"Water is an important but not an infinite resource," warns Dorothea Mergel, a project engineer at Hamburg's water company, adding that water is becoming increasingly scarce both in Germany and around the world.

Mergel is working to make sure that the drinking water supply in Germany's second-largest city is sustainable. One idea is a pilot project in which "gray water" - wastewater from our showers and kitchen sinks - is used to flush toilets and water gardens, thus saving precious drinking water.

In Tunisia, every drop counts - especially in agriculture. The country has a high proportion of desert, with temperatures reaching up to 45 degrees Celsius in summer.

Agricultural scientist Dr. Bellachheb Chahbani has developed a method of traditional irrigation that reduces water use by 70%. Dr. Chahbani visits olive farmers in the south of the country, helping to set up the water-saving system.

Industry has the potential to save enormous amounts of water, too. Water conservationist Andreas Kirketerp works at the Carlsberg Brewery in Fredericia, Denmark.

His innovative system recycles the water used to clean bottles, cans, equipment, and tanks, making it usable again. The saving amounts to 1,800 cubic meters of water every day - or the daily water consumption of around 14,000 people. The effect on the local drinking water supply has been hugely positive.