Tomorrow Today
Nature's Water Treatment Plant -- Scrubbers on the Lake Bank
Most river water is not drinkable, but that can change. Filtering wells along the banks of rivers and lakes have long been used to supply drinking water. They fill up with water which has passed through layers of sand and sediment along the banks - which serves to "clean" impurities from the water. But scientists say there's more to it than soil composition.
A lake on the outskirts of the German capital, here an important research project about purifying
Bildunterschrift: Großansicht des Bildes mit der Bildunterschrift: Berlin drinking water by means of what's called 'bank filtration' is coming to an end.
For the past three years, limnologist Günter Gunkel has been investigating the sediment at the bottom of Tegel Lake. Some of the city's drinking water comes from right here.....
Bildunterschrift: Großansicht des Bildes mit der Bildunterschrift: New extraction wells are currently being built on the north bank of the lake. Such wells have been dug in Berlin since 1875 - mainly on the banks of lakes and rivers. This is one of the 900 extraction wells used by the city's waterworks. Very high quality water has been found at 40 meters depths for over a century.
Bildunterschrift: Großansicht des Bildes mit der Bildunterschrift: The water from the extraction wells that goes to the waterworks has made a long journey. First it passes through the fine-grained sand on the banks where impurites are filtered out as the water moves through the soil.
Up to now, Berlin has relied on this natural cleaning process. But newer substances like medications or algal toxins have prompted experts to take a closer look. Whats important now is to find out exactly where the decisive processes take place, Günter Gunkel says. Which processes are important -- the biological processes or the chemical processes?
Bildunterschrift: Großansicht des Bildes mit der Bildunterschrift: Until now, Berlin's water from the depths has been so clean that it's only required a little aeration at the waterworks - mainly to precipitate the iron and manganese it contains. Then it's put through gravel filters. Many cities would give a great deal to be able to clean their water so easily. Bank filtration works best when the deep layers of sediment are fine-grained sand.
To find out how the water actually being cleaned down there Günter Gunkel collects fine-grained sediment from the bottom of Lake Tegel. 
Bildunterschrift: Großansicht des Bildes mit der Bildunterschrift: What is it about this simple sand that allows it to do such a fantastic cleaning job?
Under the microscope, the inland water expert and his team have discovered entire specialized communities of algae, fungi and bacteria. There are billions of the organisms in a single cubic millimeter, all of them removing contaminants from the water. And they are constantly renewing themselves - they reproduce and are eaten by crustaceans and other sediment dwellers. This makes for a highly efficient purification system that never gets clogged, and is confined to the upper sandy layers.
The monitoring lab at the Berlin waterworks investigates around 200 drinking water samples every day. Again and again, the results show how thoroughly the microorganisms among the grains of sand do their work. Investigations carried out at a Berlin water research center have shown that even normally hard-to-filter substances like medicinal residues, hormones and toxins produced by algae were broken down by the sand layers.
Bildunterschrift: Großansicht des Bildes mit der Bildunterschrift: Using a scanning electron microscope, Günter Gunkel was able to analyze the grains' surfaces. He discovered that the microorganisms form delicate mesh-like structures. These function both as living spaces and as nets that capture some of the contaminants. The surface of a grain of sand is a fascinating microcosm. Does it have applications for the future of water supplies?
Global climate change is bringing enormous changes to inland waters. Temperatures in Berlin's lakes have risen 2 to 4 degrees over the past few years. Researchers are hoping that this century-old purification method can be adapted to meet future challenges.
