In this month's PISA episode we take a look at Luxembourg's tap water system, how the water from the Upper Sure Lake gets filtered, as well as how the 47-metre dam was erected in 1960.
RTL Today has teamed up withPISA, the Luxembourgish science magazine, to reproduce their original videos in English for our site. Watch all English videos onRTL Play, or discover the wide range of subjects previously covered in Luxembourgishhere(there are 13 seasons, mind you! We'll try and catch up).
Some of our previous episodes covered thehistory of the tram,Luxembourg airport, explained how thecoronavirus vaccine works, askedwhy traffic lights always seem to be red,looked atLuxembourg's railways since 1859, took a dive into theMoselle Valley and its underground Dolomite minesand outlined thehistory of Radio Luxembourg...
50% from the Upper Sure Lake
Within the last four kilometres of the Upper Sure Lake lies a protected water zone, which is used for tap water, extracted near the dam. No one is allowed to bathe in this area, of course. Half of the country's tap water is derived from this lake. But it's still quite a long journey from the lake to the tap!
So where does the water come from? A 1km pipe carries future tap water from the Upper Sure Lake to the pumping station on a hill in Eschdorf, which in fact lies 240m higher than the lake itself.The process of pumping water requires significant power, with the pumps boasting an output exceeding one megawatt.
The station has two identical pumping systems with redundancy in mind. Should one side fail, due to a mechanical issue, fire or flooding, the other half can continue to operate with at least 50% capacity.
Christian Schroeder of SEBES (Dam Water Union of Esch-sur-Sure) says: "Our booster pumps generate thenecessary pre-pressure when the water level in the Upper Sûre Lake is exceptionally low. They compensate for the lack of pre-pressure required for the main pumps to transport water to Eschdorf."
There is also a plan for a pump failure or power outage, with the water flowing back down the pipe into surge tanks, which contain a mixture of water and air and act as a buffer.Consequently, even in the event of a power failure, the pipes to Eschdorf remain fully operational and well-protected.
From lake water to clean tap water
The first step in filtration is flocculation. An iron solution is introduced into the water, causing the formation of "snowflakes" that enlarge solid particles, such as viruses, bacteria and small stones. These enlarged particles can then be separated from the water in the subsequent membrane filtration stage. In conducted tests, SEBES introduced a million viruses, and less than one escaped the filtration process. The enlarged particles of viruses and bacteria become trapped in tiny straws during ultrafiltration. Over time, organic substances form a biofilm inside the straws, necessitating periodic rinsing.
While the water is free from viruses and bacteria, dissolved substances like pesticides may still be present. These are eliminated using ozone and other filters, including activated carbon filters. Since the Upper Sûre Lake water contains few minerals, lime is added.
Lastly, the water undergoes disinfection. In the old plant, chlorine was used for this purpose, but the new plant uses UV light. "Chlorine remains in the distribution network, and you can still taste it down the line. However, UV light leaves no taste. We have tubes similar to neon lights that emit ultraviolet light, effectively killing anything that's still alive."
Quality checks are conducted regularly throughout the process, which is monitored continuously from the control room.
Constructing the lake and dam
Back in 1957, the valley where the Upper Sûre Lake now lies was still inhabited. However, due to the growing demand for drinking water in the country, the residents had to relocate. The state acquired over 400 hectares of land, felling trees, constructing bypass roads and bridges, and demolishing several houses and mills. Finally, a 47-metre-tall dam was erected, and in 1960, the Sûre Valley was submerged under water.
Currently, the dam is in good condition, and its movement is constantly monitored by measuring devices. Divers conduct comprehensive inspections every decade, with the next one scheduled for this year. Based on the results, a decision will be made whether to drain the Upper Sûre Lake or if underwater repairs with divers will suffice.
The two-day blockage of 1986
A tour for visitors will be available by the end of 2023, which will include showcasing the actual start of the water treatment process: A stainless-steel movable arm that is positioned in the lake, allowing SEBES to extract water from different depths. This is crucial because the water quality and temperature fluctuate throughout the year.
One problem is algae growth. During summer, algae accumulate near the lake's surface, while in autumn and winter, they settle at the lake's bottom.
In 1986, this led to the algae catastrophe. At that time, the movable arm did not yet exist, and the algae had obstructed the pipes in the lake. This disruption caused a two-day halt in the supply of drinking water. As a temporary solution, the municipalities in Luxembourg, which solely relied on Upper Sûre Lake water, received water by lorry. This incident serves as a sobering reminder of just how dependent we are on tap water.
And what a valuable resource the Upper Sûre Lake is. Not only can the Grand Duchy's residents go there to relax in the summer, but the country also gets most of its tap water from it all year round.
Aqua tunnel beneath Luxembourg City
For this episode, Olivier also spoke to the waterworks team from Luxembourg City to guide him through their aqua tunnel beneath the city centre. Over a century ago, people faced serious challenges in the quest for clean drinking water. No wonder that many drank beer instead in the Middle Ages, as it was considered cleaner.
The tunnel was completed in 1963, but its original purpose wasn't water supply. It actually houses a sewer that collects wastewater from a section of Luxembourg City, directing it to the Beggen sewage treatment plant.
On Place d'Armes, behind the bandstand, this plaque reminds passers-by that a well was dug here in 1741.
Grand Rue also had a similar fountain, known as the Red Well. Today, the fountain with the Hämmelsmarsch statue by artist Wil Lofy stands as a tribute to the water drawn from here.
