A closed water cycle. Prerequisite for a circular economy.

The circular economy is a hot topic. Prominent examples are the use of rare earth, plastics, or biomass. A material cycle without clean water is yet inconceivable. A basic understanding, which seems to be lacking, given environmental and agricultural legislation that makes no increased efforts to protect water. Even though the quality of drinking water is suffering while drinking water quantity is getting noticeable scarce. Not only in far-away regions of the world, water is becoming contested, but also in some regions of Bavaria. The extreme weather events of recent years contribute to the perception of water as a threat rather than a resource worth protecting. Neither the socially demanded economic restructuring, nor the agricultural turn-around can be realized without a corresponding change in thinking.

Prof. Dr.-Ing. Jörg E. Drewes, chair professor of Urban Water Systems Engineering at TUM, explains why it is the right time to sensitize ourselves to the value of clean water, to break new ground in its management, and accept recycled water as an alternative resource as the planet is heating up.


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: Human activities have massively impacted the natural water cycle at many locations. Let's think about the overexploitation of groundwater, the discharge of wastewater that often significantly exceeds the self-purification capacity of receiving streams, the impact from discharge of substances that are hazardous to health into the aquatic environment, or altered landscapes and impervious urban areas that exacerbate the increasingly observed flash floods. We need to rethink our approach to water fundamentally, 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 at one place, simultaneously causes too little water at another place, and vice versa. Why is this a problem for agriculture (forestry) and agro-environmental policy in particular?

Drewes: In a nutshell, we must stop looking at water quantity and water quality separately. The two are inseparable. Massive flash floods are also a consequence 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 critically assessing demand - will not work with less and less water. How and when do we irrigate? How can we use more efficient irrigation systems that factor in real-time data (e.g., weather data, soil moisture)? Should we perhaps avoid growing certain water-intensive crops in areas with water shortage altogether? Applied nutrients or pesticides end up in our water and pollute aquifers, limiting their future use. We need to pay more attention to these interrelationships.

Adapted irrigation techniques must be developed, and alternative supply tapped to secure food and drinking water supply locally. What role does "water recycling" play in this context?

Drewes: Saving water is crucial in our times and everyone can make a valuable contribution here. We also have to ask ourselves whether we need drinking water quality for all purposes - which has been the gold standard up to now. Let's think about the 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 locally? This is one measure but 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.

Comprehensive "water recycling" produces different types of purified water to be utilized as needed and targeted. That sounds innovative...

Drewes: With this customized concept, we provide qualities 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. Although the strict regulations and controls in Germany ensure that recycled water is harmless to human 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 fashion and use technical systems to do it in a very controlled and safe manner. To protect human health and the environment, these systems have multiple barriers to retaining pathogenic germs or hazardous chemicals. Automated control makes these systems very reliable. Countries (such as Australia, the U.S., and Spain) experiencing the effects of climate change much more acutely for some time are already practicing safe water recycling in various applications for a long time. The people there welcome these alternative concepts. Now we can learn from them and find our own way.

That is to say, that water recycling is not only resource saving but also a valuable contribution to climate protection and peacekeeping. What kind of development would you like to see?

Drewes: Options like water recycling are concepts we didn’t need in Germany so far. Nevertheless, 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 mold our water management future-proof.


Prof. Drewes spoke about demand management and the 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 and the chair for Analytical Chemistry and Water Chemistry (Prof. Elsner) represented TUM at the trade fair.


Prof. Dr.-Ing. Jörg E. Drewes
Technical University Munich
Chair for Urban Water Systems Engineering
+49 (0)89 289 13713