Tons of contaminated sediment lie in the lake in front of the former chemical factory. Now the material is removed at great expense.
Uetikon may be a lake community, but the residents don’t get much from the lakefront: the area of the former chemical factory stretches along almost the entire length of the shore. It was used industrially for almost 200 years, but production has now been shut down. The factory buildings, many of which are nationally protected, characterize the townscape.
The canton and municipality have ambitious plans for the fillet property. Apartments, a high school for 1,500 pupils, commercial premises and a public park are planned for the next few years on the almost 66,000 square meter site. Parts of the area are currently being used temporarily: in the former factory buildings, for example, a bike hall, a coffee roastery, a clothes shop and the “Visible” (warning, pun!) are rented.
The deposits from the bottom of the lake are transported to land in a floating hose.
Not much is happening on the site on this Monday morning. Elaborate work has been underway since the beginning of May: Contaminated sediments have to be removed from around 75,000 square meters along the shore; this corresponds to an area of ten football pitches. Investigations showed that fertilizer production in particular had left its mark on the area. The reason: Before there were sewage treatment plants, the wastewater from the “Chemische” company, founded in 1818, was discharged untreated into Lake Zurich. water protection? Wasn’t much of an issue back then.
Lead, zinc and uranium on the seabed
And so there have been particles of lead, zinc or arsenic on the seabed for years, and particles containing uranium have also been found. In certain places, the pollutants are stored at a depth of up to 2.3 meters, but sediments at a depth of 10 centimeters are also contaminated. Nevertheless, it is “absolutely harmless” to swim in the water, says Bettina Flury, project manager at the cantonal office for waste, water, energy and air (Awel). The pollutants are difficult to dissolve. The canton, as the “owner” of Lake Zurich, is responsible for the construction work. Nevertheless, it is not possible to swim past the renovation area: For safety reasons, it is provided with buoys and an oil barrier.
The floating platform is at the height of the port of Uetikon.
The public gets very little information about the mega-construction site on the lake, even if the “heart” is clearly visible from the port of Uetikon: a floating platform, also known as a pontoon, with two bright yellow metal girders. A pump is installed on it, which sucks off a mud-water mixture at a water depth of up to 30 meters on the lake floor and transports it to land via a floating hose. The pump runs for up to ten hours a day and makes a humming noise non-stop. One of the workers on the platform shrugs: “You get used to it.”
A hose leads to the bottom of the lake to suck off the contaminated sediments.
A colossus with bright yellow metal supports: the pontoon. In the center of the basin is the hose through which the sediments are pumped ashore.
A large part of the work is automated. On the pontoon, the workers can follow the suction of the sediments with a visualization on the screen; the sucker is controlled with sonar and cameras. The wetsuits are only used if a larger object should block the pump. The situation is different in the area of the so-called block throw, i.e. the stone wall on the shore. Here divers are in action, who manually suck the sediments from the stones with a hose. In the port of Langenbaum, on the other hand, a floating suction dredger is used.
24 tons of material per container
A hall was specially built on the factory site to process the sediments. It’s so loud in there that you can hardly have a conversation. The noise comes from a machine that separates gravel, sand or wood from the extracted sediment with constant shaking. The sludge mixture then has to be treated and dewatered. What’s left goes into a yellow container that holds 24 tons of material and is chemically analyzed. The extracted water is stored in former extinguishing water tanks, receives chemical treatment and then goes back into Lake Zurich. Around 120 cubic meters of sludge mixture are removed every day and five containers are filled with sediment residues.
The contaminated sediments are processed in a specially built hall.
After the coarse sand has been sifted out of the lake bed, this watery mixture remains.
So-called chamber filter presses form the sludge into press plates.
Around 50 of the yellow containers labeled “Super heavy” are parked in front of the processing hall, waiting to be filled. The full containers are transported away by truck. For logistical reasons, it is not possible to use the historic rail system on the site and take the freight away with cargo trains, because the waste has to be disposed of at different locations depending on the level of pollution.
The refurbishment will cost a total of 25 million Swiss francs; The canton takes over 20 percent, Zeochem AG, as Chemie Uetikon is called today, 80 percent. The Marti Infra company was awarded the contract for the renovation. It was also she who removed tar deposits from the lake bed near Thalwil in 2009.
The cleaning work is expected to take about two years. However, it will not be quiet on the premises of the “Chemical” afterwards. On the contrary, that’s when things really get going – with the construction of the canton school.
