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The bottom of the lake near the chemical factory in Uetikon is contaminated with toxins. The canton cleans the floor with great effort.
The Canton of Zurich has big plans for the Gold Coast. Apartments are to be built on the site of the chemical factory in Uetikon, a park, a canton school for 1,500 pupils, commercial premises and a bike hall. Preparations for the conversion of the former industrial area are underway, in 2030, for example, high school students are to be taught directly on Lake Zurich.
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Over the next few years, the former chemical site in Uetikon will become a meeting place with parks, apartments and a school.
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Regardless of the plans on land, the canton has meanwhile built a large construction site on the lake. In a complex process, he cleans the bottom of the lake because the chemical factory’s fertilizer production has left its mark. Lead, arsenic and cadmium have been deposited because, before the era of sewage treatment plants, the contaminated wastewater was simply fed into Lake Zurich. Even small amounts of uranium have been found.
The poison at the bottom of Lake Zurich
These toxins are not dangerous to swimmers. The water is not polluted, the pollutants are several meters below the ground. But even if heavy metals such as lead or arsenic are bound in the sediments, they have to be removed, says Bettina Flury, who is in charge of the remediation of contaminated sites in Uetikon. “The pollutants are not good for flora and fauna. If we remove the sediments, it means upgrading the entire water body ecology.”
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A large part of the pollutant cleaning is done from a floating work platform.
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Divers clean the bottom of the lake on the shallow water banks, while a floating work platform is required in deeper waters. A complex process begins on this so-called pontoon, which will keep the Canton of Zurich busy for years to come. From there, a kind of vacuum cleaner sucks the bottom of the lake meter by meter, and the suction head transports the contaminated material to the surface.
Vacuum cleaner control as a computer game
Manuel Schneuwly developed the work pontoon and says that the suction work on a total area of ten football pitches in the lake is a challenge. “The suction head is sometimes 30 to 35 meters under water and has a lot of electronic drives in it. It was difficult for the machine to remain sealed even when it was carrying out its mechanical work.”
The vacuum cleaner is navigated from the work pontoon. A visualization is created using an echo sounder and cameras and the pump is controlled from the platform. “The visualization on the computer shows us where the pump is, what the floor looks like and how much we have to suck off there,” Schneuwly continues. More than two meters of rock are removed in places.
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The contaminated material is transported to the surface from the work pontoon, piped ashore and then cleaned there.
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The removed sludge-water mixture is then transported via a tube to the treatment plant on land. Coarse materials such as sand or gravel are first removed in huge containers, then the water is pressed out of the mixture. What remains is almost dry sludge, says site manager Roman Thurnherr: “We analyze this sludge and, depending on the values, it is stored or otherwise disposed of.” The cleaned water is returned to the lake.
High-tech on the water and a sediment treatment plant on land free Uetikon from the legacy of the chemical factory. 25 million Swiss francs have been budgeted for the work, which will take another two years. As the owner of Lake Zurich, the Canton of Zurich will bear 20 percent of the costs. However, Zeochem AG, the successor company to the former Chemie Uetikon AG that caused the problem, has to bear the lion’s share.