„Harvesting" rainwater instead of discharging it into the canal

With efficient stormwater management, precious rainwater is not discharged directly into the existing combined sewer system: Through the approximately 60-kilometer-long bridges, up to 90,000 m3 of rainwater can be collected and temporarily stored on site in cisterns under the roadway. The total volume of water that can be stored in selected infiltration areas close to the city is 600,000 to 800,000 m3 . On dry days, this water can then be redistributed via the ring main to irrigate the city's green spaces as needed.

Content: Which rainwater is suitable and where are potential areas for rainwater harvesting as well as temporary storage?

Rainwater from the street is usually contaminated with pollutants and tire abratson (microplastics). If one wants to avoid costly cleaning processes, it is recommended to collect roof rainwater for irrigation of green areas: both from the surfaces of the Frankfurt Bridges and from roofs of adjacent buildings and parking lot canopies.

The collection and use of rainwater in Frankfurt is currently hampered by the problem that rainwater is discharged directly at the buildings together with the wastewater into the so-called "combined sewer system". The Frankfurt Bridges solve this by temporarily storing intermittent precipitation in cisterns under the roadway along the bridge route. From these, the collected rainwater is transported via the ring main to the final storage locations (infiltration areas).

The bridges‘ ring main in combination with the bridges‘ cisterns below the roadway thus represents a "bypass" to the combined sewer system. With the comparatively clean water from the roof and bridge drainage, all of Frankfurt's green spaces can be irrigated in this way without hesitation.

Collecting and storing rainwater for the irrigation of urban greenery would be a sensible solution

Frankfurt "imports" more than 50 million cubic meters of drinking water per year, mainly from the Vogelsberg and Hessian Ried regions, to meet its total demand of around 65 million cubic meters. However, 100 million cubic meters of wastewater are discharged into the Main each year - after treatment in the wastewater treatment plants in Niederrad and Sindlingen.

The difference of 35 million cubic meters is rainwater that is currently discharged into the combined sewer system along with dirty wastewater and transported to the treatment plant instead of being used for irrigation.

"Disposal" of precious rainwater must be reduced through intelligent water management: Rainwater must be collected and infiltrated in a targeted manner so that the city's groundwater reservoirs can be filled up with it - then it can be taken back from there for irrigation as needed.

But it is only the Frankfurt Bridges that make such a cycle possible: They can "harvest" rainwater, as experts call the collection of rainwater, store it temporarily and ultimately also bring it to a "harvesting reservoir" at the end of the bridge arms.

Not all rainwater is equally usable

Rainwater from the roof is usually the cleanest, while that from the streets is usually contaminated with tire wear and harmful fluids.

Much of the rainwater that falls annually in Frankfurt should therefore not simply be used directly to water plants.

Rainwater from roofs is most suitable

Rainwater can also be contaminated with pollutants on properties: by fertilizers for the garden, by parking lots or by other pollution. Rainwater from roofs, on the other hand, is much better, with a few exceptions: slate or tiles drain the rainwater cleanly, and even flat roofs hardly emit any pollutants if no adhesive materials containing pollutants have been used.

This roof rainwater has to be collected - but there is a small hurdle: rainwater and wastewater from the buildings are mixed in most parts of Frankfurt before they enter the sewer system.

Rainwater from roofs must also be treated before use

A coarse filter is installed in the inlet pipe(s), which is installed as standard on downpipes and collects larger components such as leaves and branches. The cisterns are equipped with a bypass filter, which acts as a fine filter.

Harvesting" rainwater from roofs is easier said than done

The key is: to capture the rainwater before it mixes with the building's contaminated wastewater.

Because at present, in most parts of Frankfurt, rainwater runs into the sewage pipe together with the contaminated wastewater of the building's residents - and then from the property into the city's combined sewer system. If you want to "harvest" it, you have to tap it beforehand. And even if you have separated it from the dirty water on the property, the next problem presents itself: In front of the front door there is only the combined sewer, where the rainwater should not be discharged together with the dirty water. The construction of the Frankfurt Bridges brings the solution: The rainwater can be stored in the newly built cisterns below the road over which the bridges pass.

Experts on the Topic

Stadtentwässerung Frankfurt - AquaConsult Ingenieur GmbH - Emschergenossenschaft Lippeverband / Klimawerk - Arbeitskreis Schwammstadt Österreich - Umweltbundesamt

Frankfurt has a sewer network of around 1,700 km in length: some of it beautifully built, but most of it designed as combined sewers

Kunstinstallation ARA Niederrad 2016 Oliver Dorge

This is where the Frankfurt Bridges come into play

For the construction of the bridges, the road will be torn up anyway, which facilitates the installation of cisterns. Roof rainwater from the adjacent houses and the bridge flows into the cisterns for temporary storage - to be later either released to the plant life directly to the left and right of the bridges as needed or to be transported to larger storage facilities via the bridge ring main. In order to be able to collect a 100-year heavy rainfall event from all areas to be considered, approx. 90,000 m³ of cistern volume must be provided.

Do large water storage tanks even fit in the bottom of the road?

Since ancient times, cisterns have been used to store water - especially in areas where rainfall is irregular. So it makes sense to store rainwater in cisterns.

But laying underground cisterns in a city like Frankfurt is a challenge, because a wide variety of supply lines for water, gas, electricity, communications and much more already run under the sidewalks.

There is, however, an unused space: the space under the roadways. There are usually no longitudinal pipelines underneath the roadways, only the sewer, if any, and this is usually several meters deep underground.

Lines are rarely laid under roadways for good reason: Otherwise, traffic would have to be closed in the event of malfunctions or maintenance work on the lines. The low-maintenance cisterns, on the other hand, can be accommodated here in the course of the construction work for the Frankfurt Bridges and can be accessed by connecting to maintenance shafts in the pedestrian area.

Planned are cisterns with a diameter of 2 to 2.5 meters

Accordingly, at least two of these water tanks can usually be placed next to each other underneath multi-lane roadways if no other pipes are laid there or run across the road.

The cisterns, however, are merely a temporary storage facility

The length of the cisternsrows varies - depending on the space available - and can be between 20 and 200 meters. Several cisterns can also be connected to each other via pipelines if desired.

The planned capacity of the cisterns under the course of the bridge is about 90,000 cubic meters over the entire length of the bridge, because it is not possible to put cisterns in the ground everywhere.

While this storage capacity is not enough for the 600,000 cubic meters of rain harvesting planned, they serve as the closest temporary storage after heavy rain events. For safety, cisterns can be equipped with an emergency overflow into the existing sewer or into the ground.

As soon as a certain filling of a cistern is reached, it conveys its water to the ring main, which brings it to the storage locations, and is thus emptied again for new rainwater.

However, each road section is different - so the cisterns cannot simply be placed schematically in the ground everywhere the bridges run

Depending on the course of the road and the existing infrastructure in the ground (intersection sections, for example, are often very full of pipes), cisterns with a diameter of 2 m or 2.50 m can be installed. To store the annual precipitation volume of an example section (max scenario for 4-lane roadway), the following dimensions are required mathematically: If a cistern diameter of only two meters is chosen, the length of the cistern row-string must be 72 meters; if a diameter of 2.50 meters is chosen, the same volume can be stored with a cistern string of only 46 meters in length.

In the example on Kennedyallee, nine long and four shorter cistern row-strings with different diameters but a uniform capacity of 225 m each3 can be installed in a section of road approximately 600 meters long, taking into account the location of pipes. This corresponds to a total cistern row-string length of 832 meters and a storage volume of around 3000 m3 of water.

Other potential areas for the installation of cisterns are to be defined in the course of further planning.

Repairs and maintenance are carried out via inspection shafts

Each cistern requires an access for revisions. This is located above the road surface. The exact location along the cistern string is freely selectable.

In addition, each cistern requires connection to a pump to pump water into the ring main. This is located together with a filter in a pump shaft, which should be placed outside the roadway if possible, in order to be better accessible. The shaft diameter for the pump connection is 1.50 m, and depending on the situation, up to four cistern row-strings can be connected to such a shaft.

For the inlet pipes from the bridges as well as from other roof areas along the bridges, the connection points to the cisterns can be freely selected or adapted to the local conditions.

Google Earth / Stiftung Altes Neuland Frankfurt GNU

Experts on the Topic

Berliner Regenwasseragentur - Rheinisch-Westfälische Technische Hochschule Aachen Institut für Siedlungswasserwirtschaft Bereich Siedlungsentwässerung - Technische Universität Kaiserslautern Institut Wasser Infrastruktur Ressourcen Fachgebiete Siedlungswasserwirtschaft - Technische Universität Berlin Institut für Bauingenieurwesen FG Siedlungswasserwirtschaft

Drainage of buildings and sealed areas plays a major role in Frankfurt: For building owners with large areas, connection to the rainwater cisterns of the Frankfurt Bridges is attractive

Currently, the monetary incentive to unseal one's property or to green one's roof in order to save precipitation water charges is rather low: only 50 cents per square meter per year cost the owner of a property sealed land areas.

For example, for a DIY store with a roof area of 6,000 m2 , this means a payment of € 3,000 per year for the rainwater on the roof. No problem for a DIY store. They would rather pay that than invest in a much more costly green roof.

To drain the roof rainwater into a cistern in the street, on the other hand, is much more attractive, especially if the costs for the only important investment for this are borne by the bridge operating company: the separation of the wastewater pipe and the rainwater pipes on the property or at the building.

Once you have endured this construction measure as an owner and/or tenant, you can save thousands of euros and at the same time you have done something good for the city as a company - a marketing effect for free!

A 6,000-square-meter DIY store with a corresponding parking lot can save around 70,000 euros over ten years - and offer its customers significantly more convenience in parking

For the example DIY store with a 6,000 m² roof area, the possibility of connecting to the bridge cisterns not only means €30,000 saved over 10 years: Rather, there is also the offer from the bridge company to equip the DIY store parking lot with photovoltaic roofing, the rainwater from which can also flow into the cisterns.

For a parking lot of, say, 8,000 square meters, that's savings of another €40,000 over 10 years. This means that it is slowly starting to become attractive even for a large DIY store.

What's more, the parking lot canopies protect DIY store customers with cars from rain and snow or mean they no longer have to climb into blazing hot vehicles in the summer - a real customer loyalty campaign for all companies that take part.

Along the bridges, roof areas and covered parking areas of about 1.2 million m2 in Frankfurt are eligible for rainwater harvesting

Publicly or semi-publicly owned buildings and company buildings with more than 1000 square meters of roof space along the bridge route were considered for rainwater harvesting. Where possible, parking lot areas were also identified to be covered to collect rainwater and install photovoltaic systems on them.

These roof areas along the bridge can collect about 625,000 m3 of rainwater per year

Based on an average rainfall of 650 l/(m² x a) in Frankfurt, the potential rain harvest can be estimated at 520 l/(m² x a). Consequently, up to about 625,000 m³/a of rainwater can be captured.

Some of the roof and parking areas are likely to be not suitable for rainwater harvesting for a variety of reasons, for example because the owners do not consent or because there are substances on the roofs that are harmful to irrigation water - adhesives on bitumen roofs in particular are a common source of pollutants.

Accordingly, the theoretically available roof area is likely to be reduced. However, even with a reduction from around 1.2 million to around 1 million m², the irrigation requirement of 600,000 m³ of water for the new green spaces being created and revitalized in Frankfurt would be covered by the rainwater from these remaining roof areas alone.

The principle of rainwater harvesting with storage in cisterns can also be applied independently of bridges to buildings with large roof areas - for example, at Frankfurt's main train station: its forecourt could be lushly planted with the rainwater from its ample roof.

The bridge area must also be drained: Less evaporation, approx. 500,000 m3 of rainwater can be harvested there and partly used directly

When it rains, the water-bearing layer under the bridge beds, known as the retention layer, is used for drainage. A large part of the roadway as well as all non-green roofs of buildings on the Frankfurt Bridges can be considered sealed surfaces. From these surfaces, much of the precipitation flows over said retention layer through downspouts on the bridge columns toward the ground. The rainwater that falls on the footpaths of the bridges is drained into the green areas on the bridges. Thus, part of the rainwater remains in the beds and is available to the plants there.

Streets covered by the bridges are no longer directly sprinkled. In the event of downpouring rain, this reduces the load on the municipal sewer system. If the precipitation load on the bridges is extremely high, however, the rainwater is also discharged from there directly onto the road on the right and left via the bridges‘ shoulder and still reaches the sewer system in parts during such extreme weather events.

Drainage of the bridge deck takes place via center green strips planted in the middle of the bridges‘s driving roads

Rain that falls on the roadways of the bridges is diverted toward the center, where it infiltrates into the green median strip that can be found in all roadways on the bridges; or else, the rainwater is also diverted into the retention layer in the center.

In part, the rain diverted from the bridges is used directly on site for the irrigation of greenery

Precipitation water is discharged from the retention layer directly into the cisterns under the streets when fully saturated. The cisterns, in turn, irrigate the greenery directly along the bridges, on the one hand, when needed; but as soon as they are full, they also release their water to the ring main, which then transports it to larger storage locations.

In areas with sufficient space next to the roads - mostly outside the city center - infiltration can also be carried out directly in swales at the roadside. Where neither a cistern is available on site nor infiltration is possible, discharge into the sewer system continues.

Conclusion: The Frankfurt Bridges make rainwater harvesting possible on a larger scale in the city

Around 35 million cubic meters of rainwater are discharged annually through the combined sewer system into the Main River via the wastewater treatment plant.

The aim is to capture as much of this as possible and reuse it. The Frankfurt Bridges are like a bypass parallel to the combined sewer system: Rainwater can be collected from adjacent roofs, temporarily stored in cisterns and transported to the final storage locations.

Such a "bypass" could also be installed in other parts of the city with numerous large roof areas.

The Frankfurt Bridges infrastructure model for rainwater harvesting is an important step towards making Frankfurt less dependent on water imports from the surrounding area.

Preapring the City for Challenges

The Bridge Ring Line

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