Basel tests brewery wastewater to keep green roofs cool

Basel is testing an automated system that uses treated brewery wastewater to irrigate green roofs, after an exceptionally hot summer left roofs dry and stripped them of their cooling effect, SRF reported.

On the roof of the Biozentrum, 73 metres above the ground, what should have been a green meadow became parched. Stephan Brenneisen, a lecturer in urban ecosystems at the Zurich University of Applied Sciences, said the roof dried out quickly and no longer cooled its surroundings.

The drought also devastated wildlife on the roofs, Brenneisen said. Thermal-camera images showed that the dried-out green roofs heated up even more than conventional gravel roofs. Some planted roofs had dried out completely after two months and remained that way for months.

Jascha van Gogh, who leads projects at Oekoskop AG, is now testing an automated irrigation system on a hotel roof in Basel’s Gundeldingerfeld. Fine drip hoses deliver water directly to plant roots, while soil sensors regulate the supply. Van Gogh said sprinklers would waste water because much of it could evaporate in the hot air before reaching the roots.

The project does not use drinking water. Instead, it draws on wastewater from a neighbouring brewery, which is treated in the hotel basement. A sand filter removes coarse suspended particles to prevent blockages in the hoses, and the water’s pH is monitored because water that is too acidic or too alkaline could kill the plants.

The trial on the former industrial site is funded by foundation grants. Sarah Barth, the site’s board chair, said the project is intended to cool both the hotel rooms and the surrounding neighbourhood.