is no electricity grid, so conventional desalination plants are out of the question,” states Joachim Koschikowski of the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg. In various EU-funded projects over the past few years, he and his team have developed small, decentralized water desalination plants that produce fresh drinking water with their own independent solar power supply.

“Our plants work on the principle of membrane distillation,” explains Koschikowski. This can best be explained by the principle of a Gore-Tex jacket, in which the membrane prevents rainwater from penetrating through to the skin. At the same time, water vapor formed inside the jacket by perspiration is passed through to the outside. “In our plant, the salty water is heated up and guided along a micro-porous, water-repellent membrane. Cold drinking water flows along the other side of the membrane. The steam pressure gradient resulting from the temperature difference causes part of the salt water to evaporate and pass through the membrane. The salt is left behind, and the water vapor condenses as it cools on the other side. It leaves us with clean, germ-free water,” says Koschikowski.

The researchers have so far built two different systems, both with their own energy supply. “Our compact system for about 120 liters of fresh water per day consists of six square meters of thermal solar collectors, a small photovoltaic module to power a pump, and the desalination module itself,” explains Koschikowski. In the dual-circuit system, on the other hand, several desalination modules are connected in parallel, enabling several cubic meters of water to be treated every day. One cubic meter of drinking water – 1000 liters – will cost about 10 euros. “When you think how much the inhabitants currently have to pay for the same amount of bottled water or soft drinks, the plant will pay off very quickly,” claims Koschikowski. The test plants in Gran Canaria and in Jordan have been operating successfully for some time. The researchers are therefore planning to market the plants through a spin-off known as “SolarSpring” from the middle of this year.