Underwater wastewater piping in alpine lake

The Traunsee is considered one of the purest lakes in Austria. To keep it that way, a 47-year-old sewer pipe was replaced by a modern underwater pressure line. The community of Altmünster played it safe and decided on AGRULINE HDPE 100-RC pipes with protective layer of OD 14 in (355 mm) in SDR 17. Except for the project planning by civil engineering office DI Putre, which is based in the province of Salzburg, the complete construction project is “made in Upper Austria”. From AGRU, the pipe supplier, to RBS Rohrbau-Schweißtechnik GmbH, the company responsible for the welding operations, and PEER Wasserbau GmbH & Co. KG, the hydraulic engineering company, all participants originate from the province of Upper Austria.

47-year-old sewer line in alpine lake

The community of Traunkirchen and the market town of Altmünster are home to 17,000 people. Its sewage, about 35 ft³ ( 1,500 m³) daily, is directed to the sewage treatment plant of the Traunsee north clean water treatment plant in the city of Gmunden via a common sea-pressure pipeline in the Traunsee lake. Already when it was built in 1970, two sewage pipes OD 12 in (300 mm) had been laid side by side in the lake over a length of 1.4 mi (2.3 km) in order to be able to maintain operation at least with one pipe in the incident of any rupture. The wastewater is pumped alternately through one of the two pipelines every day. This ensures that the abrasion in both pipes runs evenly. In the first few hundred meters of the PE pipeline, pump pressure is built up several times a day to set the several mile long water column in motion. Specifically, the beginning of the line had been stressed for several decades. Therefore, a replacement of the old pipe was decided by a new, 10548 ft (3215 m) long double sewage pressure pipe. Two parallel laid PE 100-RC pipelines were installed in two phases. The project started in 2015/2016 with the new installation of AGRU PE 100-RC pipes with protective layer in the eastern, 2625 ft (800 m) long section.  After the two sewer pipes were welded from 40 ft (12 m) long AGRU Sureline III pipes with protective layer, two 2625 ft (800 m) long pressure pipelines were stored in the water along the river until they were installed.

Horizontal directional drilling solves sewer gas problem

So that sewer gases can escape at both ends of a submerged sewer line through special ventilation valves and a buoyancy of the plastic pipe is prevented by gas cushion, a modern sea pressure line must be laid in a steady gradient. In this case, a wash-boring in the area of the sea outlet was required to lay the maritime pressure line in an even gradient. BS Rohrbau-Schweißtechnik GmbH performed this wash-boring with high precision. In addition to the accurate drilling curve, a major challenge was the anchoring and bracing of a pontoon floating on the water surface. On this platform, the horizontal directional drilling system was installed. The pontoon had to withstand a pulling force of 40 tons and was correspondingly secured by means of ropes from the bridge piers and from the shore.

After completing the horizontal directional drilling operations, the sewer pipe was pulled into the underwater tunnel with several tons of traction. Of course, the pipeline is heavily stressed in such a power act. Therefore, crack resistant PE 100-RC pipes with protective layer from the Upper Austrian plastics manufacturer AGRU were used, which were developed precisely for these extreme applications. They offer double protection: on the one hand, they are made of the extremely point-load- and crack-resistant material PE 100-RC, and on the other hand they have an extra protective layer made of scratch-resistant polypropylene. The complete sewer pipeline was laid without protective sand bed directly on the bottom of the lake. The protective layer pipes absorb the point loads caused by stones without any problem.

Hydrographic surveying of the lake floor

Now the installation around the Toscana peninsula is about to begin. While the old sewage pipe in this area has been laid onshore, the new maritime pressure pipeline will be placed on the lake bottom by means of concrete weights. The AGRULINE PE 100-RC pipes with protective layer in OD 14 in (355 mm) and in SDR 17 were butt-welded on site by RBS Rohrbau-Schweißtechnik GmbH and temporarily stored at the water surface of the lake. In March 2018 they were towed in the planned route and ballasted with concrete elements, which are clamped every three meters around the pipe. Lake water is then pumped into the pipes from one side, allowing the air to escape in a controlled manner on the opposite. The PE 100-RC pipes are carefully lowered to the bottom of the lake in an S-shaped curve. The pipeline route runs gradually from the water surface to a depth of 66 ft (20 m). Starting from this low point of the pipeline, the two pipes then reach the surface again along the route. Thus, digester gases can escape at both ends of the line through special ventilation valves, a buoyancy of the line by gas cushion is prevented. In order to create an accurate elevation profile of the underwater landscape, the seabed in this area was measured three-dimensionally. This showed that the eastern slope of the Toscana peninsula drops off almost vertically under water. In this steep slope, the double line is to prevent from slipping into great depths. Therefore the experienced divers of Peer Wasserbau KG have to permanently anchor the maritime pressure pipeline in the slope.

Protective layer pipes for the next generations

Josef Leidinger is a water engineer and manager of the Altmünster sewer department. He is particularly interested in the crystal-clear lake Traunsee. Known as a jewel of nature and a bathing lake beyond the national borders, no deterioration in the quality of the water due to pipe bursts or leaks may take place in future generations. “I selected AGRU’s PE 100-RC pipes with protective layer because I think they are the safest alternative on the market. If you pump wastewater through drinking water, you cannot compromise. I think in generations, not legislature periods. Our polyethylene PE 80 waste water piping system, installed in 1970, has suffered a few scratches over the decades and has been in use for 47 years despite incidents. Thanks to the much more robust and crack-resistant HDPE 100-RC, the computer-controlled welding technology and last but not least the additional scratch-resistant PP protective layer, that AGRU pipeline will be in service for the next 100 years without any problem. The benefits of greater operational safety, longer service life and better protection of our drinking water reservoir Traunsee far exceed the slight additional cost of pipes with protective layer. On behalf of Altmünster, I would also like to thank the civil engineering office DI Putre for the excellent planning of the maritime pressure line.”

Automatic alarm in case of leakage

In the course of the work on the renewal of the existing sewage pressure pipeline in the first section, a measuring and inspection shaft was set up at the outfall, and a pipe burst monitoring system using IDM was provided via a wireless remote control system. There is a permanent comparison measurement between incoming and outgoing water volumes at the beginning and at the end of the sewage pipeline. In the event of non-compliance, an alarm is automatically sent to the head office of the pipeline operator! This represents a special contribution to the required operational safety as well as to an effective water protection in the Traunsee lake.