The city of the future wastes no water

Only a water-sensitive city is armed against periods of drought, heavy rain, tree death or flooding. For a long time, the goal of cities was to dispose of accumulating water as quickly as possible to avoid potential damage. In the future, however, the goal must be to keep any water in the city and use it as efficiently as possible. Infiltration, treatment and intra-urban storage are some important elements along the way.

Content: The following packages of measures are described for Frankfurt on its way to becoming a water-sensitive city

Surface waters in the city represent reserve and intermediate storage for the watering of plants. Like rainwater and groundwater from construction sites, water from the Main River or from stillwaters must be treated before it is fed into the ring main and before it seeps into the groundwater reservoirs.

Another important untapped source of water is wastewater: Similar to most other major cities in the world, the greatest reserve potential for the city of Frankfurt lies in the reuse of wastewater. The need for this could arise in the context of climate change.

In Frankfurt, there are also numerous additional smaller measures to irrigate urban greenery and thus improve the urban climate, such as the creation of swales or the installation of cisterns or infiltration trenches - all milestones on the way to becoming a water-sensitive city.

River water from the Main can be used as a reserve to replenish the reservoirs

If the rain and groundwater volumes are not sufficient, there is a safety net: the Main River. However, water can only be taken from the Main at high levels and must then be stored for later.

In Frankfurt, it makes sense to have the extraction point upstream in the direction of Fechenheim, since further south the municipal wastewater treatment plants (ARA) Niederrad and Sindlingen discharge the (multi-stage treated) wastewater into the Main.

At the northern end of the bridge, a bathing lake can be newly created close to the city as a further reservoir

Frankfurt does not have its own bathing lake that is easily accessible by public transport. Such a bathing lake is being created in the Niddapark on the former Federal Garden Show site on the northern spur of the Frankfurt Bridges.

The bathing lake is not only an ideal reservoir for irrigation water, it also increases the value of this local recreation area and provides an important habitat for various animal and plant species.

The lake is easily accessible by local bridge passenger transport and drop-offs lead directly from the bridge down to the lake. For families without a car, senior citizens and children, who until now could only get to a bathing lake with a lot of effort or in the company of adults, this is a new leisure attraction in Frankfurt.

The bathing lake also has a positive influence on the city climate: due to the evaporation of the water - at least in some weather conditions - the air near the lake is cooled down and can move into the city center.

The bathing lake on the Nidda - water reservoir, climate cooler and recreational area

Scope: 820 m

Surface area: 45.000 m²

Storage capacity: around 120,000 m³

Extraction capacity: 13.500 m³

Accessible via autonomous traffic on the bridges - every minute as needed

Replacement for dog run: The complete area south of the lake

The people of Frankfurt have always appreciated natural open-air swimming pools in the city, as shown in this historic photograph of the Main-Swimmingplace at the Eisernen Steg from 1930

The bathing lake is created in a nature-friendly and sustainable way

During the planning phase, hydrogeological investigations must be carried out to determine the extent to which the lake can be fed by groundwater. The remaining water enters the lake through collected and treated rainwater via the bridges‘ ring main.

The lake has a large storage volume; depending on the depth selected, it can hold up to 120,000 cubic meters of water.

Not all the lake water is discharged for irrigating of Frankfurt‘s greenery, but the so-called "water usability" in dry periods is limited to 30 cm of the water level (circa 13,500 m3 of water): This ensures that the bathing lake acts as a natural air conditioner for the city even in dry periods – and as a great recreational swimming place for Frankfurt‘s citizens.

The Frankfurt Bridges Society is responsible for the water quality of the lake, the cleanliness of the overall area and the safety and care of bathers.

The withdrawal of water from bathing lake waters

Due to the quality of bathing water, the extraction can be done directly by a dry installed self-priming pump in the shore areas. A submersible pump should not be installed because of the risk of electric shock.

To prevent coarse material from being sucked in, the suction lance should be surrounded by a coarse to medium gravel pack. A cover can minimize algae growth and a subsequent fully automatic filtration system ensures extensive reduction of undissolved contaminants.

The pipe for bed and park irrigation should be installed so that when the pump is switched off, it is automatically drained. The plant can be equipped with a hygienization stage (UV irradiation or chlorination and/or an activated carbon filter).

All components of the plant can be installed in a simple functional building/container. During operation, as-needed checks and annual cleaning of the water intake are required.

Experts on this Topic

Bundesministerium für Wirtschaft und Klimaschutz - International Water Association - Water-Wise Cities Group - Arbeitskreis Schwammstadt Österreich - Umweltbundesamt klimaresiliente Schwammstadt - Bayerisches Umweltministerium wassersensible Stadt

In addition, other still waters in the Frankfurt urban area can be integrated into the water system of the Frankfurt Bridges

In addition to the bathing lake described, other surface waters can be integrated into the system. Depending on the location, it would be conceivable for these so-called still waters to function in a similar way to the cisterns, i.e. to collect and temporarily store rainwater from roof surfaces and then release it into the ring main; or, like the groundwater and the bathing lake, they can serve as storage until the supplied water is withdrawn for irrigation needs.

In the case of the stillwaters, as with the bathing lake in Niddapark, the level fluctuation is also limited to 30 cm: Accordingly, a total of up to 12,775 m³ of water can be temporarily withdrawn.

Thoroughly treated wastewater will provide another irrigation water reserve in Frankfurt in the future

Currently, water generated in the city, whether stormwater, groundwater or wastewater, is primarily disposed of.

The Frankfurt Bridges can offer the solutions already described for the use of rainwater and groundwater. That leaves wastewater: from households and commercial units.

Around 65 million cubic meters of treated wastewater are discharged annually into the Main River from Frankfurt's wastewater treatment plants in Niederrad and Sindlingen.

The treatment of this wastewater is already very thorough: In a three-stage process, coarse solids are first removed in mechanical treatment. Biological and chemical treatment then primarily removes carbon and the nutrients nitrogen and phosphorus. Only then is the water disposed of into the Main River.

In order to be able to use it without hesitation as irrigation water over a wide area, the so-called "4th purification stage" is still required.

The fourth treatment stage includes more extensive elimination of phosphorus and removal of micropollutants.

More and more cities are implementing this treatment stage in their water management.

The implementation of this advanced wastewater treatment is also already being planned in Frankfurt.

In the 4th purification stage, sand and activated carbon filters, ozonation and membrane systems as well as combinations thereof are used.

Currently, there are no binding legal requirements for the "4th purification stage". However, it can be assumed that this circumstance will change in the near future.

After the first large-scale implementations of such plants in Switzerland, Germany has followed suit, especially in Baden-Württemberg and North Rhine-Westphalia.

The first experimental plant in Hesse was realized with the support of Darmstadt Technical University at the Langen wastewater treatment plant. The first industrial-scale plant on Hessian soil is currently being built in Bickenbach.

Different treatment processes are necessary for the different water sources and stations of the bridge water system

The ring main serves as the central distribution organ. All water from the sources to the reservoirs and from there to the plant beds and trees flows through the ring main, for which any water must be of minimum quality, so as not to damage them. There are no fixed limits for irrigation water itself. The decisive factor is that it is largely free of solids and germs. Depending on the initial water quality to be expected, different treatment processes are required.

The water-sensitive city of the future uses every drop of water

Due to climate change, water shortages are now increasing in some years even in the previously water-rich countries of Central Europe, especially in the summer months.

Accordingly, the reuse of treated wastewater has been defined at EU level as an additional source of water - and is now to be further promoted in the member states.

Germany is also working on this: an expert committee of the German Association for Water, Wastewater and Waste (DWA) is currently drawing up a set of rules for the German region, which is to be published in 2023. It explicitly states that the treated wastewater should not only be used in agriculture, but also for irrigating urban green spaces.

This is already the case in other industrialized countries with water shortages, such as the USA, Australia or Israel, and is likely to become inexorably widespread in Germany as well.

Reuse of more extensively treated wastewater will become a future component of the bridge project

The plan is for the wastewater to be used specifically for groundwater recharge, i.e. to travel the same route via the ring main to the infiltration facilities as the collected rainwater, excavation pit water or river water.

For this purpose, a connection will be created from the outflow of the Niederrad wastewater treatment plant to the nearest bridge section. Since sewage treatment plant drains are a permanent source of water compared to the other water sources in the bridge project, water can be transported from the sewage treatment plant via the ring main to the beds only when there is a need for irrigation, depending on the weather. Thus, if necessary, the "detour" via groundwater can be avoided, shortening the water transport and reducing the energy demand of the irrigation system.

The prerequisite for such an application is the realization of the 4th purification stage at the Niederrad wastewater treatment plant on a large technical scale. This has not yet been done at present, with the lack of space on the site posing a challenge in addition to the necessary investment. Furthermore, there are no binding specifications as to how much of the water produced must be further treated to be used as irrigation water.

However, it is expected that by the time the Frankfurt Bridges are completed, the 4th treatment stage will also be implemented in Frankfurt and the distribution function of the bridges will bring the treated wastewater to storage locations and, after removal, make it directly usable as irrigation water for the city.

Basic dimensioning of a rapid filtration system (two-layer rapid filter)

Experts on this Topic

Sieker die Regenwasserexperten - TU Dresden Hydrowissenschaften

The package of measures on the way to becoming a water-sensitive city also includes small-scale decentralized measures such as the creation of swales

The design of green areas in a swale character with a maximum height difference of 30 centimeters to the surrounding surface allows for decentralized collection and infiltration of rainwater, especially during heavy rain events.

If the water from sidewalks or other sealed areas is directed to such swales, this relieves the burden on the sewage system. At the same time, the water is available to the plants, which must be specially selected for swales. In this way, rainwater is returned to the natural water balance on site.

Especially in summer, increased heavy rainfall events can be better absorbed, and the evaporation effect provides a pleasant microclimate afterwards.

When the permanently vegetated swales are professionally installed, the water seeps away within 24 hours, preventing waterlogging, damage to plants, insect breeding grounds, or other unpleasant side effects.

The swale concept can be implemented not only along the bridges, but throughout Frankfurt's urban area

The installation and dimensioning of swales must be professionally planned and calculated in advance

A cistern statute should be introduced in Frankfurt

Wherever the bridges pass large blocks of houses whose facades are suitable for greenery, a cistern should be installed on the property of the blocks of houses to irrigate the facade greenery.

The bridge cisterns under the roadway "in front of the front door" serve as a backup: If the property's own cistern is completely full, it can empty excess rainwater toward the bridge cistern. And if the in-house cistern does not carry enough water, it can draw this from the bridge cistern.

The installation of a cistern on plots of land with a certain minimum size could also be made mandatory in Frankfurt (similar to other municipalities), especially for new buildings, in the form of a "cistern statute".

As a water-sensitive city, Frankfurt can contribute to the sustainable development goals of the United Nations

Today's standard wastewater treatment will be expanded in Frankfurt in the future to include the 4th treatment stage, which eliminates micro-pollutants:

Microplastics such as tire abrasion, hormonally active substances and pharmaceutical residues or antibiotic-resistant germs are removed in the process.

Accompanying measures such as the creation of infiltration troughs or the standardized installation of cisterns and infiltration trenches on courtyards and open spaces are further building blocks on the way to a water-sensitive city.

In this way, Frankfurt can keep not only the rainwater and groundwater that accumulates in the city, but also the wastewater in a municipal water cycle and no longer has to deprive other communities of drinking water for water imports to Frankfurt.

Conclusion: The Frankfurt Bridges are an important milestone on the way to becoming a water-sensitive city

The Frankfurt Bridges will create a system to collect, store and redistribute previously unused water.

Not only rainwater or construction pit water can be used as water sources for the unsealing and greening of Frankfurt, which is important for the city's climate; in the future, it will also be possible to transfer further treated wastewater from the bridges to storage locations or green areas.

With the help of the Frankfurt Bridges, a wide variety of water flows, which historically has always been transported out of the city, can then be used to irrigate urban greenery and thus be returned to the city's natural water cycle.

Preapring the City for Challenges

The Bridge Ring Line

Harvesting" rainwater instead of discharging it into the canal

Store water in the soil through infiltration

Water Storage near the City

Unsealing of the city center

Vitalization of the urban green

The green metropolis of the future