Sewer made with double containment piping system

The great Brombachsee is located about 40 km south of Nuremberg. Inaugurated in 2000, the lake is one of the largest dams in Germany, both in terms of surface area and storage volume. Due to the rising number of tourists, in February 2018, 20 floating houses were built on the shores of the lake. In addition to various other equipment options, each house has a well-coordinated sanitary installation. The sewage pipes were welded from the double containment AGRU Poly-Flo piping system, made of PE 100-RC and delivered by Frank GmbH.

The wastewater is safely transported to a collecting tank on the mainland via a wastewater pressure pipe laid in Brombachsee. From here, the collected wastewater is forwarded to the sewage treatment plant. The construction and operation of sewers in water catchment areas such as the Brombachsee requires compliance with special precautions. In this zone, pressure pipes with a very high-risk potential must be double containment systems. Another challenge in the planning and implementation of the project was the difference in water levels. Because the pipes connected to the pontoons, which serve as a floating basement for the houses, reach out to the main land. Therefore, the pipeline had to be flexible enough to compensate the difference in water levels, which can be up to 8 m in height.


FRANK GmbH was able to present an economical and technical solution for all these problems with the AGRU double containment piping system "Poly-Flo" made of the thermoplastic material PE 100-RC. A double-containment pipe extruded as a mono strand, which offers high system security, characterizes the Poly-Flo piping system. Possible leaks on the inner pipe can be detected in the existing annulus by the leak detection system installed there. The outer pipe is used to restrain the present flow medium so that it cannot get into the lake. In the leakage monitoring of the double piping system, the decision was made to use an overpressure leakage system. In the overpressure process, both walls of the double-walled system are monitored for leaks. The monitoring pressure is operatively higher than any other internal or external wall applied pressure, so that a pressure drop indicates leaks.

The double wall Poly-Flo pipe system provides a high level of system safety.

In the annulus of the Poly-Flo piping system, a permanent monitoring pressure of 1.5 bar is in operation. The pipes and fittings from this extremely compact double containment piping system are completely extruded (pipes) or moulded (fittings) in one production process.

Continuously extruded webs make the alignment of the inner pipe and the outer pipe easy. An advantage of the continuously extruded webs is that the pipes can be separated at any point, if required by isometry. Due to the considerably reduced component dimensions, a simple assembly is possible even in confined spaces. The connection of the individual double pipe components was carried out by means of simultaneous welding.

Since in this process both inner and outer pipes are welded simultaneously in one-step, this was an optimal solution both economically and from a processing point of view. The double containment pipe made of PE 100-RC has a high flexibility, so that the large height difference between the transition point on the mainland and the pontoons could be overcome without additional fittings.

Why PE 100-RC?

For the material, the choice fell on PE 100-RC. The innovative material PE 100-RC (resistant to crack) has a much higher resistance to slow crack growth than PE 100. Due to this increased resistance to stress cracking, piping systems made of PE 100-RC can also be used for sand bed-free laying. In addition to the corrosion resistance to the surrounding seawater, the resistance to changing temperatures in the Brombachsee was an advantage of PE 100-RC over other materials. Since the existing flow medium sewage can also contain larger constituents of solids, the very good abrasion resistance of PE 100-RC was also decisive for the choice of the material.


The pipes, which serve as a connection sewer line between the individual houses, were made of Poly-Flo pipes PE 100-RC OD 90/63 manufactured by AGRU Kunststofftechnik. After assembly, the accumulated wastewater of all floating houses is pumped towards the main line. The main line of Poly-Flo pipe PE 100-RC 160/110, as well as the connecting lines were welded directly on site. As there is a 10 m wide public pedestrian section between lake and the wastewater reservoir on the mainland, a crossing through the soil was necessary. The so-called horizontal directional drilling method (HDD method) was applied. To allow the pipe section to sink slowly to the bottom of lake, it had to be weighted. This was done by filling the pipe with water and additionally fitting it with pipe clamps embedded in reinforced concrete. The project was realized in the previously planned period. The "Floating Houses" are already enjoying great popularity shortly after completion and provide the tourists a comfortable accommodation on the great Brombachsee.