Circular economy is a hot topic. The use of rare earths, plastics or biomass. However, a material cycle without clean water is inconceivable. A basic understanding that seems to be lacking in the face of environmental and agricultural legislation that makes no increased efforts to protect it, even though the quality of drinking water is suffering, and its availability is measurably scarce, which is why water is becoming increasingly contested in some regions of Bavaria, not just the world. The extreme weather events of recent years contribute to the perception of water as a danger rather than as a resource worth protecting. At the same time, neither the socially demanded economic restructuring nor the agricultural turnaround can be realized without a corresponding change in thinking.
The current trend towards a circular economy illustrates the importance of closed resource and material cycles. Is it true that the natural water cycle, which lies at the core of a healthy Earth system, is comparably disturbed?
Drewes: The natural water cycle has been influenced massively by human activities at many locations. Let's think of the overexploitation of groundwater, the discharge of wastewater that often significantly exceeds the self-purification capacity of flowing waters, the discharge of substances that are hazardous to health into the aquatic environment, or altered landscapes and sealed settlement areas that exacerbate the increasingly frequent flash floods. We need to fundamentally rethink our approach to water while simultaneously addressing many fronts. What lies ahead is a generational task!
In the light of natural disasters, like the Ahr Valley flood, water increasingly appears as a dangerous element. Yet it is an endangered resource in need of protection. Paradoxically, too much water in one place simultaneously causes too little water in another place and vice versa. Why is this problematic for agriculture (forestry) and agri-environmental policy in particular?
Drewes: Basically, we have to stop looking at water quantity and water quality separately. The two are closely coupled. Massive flash floods are also the consequences of altered natural landscapes, in which water cannot be retained and seep away in a decentralized manner. Many of our landscapes today are a “run-off'” model. Simply calling for irrigation water during droughts, without taking a critical look at demand, will not work with less and less water. How and when do we irrigate? How can we use more efficient irrigation systems that take into account real-time data (weather data, soil moisture)? Should we perhaps avoid growing certain water-intensive crops altogether? Applied nutrients or pesticides end up in water and pollute aquifers, limiting their future use. We need to pay more attention to these interrelationships.
In order to secure our food and drinking water supplies locally, adapted irrigation techniques must be developed, and alternative sources of supply must be tapped. What role does water recycling play in this context?
Drewes: Saving water is very important in these times, and everyone can make a valuable contribution here. We also have to ask ourselves whether we always need drinking water quality for all purposes - which has been the case up to now. Let's think of irrigation of urban green spaces, agriculture, flushing toilets or car washing. When water becomes scarce, we must be open to thinking about alternatives. Why not collect and use rainwater decentrally? Well, that cannot be the only solution during prolonged dry spells. One resource that is always available, regardless of droughts, is the run-off from our municipal wastewater treatment plants. With appropriate advanced water treatment, this can provide a water quality that allows safe water reuse. Such approaches will not be necessary across the board in Germany in the foreseeable future, but in regions where water resources are already severely strained - just think of Lower Franconia - this can be a reliable and safe alternative.
The idea of a comprehensive “water recycling,” produces different types of purified water that can be used as needed and targeted. That sounds innovative...
Drewes: With this customized concept, we provide qualities that are adapted to the respective use, so the user gets the optimum quality, and we conserve resources through targeted treatment. We don't overshoot the mark and treat more than is necessary.
The thought of treated wastewater may upset some people. Even though the strict regulations and controls in Germany ensure that recycled water is harmless to health. How would you encourage an open approach towards alternative water resources?
Drewes: Water is always in a cycle, sometimes larger, sometimes smaller. Used water does not just disappear. Even in the days of the dinosaurs, water was recycled. Today we do it in a planned way and use technical systems to do it in a very controlled and safe way. To protect human health and the environment, these systems have multiple barriers for the retention of pathogenic germs or hazardous chemicals. Automated control makes these systems very reliable. Countries (such as Australia, the U.S., and Spain) that have felt the effects of climate change much more acutely for some time are already practicing water recycling in a variety of applications. The people there welcome these alternative concepts, now we can learn from them and find our own way.
That is to say, water recycling is not only resource saving, but also a real contribution to climate protection and peacekeeping. What kind of development would you like to see?
Drewes: Options like water recycling are concepts that we haven't needed in Germany so far. But times are changing drastically, and we should be open to exploring and breaking new ground. Relying solely on concepts that were sufficient in the past will not help us to make our water management future-proof.
Prof. Drewes spoke about demand management and use of alternative water resources in the face of declining supply during the interdisciplinary IESP Forum “Drought and Heat - Irrigation under the Framework of Climate Change” at the IFAT Munich 2022, the World’s Leading Trade Fair for Water, Sewage, Waste and Raw Materials Management. His chair of Urban Water Systems Engineering alongside the chair of Analytical Chemistry and Water Chemistry (Prof. Elsner) represented TUM at the trade fair.