It is sensible to make use of groundwater from excavation pits in urban areas

Like most German cities, Frankfurt is blessed with sufficient quantities of groundwater and rainwater. For this reason, construction projects in the inner-city area of Frankfurt come up against the groundwater relatively quickly. This is currently fed into the Main River through specially laid pipes across the city: at times up to two million cubic meters per year. Using comparatively simple treatment methods, however, this water can be purified into irrigation water and fed into the ring main at the nearest point, so that it can be made available to green areas in need of water.

Content: Use of excavation pit groundwater for irrigation of urban greenery

In Frankfurt, groundwater from excavation pits represents an untapped potential of up to two million cubic meters of water per year for city-owned irrigation. Until now, these enormous quantities of water have simply been discharged into the Main River or other nearby watercourses.

With the help of the Frankfurt Bridges, this water potential is to be used for the irrigation of the city greenery.

However, before excavation pit groundwater can be fed into the water system of the Frankfurt Bridges and subsequently used as irrigation water, it must first be thoroughly treated.

Groundwater from large excavation pits: an untapped potential for city-owned irrigation

Anyone driving through Frankfurt can see it: There is a lot of construction going on here. Apartment buildings, high-rises, underground parking garages and even subway shafts. Construction projects of this magnitude require digging deep into the ground. Groundwater often collects in the resulting excavation pits, because in Frankfurt it is usually found at a depth of only 3 to 7 meters, depending on the location.

This groundwater must be pumped out during a construction project to keep the excavation pit dry. In Frankfurt, experience shows that this involves 400,000 to two million cubic meters of excavation pit groundwater per year. Currently, this water is discharged unused into the Main River. In view of falling groundwater levels, this is a waste that a city like Frankfurt will no longer be able to afford in the future.

Since most of the larger construction projects are located in the inner city area of Frankfurt, these construction sites can be connected to the Frankfurt Bridges‘ ring line with relatively little effort.

The colorful pipes transport groundwater from construction sites to the Main River

For drinking water treatment, the excavation groundwater is too heavily polluted and, above all, often contaminated with pollutants. This is the legacy of a time when pollutants were handled much more thoughtlessly than today and some pollutants were released into the soil. As irrigation water, however, excavation groundwater can be used with little effort.

Discharging groundwater from large construction sites so that it can be stored as irrigation water involves some effort on the part of real estate builders

Large construction sites are a real challenge - watering flowerbeds is understandably not usually high on the agenda for real estate builders.

Regulatory requirements and complexity of large construction sites are already hard to manage- yet another requirement from the municipality about groundwater treatment will therefore not meet with great approval - but it is possible in principle.

Groundwater from construction sites usually has to be treated before it can be used as irrigation water - or even be discharged into the Main River

The quality of groundwater varies greatly: in some areas of Frankfurt it is heavily polluted by former industry, while in other places it is relatively clean.

The groundwater does not rest like a lake under Frankfurt, but flows in so-called aquifers.

Some of these aquifers are interconnected, but some are not, so there may be differences in quality at different locations.

In addition, if groundwater is pumped at a construction site, there is often contamination from site operations.

Groundwater from excavation pits must be thoroughly treated - including with flat-bottom aerators

The plant for deferrization and demanganization for 50m3 /h is shown below as an example. These plants can be supplemented as required, i.e. their performance can be scaled. The filter dimensioning and the selection of the filter material can only be made for each construction site after a detailed water analysis. The filter systems are installed in a container.

Experts on this Topic

Helmholtz-Zentrum für Umweltforschung Department Hydrogeologie - Kompetenz Wasser - Hochschule Koblenz Landau - Technische Universität München Lehrstuhl für Siedlungswasserwirtschaft - Uni Stuttgart Lehrstuhl für Siedlungswasserwirtschaft und Wasserrecycling - Universität Bonn Institut für Geowissenschaften AG Hydrogeologie

Along the bridges, there are special purification containers for excavation groundwater: This allows the water to be treated, transferred and stored directly after extraction at the construction site

Google Earth / Stiftung Altes Neuland Frankfurt GNU

For large construction sites in the extended urban area, mobile "tailor-made" cleaning containers are installed - whereas in inner city locations, which are always characterized by construction activity, they can be permanently installed

The permanently installed cleaning containers for water treatment are all planted with greenery

Containers can also be sunken into the ground, so that they are hardly noticeable in the respective landscape.

Conclusion: Groundwater that accumulates at construction sites can be returned to the natural water balance in Frankfurt

Up to two million cubic meters of groundwater have to be pumped out every year for major construction sites in Frankfurt.

Instead of discharging it into the Main River as before, the infrastructure of the Frankfurt Bridges makes it possible to treat this groundwater and infiltrate it back into the groundwater near the city.

The groundwater reserves thus obtained can be used for intensive irrigation of the urban greenery: Valuable water is thus preserved in Frankfurt'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

The city of the future wastes no water

Unsealing of the city center

Vitalization of the urban green

The green metropolis of the future