Water distribution with the help of the bridge ring main

The bridges stretch along the main „traffic arteries“ through the city, not only creating an additional level of living and mobility for people, but also helping Frankfurt's water system to achieve an additional level: the ring main. This main collects water along the course of the bridges, transports it to storage locations, and when it is taken from there again as needed, the ring main distributes it again for the irrigation of Frankfurt's plant life - on and next to the Frankfurt Bridges.

Content: Tasks of the ring main, its structure and information about its operation

Just when the plant life needs water in dry phases most urgently, no rainwater accumulates, and the river water level of the Main is usually too low to take water from, as well. Once there is a heavy downpour, rainwater is distributed over the entire urban area or flowing into the riverbed, while the dry earth can not absorb it quickly enough.

In order to be able to always water exactly when plants need it on dry days, and to bring water to where green spaces need it most, a piping system must be created that collects decentralized water, brings it to storage locations, and from there brings it back out for distribution to the plant life.

With the help of the Frankfurt Bridges ring main, water can be collected, stored and redistributed. It is integrated into the structure of the Frankfurt Bridges along the entire route. For its operation, cleaning, controll, maintenance and frost protection must be ensured.

The conventional wastewater and sewer system in Frankfurt does not allow rainwater to be used as irrigation water on a large scale

As in the vast majority of cities, most surfaces in Frankfurt are sealed. This means that rainwater cannot seep away, but flows into the sewage system. Since Frankfurt has a mixed system in the inner city area, in which wastewater and rainwater are discharged together, this water is lost for the irrigation of green spaces. An incredible waste.

Frankfurt's canal system was built in the 19th century, at a time when no one could have imagined that rainwater could one day become precious. The main goal at the time was to channel rainwater and sewage out of the city as quickly as possible to prevent flooding and epidemics. Once constructed, however, sewer systems can be difficult to adapt to changing conditions. Separating wastewater and stormwater would require major construction work. Gradually, all streets would have to be broken up and a parallel pipe system built: one pipe for wastewater, the other for rainwater. Nowadays, this so-called separation system is mostly used in new housing estates, but it is hardly ever used in existing housing estates.

The Frankfurt bridges offer the possibility to solve this problem in a different way: Independent of the existing sewer system, a water ring main runs in the bridges for the collection of potential irrigation water, across the entire rout.

Collecting, storing, distributing water independently of the existing sewer system: the Frankfurt Bridges ring main makes it possible

Rainwater can be collected and stored over the entire course of the bridge and on adjacent roofs. In subsequent dry phases, it can be made available again to the plant life on and next to the bridges.

Google Earth /Stiftung Altes Neuland Frankfurt GNU

The bridges collect water and transport it to storage locations . . .

. . . later they take it from there and distribute it again

The bridge ring main is an additional layer for the transport of water through the city

The bridges stretch along the main arteries through the city, not only creating an additional level of housing and mobility for people, but also helping Frankfurt's water system to achieve an additional level: the ring main.

The ring line will be integrated - almost invisibly - into the structure of the Frankfurt Bridges over the entire course of the line

In the case of bridge sections of concrete construction, the ring main can be installed in the body of the bridge. In metal structures, it is integrated into the structure of the bridges and visually disguised like a dark cast-iron pipe. And in some places it is also hidden in the plant substrate on top of the bridges.

Experts on the Topic

FH Münster Lehrgebiet Wasserversorgung - Universität Trier Fachbereich Hydrologie - Bauhaus-Universität Weimar Professur für Siedlungswasserwirtschaft - Bergische Universität Wuppertal Lehr- und Forschungsgebiet Siedlungswasserwirtschaft - Universität der Bundeswehr München Institut für Wasserwesen - Professur für Siedlungswasserwirtschaft und Abfalltechnik

For the irrigation of the green areas through the ring main, a sophisticated control system is required

You can't just pour water on the plants to the right and left of the bridges: In addition to pumps and fittings, various measuring points and variables are required to control the water system. The main measured variables are pressure, filling level, degree of moisture in the soil, flow rate and temperature.

This system with its numerous valves and interfaces must be kept continuously clean

As with any complex piping system, it must be ensured that there is no contamination from solids, pollutants or germs. While there are filter systems for solids and pollutants, germs can be effectively rendered harmless with UV treatment.

Explanation of UV treatment

The modules are designed so that the water flows along the UV lamps at a small distance.

In addition, a turbulent water flow is created so that every water particle receives UV treatment. The UV radiation thus destroys the genetic material of the microorganisms, making reproduction impossible.

The UV light is emitted by low-pressure mercury vapor lamps located in transparent protective tubes.

Radiation wavelength: circa 254 nm

Power consumption: circa 50 Wh/m³

Heat exchangers and heating tapes together with waste heat from bridge cable ducts ensure frost protection

During the coldest weeks of winter, the ring main usually does not carry water. However, if surprising frost events should occur in the fall or spring, frost protection for the ring main is provided differently depending on the bridge section:

Sections that pass by data centers can be protected with a heat exchanger system using the year-round waste heat from these building complexes.

On some sections, the ring main can run under the sidewalks next to the cable shafts under the bridges. The cable shafts also release sufficient thermal energy in their ducts in winter that even rather thick waterproof protective walls can protect an adjacent ring main from freezing.

Solar heat stored in storage probe fields under the Frankfurt Bridges is also conducted through the body of the bridge in winter to keep the roadways frost-free, which, depending on the constellation, can also help keep the ring main free of frost.

On sections of the line where it is not possible to use waste heat from data centers or cable ducts, frost protection of the ring main is provided by demand-controlled heating strips.

The design and structure of the ring main system, as well as the control of its operation, pose a challenge even at the planning stage

Ring circuitfactsheet

Inner diameter of the pipe: 25 cm (DN 250)

Length: 2 x 50 km

Average operating pressure: 4 bar

Laying: In the bridge body

Redundant design for more operational reliability

Year-round operation (frost-free conditions are guaranteed)

But the effort is worth it: With the help of the ring line, areas further away from the bridges can also be planted and revitalized

Conclusion: The Frankfurt Bridges ring main is a useful addition to the city's existing water system

With its network-like route, the Frankfurt Bridges‘ structure is a suitable support for a complementary water infrastructure: the ring main.

With the appropriate control and cleaning system, the ring main can make a contribution to the sustainable water management of the city of Frankfurt - similar to the existing sewer system.

In addition, the waste heat from the cable ducts integrated in the bridge body can be used as frost protection.

Preparing the City for Challenges

Harvesting" rainwater instead of discharging it into the canal

Groundwater from excavation pits should be used

Store water in the soil through infiltration

The city of the future wastes no water

Unsealing of the city center

Vitalization of the urban green

The green metropolis of the future