What does evaporation have to do with spa towns?
As you might have noticed by the lack of an experiment last week, I’ve been on a vacation and enjoyed some time away from work, Instagram and my blog. But I can tell you – my mind never wanders far from fascinating physical phenomena and so when I went on a stroll in the region I grew up with, I came across the graduation towers and enjoyed the fresh breeze filled with salty smell just like the ocean. We used to go there often when I was a kid and I really enjoyed talking a walk there. They don’t have anything to do with a school graduation but are used for increasing the concentration of saline solutions. And that’s exactly what I want to talk about today, so this will be a more theoretical blogpost with a little chemistry and physics backgrounds. Enjoy.
When you think of Germany, salt evaporation ponds probably do not come to mind. And for a good reason. The Baltic Sea does not have a high concentration of salt in it and the climate at the North Sea barely makes for good evaporation ponds. But salt extraction can also happen from brine, water with high salinity. And from that we do have some springs in Germany, which we historically came to use with these graduation towers.
These graduation towers are made with a wooden structure filled with brushwood. The water then runs down the brushwood twigs and with sun and wind, some of the water evaporates. This is the most interesting part, so let’s talk about that in more detail.
Process of evaporation
The salty water that drips down the twigs of the tower does not do so in a steady stream but in droplets with less volume and therefore a bigger surface that is exposed to the wind and the sun. So you probably don’t have a hard time imagining, that evaporation does happen way faster than on a pond in the same conditions.
You can do a little experiment with regular water yourself: pour some water on a clear surface and at the same time, spray some water on the same surface in drops. If you observe the water for some time (or come back to it later as evaporation takes time) you can see how the sprayed water evaporated more quickly than the little pond. And it’s exactly the same with the graduation towers.
Increasing the salt concentration
When I first thought about the evaporation process at the graduation towers, I was very confused about the increase of the salt concentration at the end. I thought about solar desalination, which I had heard of (and tried out) as a kid. Catching the evaporated water with a cloth leaves you with fresh water to drink while the salt stays where you poured the water onto in the first place. And that was exactly what I thought about, because all I could think of was the water that evaporated. My mistake was in not thinking about the saline water that kept pouring over the twigs into the pools at the bottom of the towers. And this actually is where the magic happens.
So let’s say you have 100 liters of salty water and the salt concentration is at about 10% (10 liters of the volume is salt). Now let’s pour this water over some brushwood twigs and let physics do its magic and some of the water evaporates. Not all of it (as I originally thought of) but just about 10 liters. The rest of the water still washes over the twigs and into the pool at the bottom. What you now have is 90 liters of salty water with the same amount of salt in it than before! After that evaporation process, you are now left with 10 liters of salt on 90 liters of water in total. Which makes a concentration of about 11%. And there you have it – a higher concentration.
Ultimately, I have no clue how much water really evaporates and how much runs over the twigs. Thinking about my numbers example really helped me to understand how increasing the salt concentration works with the graduation towers.
Today, graduation towers are rarely used for salt extraction anymore. Especially in my home region, they function as spa facilities, as the salt water drops are really good for breathing. Especially for people with asthma or allergies, the air around the graduation towers is a treat and really helpful against the symptoms. When I was a kid, I used to have bronchitis a lot. Which is why my parents took us to the parks a lot and we enjoyed the fresh air and ocean-ly smell. I still like to visit the parks and wander around the tall graduation towers, enjoying the small ocean breeze on the mainland of Germany.