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Water in the slow sand filter of the lake waterworks facility Lengg in Zurich, Switzerland, pictured on August 6, 2015. The waterworks facility Lengg treats up to 250'000 m3 of drinking water a day. The raw water is run through natural sand and activated carbon filters in order to hold back dirt and degrade it biologically. The water is then treated with ozone in between filtrations, which inactivates microorganisms, oxidises organic substances and thus ensures healthy drinking water. The ozone itself is then removed in the activated carbon filters. (KEYSTONE/Christian Beutler)