"Cities are increasingly reaching their limits"

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Jörg E. Drewes is researching new methods for purifying treated wastewater.    I
Jörg E. Drewes is researching new methods for purifying treated wastewater. Image: S. Reiffert / TUM

Extreme dry spells, shorter spring periods: Groundwater tables in Germany are declining. Prof. Jörg E. Drewes of the Chair of Urban Water Systems Engineering at the Technical University of Munich (TUM) explains why the use of purified wastewater effluents could be a solution to future potable water shortages.

Professor Drewes, where does our drinking water come from?

Most drinking water in Germany is taken from groundwater. This is ideal, since the natural subsurface already filters out pathogens and harmful substances from the water. And groundwater resources are well protected and relatively stable with regard to environmental factors such as temperature. But groundwater resources are limited at many locations in Germany. In these cases the groundwater is artificially augmented with surface water from rivers or lakes, or drinking water is acquired directly from large surface water reservoirs.

Is the amount of water in Germany constantly dropping?

Extreme weather events like floods and extended extreme dry periods will become more common in Germany. At the same time the spring run-off period will grow shorter. As a result, snow packs will melt much quicker and instead of recharging the groundwater will flow directly into the rivers. This can result in a rise in the number of floods, while at the same time the natural groundwater recharge will decline. This is a long-term trend which we can, however, already observe today. In the northern part of Bavaria in particular these developments are having very alarming impacts. The Franconian dry plateau ("Fränkische Trockenplatte"), which includes the cities of Würzburg and Schweinfurt, is traditionally an arid area with very limited groundwater reserves. This region is surrounded by highlands where many clouds shed their moisture in the form of rain. As a result, the groundwater reserves can’t be recharged as quickly. Together with the impacts of climate change, today we’re already seeing increasing conflicts on use between agricultural irrigation requirements, the requirements of public drinking water supplies while also ensuring minimum ecologically base flows in rivers.

So this means we should conserve water?

Conserving water makes good sense in general. But we can’t forget that our conservation measures also have to be compatible with the existing water infrastructure. If we save too much, less water flows through the pipes, which might mean water stagnation that results in hygienic problems with drinking water. And less water also means more concentrated wastewater, which can lead to formation of deposits in sewage systems and thus to high levels of corrosion. A minimum flushing effect is needed in order for the system to work properly. Of course other solutions would also be conceivable, but retooling this infrastructure, which has evolved over the course of 100 years or more, is no easy thing. And in many cities these infrastructures are also outdated and investments are urgently needed that are however only being pursued half-heartedly.

,,Do we really need the highest possible water quality when cleaning our houses, flushing toilets and irrigating greenery or in agriculture? "

What are possible solutions?

Every location is different and therefore calls for water solutions that are adapted to local circumstances. This means that future-oriented solutions for our water infrastructure also look very different. We can also question whether we have to always use potable water for every application. Do we really need the highest possible water quality when cleaning our houses, flushing toilets and irrigating greenery or in agriculture? Instead we should provide a water quality that fits the application. Today purified wastewater effluents have such a high quality that they can be directly discharged into our rivers. Of course, there are still several substances in these discharges that we don’t want to have. For example, the conventional wastewater treatment processes do not entirely remove residual pharmaceuticals and pathogenic germs. In order to produce a quality level suitable for a large number of possible reuses, this means the water has to be treated further. We’ve developed new processes for this purpose.

What’s special about these processes?

We’re driven by the desire to develop processes that are taking advantage of natural principles, are energy-efficient, have a low carbon footprint, and are low in producing waste materials. In the SMART process we are modifying operational conditions in such a way that we select for high-performance bacteria that are very good at breaking down trace organic chemicals and pathogenic germs that would survive in conventional treatment systems. Where very flexible solutions are required for instance due to seasonal demand variations, we combine physical separation methods like ceramic membranes with chemical processes such as ozonation. Ceramic membranes are expensive to buy, but their 20-year life expectancy is comparatively long. This results in lower costs over the entire life cycle, so the investment pays off in the end. The membrane is a very reliable barrier and filters out the pathogens, while any remaining trace organic chemicals are removed by ozone.

Are these procedures already in use?

We’re conducting a feasibility study in the Schweinfurt region where the need for alternative solutions has been recognized and there is great interest in adopting this approach. In order to implement unconventional solutions requires a dialogue with all stakeholders to discuss advantages and disadvantages with them. We’re also currently preparing a demonstration project to investigate the technical feasibility for the region. Agriculture is an important stakeholder, since fruit crops and medicinal herbs have been grown in Schweinfurt for over 100 years, and these crops require irrigation. In order to address seasonal demand variations these irrigation systems and subsequent water treatment processes have to operate in a highly dynamic way and ideally will function remote less. This is why we’re employing the latest in sensor technologies and cloud-based approaches which is considering weather forecast data and measured values in real-time and integrate them in control processes.