Digesters in Dueren’s Sewage Plant Post-Tensioned using Prefabricated Tendons

Context

Since 1974, municipal and industrial sewage has been treated in a single sewage plant owned by the Eifel-Rur water board (WVER). In 2007, this sewage treatment plant processed approximately 24 mio.m³ of wastewater.

In 2003, the water board decided to modify its process to include the digestion of sewage sludge. Thanks to digestion, the volume of sludge is reduced by approx. 30%, and the sludge is stabilized, which considerably facilitates its disposal. In addition, the sewage plant can cover approx. 70% of its energy demand with gases such as methane, which are created during anaerobic sludge stabilization. Construction began in 2008 and was completed in the first quarter of 2010.

Anaerobic sludge stabilization takes place in three new 6,000m³ digesters, which can be operated individually or simultaneously. The digesters will produce an annual volume of approx. 5 mio.m³ of gas, which roughly equals the power requirement of 3,000 households and the heat energy necessary for 1,000 households.

Solution

An 8m deep excavation was needed for the construction of the digesters. A sheet pile wall was embedded in the excavation in order to make it watertight. The pump and coagulation buildings necessary for the construction of the digester plant were constructed using the cast-in-place method, thus creating a “white tank”.

The three digesters are oval because this shape is ideal for an even mixing of the sludge, which is thoroughly circulated in the three towers. The digesters’ outer shell was post-tensioned using prefabricated SUSPA Systems Tendons. On the whole, DYWIDAG supplied 108t of prefabricated SUSPA Tendons with 6-2 to 6-9 strands and the necessary accessories such as anchorages and anchor plates. Thanks to the technical assistance DYWIDAG employees provided, tensioning work could be successfully completed on schedule.