Dr. Andreas Matzinger, born in 1974, studied environmental natural sciences at the Swiss Federal Institute of Technology (ETH) in Zurich and holds a doctorate in the field of freshwater research. Since 2007, he has been a research assistant at Kompetenzzentrum Wasser Berlin.
The summer of 2018 was one of the hottest ever, the summer of 2019 seems to be similar. Climatologists speculate that such temperatures could become the norm. How can Berlin protect against this?
Andreas Matzinger: It’s particularly important to make sustainable use of the available water in the city. Rainwater is a valuable resource. We should use it more for vegetation and thereby for cooling the air.
Many experts are calling for Berlin to become a “sponge city”. What does that mean?
Andreas Matzinger: The term was coined in China. There, in the course of rapid urban development, aspects such as drainage and sewerage were given too little consideration. Now the Chinese megacities are struggling with flooding and enormous water pollution. Therefore they want to keep water increasingly in the urban space, as if in a sponge. I think the term “sponge city” is not a particularly fortunate one. In Berlin, we should rather strive to bring the situation closer to the natural water cycle
What do you mean?
Andreas Matzinger: In the surrounding forests about 80 percent of the rainwater evaporates: it first seeps into the soil, is absorbed by plants and finally evaporates through the leaves. Only about 20 percent flows into the groundwater. In the city, on the other hand, up to 50 percent of the rainwater is discharged via channels into waterways such as the Spree or the Landwehrkanal. This is not just a waste, it also causes problems there.
Andreas Matzinger: Suddenly huge amounts of water get into small rivers, the river sediment is churned up and living species are disturbed or washed away. If this happens too often, no ecological balance can be established there. And one more thing to remember: as a layman, you think rainwater is clean. What arrives in the waterways, however, is an often heavily contaminated broth, not just from dog waste or cigarettes from the roadside gutter. A largely underestimated factor is chemicals from roofing membranes and facade paints, such as toxins to counter algae growth. Berlin emits as many pesticides per square meter as an intensively farmed agricultural area.
So shoudl rainwater also be routed to sewage treatment plants?
Andreas Matzinger: In the centre of Berlin, roughly within the S-Bahn ring, rainwater and waste water are collected in the same conduit. This means that the rainwater is also purified in the sewage treatment plant, so that fewer pollutants and microplastics get into the waterways. However, this so-called mixed water system also has disadvantages: heavy rainfall leads to the notorious overflows – and unpurified domestic sewage also spills out of sewer shafts into bodies of water. This can cause fish death at high temperatures in summer. And climate change is not just increasing average temperatures. Even extreme rainfall may become more frequent. In June 2017, it poured down for days in Berlin. In low-lying areas roads were flooded; the water was also running into some underground stations. This is another reason why it’s so important to retain as much rainwater as possible and make it usable. Green roofs, for example, can absorb up to 70 percent of the annual rainwater there.
Why are only three percent of Berlin#s roofs greened?
Andreas Matzinger: Three percent is already a good success! No other city in Germany has achieved as much. Even Hamburg, which is often referred to as a prime example, has achieved significantly less. But you’re right: there is still room for improvement in Berlin. Up to 40 percent of roofs in the capital could be considered for greening. And green roofs not only cool the rooms below. On low school buildings they also help to stop the schoolyards heating up so much in the summer by evaporation.
Where is the rainwater to go if you have neither a garden nor a flat roof?
Andreas Matzinger: In avenues you can target the trees. Australia and the US have already had good experiences with this approach. Even toilet flushes can be operated with rainwater. For example, this was successfully implemented at the ufaFabrik cultural centre in Berlin-Tempelhof. You need extra water pipes for this. As a rough estimate, such an installation can also be expected in residential buildings in about ten years.
Within the Berlin S-Bahn ring it gets up to five degrees hotter in summer than outside it.
Andreas Matzinger: Right. In particular, the heat builds up on concrete open spaces such as Alexanderplatz, where shading is largely absent. Studies by the World Health Organisation (WHO) clearly show that a high level of heat stress damages health, especially in the elderly. We should therefore increasingly return green spaces to the city, because then the water is absorbed by plants, evaporates – and, together with shading, ensures cooling. In general, people should be able to experience water in public space with their senses.
Do you dream of the Paradise Gardens on Alexanderplatz?
Andreas Matzinger: Often simpler measures help. Perhaps you know the concrete pool on Potsdamer Platz. It is fed exclusively by rain. In summer, people sit on the edge and bathe their feet in the cool water.
But what’s to be done if it does not rain for months, like last summer?
Andreas Matzinger: Wherever possible, we should use water several times over. So-called grey water – i. e. slightly polluted waste water from baths, showers or washing machines – can be treated and used, for example, as process water in buildings or perhaps even for irrigation. There is still a lack of larger scale applications and there is still a need for research, for instance on pollutant levels. But grey water is always available, and especially in hot summers, when the plants in parks are endangered by drought, people shower even more often than usual.
Interviewer: Till Hein, Journalist
Pictures: KWB and Andreas [FranzXaver] Süß
Headshot: Silke Reents (Ahnen&Enkel)