After burning in a waste incineration plant, almost a quarter of a filled garbage bag remains left over – most of it is slag (19%). It is not immediately dangerous, but contains toxic heavy metals, especially lead, copper and zinc in considerable quantities. These can be activated by acid rain. Landfills are therefore secured with drainage pipes which are constantly monitored. They run into sewage treatment plants; the heavy metals fall out in the sewage sludge, and then remain in the ash ‘biscuits’ after incineration. This is stored.


In landfills with the most modern scrapping systems, however, most of the metals are extracted, that is about 10% of the slag, of which 2% is non-ferrous metals which may, for example, contain components of smartphones which may have been disposed of in household rubbish. In contrast, metal salts remain behind in the stored slag.

Non-ferrous metals, such as aluminum and heavy metal mixtures (copper, zinc, lead, gold, silver), are refined in the latest processing plants, like those operating at the DHZ AG Lufingen landfill, and are then sold directly to smelters such as Umicore or Aurubis. Slag is big business or as they say in Yorkshire, England “where there’s muck, there’s brass”: since it has about the same copper concentration as copper ore from a mine in the Democratic Republic of Congo, it is considered as “urban mining Eldorado”.

DHZ Lufingen

Sources: Discussions with Johannes Allesch KVA Basel (, Benjamin Blumer DHZ Lufingen (30.1./7.3.17), Rolf Widmer EMPA (6.3.17); Environmental Report KVA Basel 2016.

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