WaLu – Producing Drinking Water from Air Humidity

In regions with a dry (arid) or mainly dry (semi-arid) climate the production of drinking water is an existential problem. On average, more water evaporates here than is compensated as a result of precipitation. The ground is therefore dried out and the generally salty groundwater is often only found at great depths. Additionally, in many cases the groundwater level is steadily dropping or so-called fossil, non-renewable aquifers are used. A sustainable production of drinking water from groundwater that can also be used for future generations is thus not possible. The use of surface waters is also difficult in these regions, especially at a great distance from the sea.

Even though there is a shortage of surface or groundwater in arid regions, in often considerable quantities of water are to be found in the air (see example in the box). Moreover, as a result of global warming it is to be expected that the water content of the atmosphere will increase further because of the rising temperatures.

So that this water resource can be developed as a source of drinking water, the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB is working in cooperation with the Institute for Interfacial Engineering IGVT of the University of Stuttgart as well as three medium-sized industrial partners on a new process concept. The entire process consists of two parts. First, the humidity from the air is absorbed by a highly concentrated saline solution (brine) and thus bound. Then this diluted saline solution is distilled and the water separated from the saline solution is condensed as drinking water (desorption).

In order to make the absorption of the air humidity in the saline solution as efficient as possible, a large interface with the air and a long contact time are necessary. This is done by allowing the saline solution to flow slowly down sorption strings in tower-shaped, naturally ventilated plant modules and to absorb the water from the air. By means of a special design of the sorption strings an efficient mass transfer is achieved and the saline solution is diluted by the substantial absorption of water.

The water has to be separated from the circulating saline solution (desorbed), and so a distillation process follows. The distillation is effected by means of gravity-assisted, multiple-stage vacuum evaporation. To do this, the saline solution, diluted with water, is subjected to a vacuum, which considerably reduces the evaporation temperatures. The advantage of this is that these temperatures can be achieved with simple solar thermal collectors or also with waste heat. Since the plant works with a negative pressure, it is also possible to use the thermal energy employed several times in various evaporation stages with different pressures. The water vapor produced in the distillation is condensed and can be used as high-quality drinking water.

A combined tower-construction for the sorption and desorption makes it possible to use the gravity of the process flows to create the required vacuum. Energy-intensive vacuum pumps are no longer needed.

The method described enables a sustainable production of drinking water from air humidity in decentralized and independent facilities. This is especially important in the arid or semi-arid regions where the building density is loose and the infrastructure poorly developed. The various system components display very good synergies. Both the gravity-assisted vacuum evaporation and the absorption are designed for energy efficiency and the careful use of resources. The energy supply can be effected purely by renewable energy sources. In such cases, the electric components such as pumps and process control are supplied by photovoltaics or by wind power. The thermal energy required is provided by solar thermal collectors. The plant does not produce any wastewater or salt concentrate that has to be disposed of, as in the case of the desalination of sea or brackish water. There is 100 percent circulation of the sorption medium. As a result of the combination with a renewable energy supply, the technology is CO₂ neutral and does not cause any emissions. The technology is robust, without any demanding requirements as regards operation and maintenance; it can be used universally and is completely self-supporting.