Lake Schäfersee in Berlin, once choked by sludge and stench, is now thriving thanks to a surprising mix of oxygen, calcium nitrate, and smart engineering.
00:01It might look like a leisurely outing, boating on the Schaefersee Lake in Berlin.
00:07But for Hartmut Wassmann and Roman Klems, it's all business.
00:11The duo are from a small engineering firm that wants to examine the condition of the water.
00:16To do that, they're collecting samples from more than six meters deep.
00:22Smell anything? Nothing.
00:25Absolutely nothing. Nothing at all. Great. Then it worked.
00:31They've been taking care of this lake for over ten years.
00:35Tucked away in the north of the German capital, it's just 4.5 hectares in size.
00:40But the lake's catchment area, the surface from which rainwater flows unfiltered into the lake, is over 50 times bigger.
00:50Hartmut Wassmann recalls the day that they first examined the lake.
00:56We've worked with a lot of lakes, but we've never seen one this bad.
01:00That spring, after the ice melted, we took our first samples and found fish scales in the deep water.
01:06There must have been a massive die-off under the ice.
01:09We'd never seen anything like it.
01:12When leaves and other organic material get washed into the lake by rain, bacteria starts breaking it all down.
01:18That process uses up oxygen.
01:22Once it's depleted, other gases form, like methane and hydrogen sulfide.
01:27That creates a layer of foul-smelling sludge.
01:30The water starts to stink.
01:32It's a problem that many cities around the world are dealing with.
01:36Just last year, the lake in Roshanarabagh, a historical park in Delhi, was dredged at a cost of over 1 million euros.
01:47Now it's back to smelling foul again, and it's not an isolated case.
01:51Back in Berlin, the water in Schäfesee is roughly cleaned, and then released back into the lake.
02:01To improve the water quality, they're treating it with calcium nitrate.
02:07The idea of using calcium nitrate to fight odours isn't new, and applying it like this in a body of water is also not unheard of.
02:20But combining it with oxygen, that actually came about more or less by chance, and it had a huge impact.
02:27Schäfesee was the first place we really saw the full positive effects.
02:31For over a decade, their small company has treated the lake using oxygen pumps and calcium nitrate.
02:43Large plastic containers are hauled to the lake's centre several times a year.
02:48They pump the chemical into the deepest areas over the course of a week.
02:52But calcium nitrate has a bad reputation.
02:56Frequently used in industrial livestock farming, it's known as a water pollutant.
03:00So how can it help the lake to recover?
03:04We use very low concentrations in our process, well below the legal limits for drinking water.
03:11And the nitrate is simply converted into nitrogen gas by bacteria.
03:19The company digitally monitors all the lake's data, and they're tracking information from other lakes they're also treating.
03:25Along the way they noticed something else, the phosphorus levels in the lake were dropping.
03:30And phosphorus is the nutrient that fuels dangerous algae blooms.
03:35So they brought in Professor Thomas Neumann, a geochemist from the Technical University of Berlin.
03:41He and his team studied sediment samples from Schaefer See.
03:49They found that calcium nitrate was helping to bind the phosphorus and trap it in the sediment.
03:57That's good news for the lake, because it has too many nutrients, especially phosphate, and that causes problems.
04:10If there's a way to lock that phosphate into the sediment for years, even decades, that's a great outcome.
04:21But that still means they're only treating the symptoms here at Schaefer See.
04:27What would help most is cleaning the rainwater before it enters the lake.
04:32But a treatment plant takes space, something that's hard to come by in a built-up city.
04:36About 15 kilometres away, next to Berlin's city highway, there is space, right by Hallensee, a small lake with a public swimming area.
04:49Here, rainwater that runs off the highway gets clean before it reaches the lake.
04:55Every month, a ton of sludge is filtered out. The rest is taken care of by a reed bed.
05:06These are fine particles, and they carry germs like Enterococci and E. coli.
05:14Not great if you want good water quality for swimming.
05:18The water we return is safe to swim in, because we remove the germs, we remove the phosphorus, and the water flows crystal clear into the lake.
05:31Berlin only has two of these treatment facilities at the moment.
05:35That's why the city still struggles with oxygen-poor lakes.
05:40Which is also why more and more lakes are being treated with calcium nitrate.
05:45Urban lakes face all sorts of issues, and each one requires a different treatment.
05:51You have to match the method to the problem.
05:55We're kind of like doctors for lakes.
05:58First comes the examination, then the diagnosis, then the treatment.
06:04For small, shallow, pond-like urban lakes, the Schaefersee method is a good fit.
06:10The nitrate breaks down the organic matter and triggers the lake's natural self-cleaning processes.
06:16For Roman Klems and Hartmut Wassmann, the work at Schaefersee is far from over.
06:27And as well as looking after a number of other lakes across Germany, they're even getting inquiries from abroad.
06:33And Schaefersee, where it all started, has lent its name to their process, the Schaefersee method.
06:38Whatever it's called, people around here are just happy that what used to be a stinky swamp is back to being a real city lake that people can enjoy.
