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World's largest solar-thermal sewage sludge drying system

12 kilometre-long thick-walled steel pipe installed in the world's largest solar-thermal sewage sludge drying system A solar-thermal system, most likely the largest in the world, for the drying of sewage sludge is currently under construction on the B224 federal road in Bottrop, Germany. In line with the size of the project, the engineering required to implement it is similarly challenging in every respect – complete with the detailed planning of the building services engineering work throughout the construction progress.  


One typical example: the linking up of the many hundreds of heating coils in the 32 drying halls. Because the halls are situated on unconsolidated ground, these coils must have long-term resistance to considerable thermal, corrosive and also mechanical influences. Therefore, thick-walled steel pipe in the dimensions 1 ¼ to 4 inches (DN 32 to DN 100) was used almost in the conventional way. However, in combination with connectors of the"Megapress" (3/8 up to 2 inch) and "Megapress S XL" (2 1/2 up to 4 inch) systems from Viega, which, in typical Viega style, had been "cold" pressed, not welded.

Objective: To save energy

The wastewater treatment plant in Bottrop is one of the largest in Germany. It purifies, treats or incinerates not only the waste water produced by around 5 million people from the entire catchment area of the Emscher river, but also external sludge that is delivered to the plant. A process that is at once complex and energy-intensive – to be able to incinerate the 120.000 tonnes of sludge per year in the two furnaces in the neighbouring combined heat and power plant at approx. 450 °C, the sludge must have a defined degree of dryness. The optimum is a dry matter content of between 60 and 70 percent. Previously, this was achieved by adding around 20,000 tonnes of hard coal during the incineration process. As part of the overall "Emscher hybrid power plant" package, the solar-thermal drying system (STT) is set to reduce this quantity as far as possible in the future – relieving the burden on the environment and preserving valuable resources at the same time.

To achieve this, the sewage sludge will be pre-dried in 32 halls. Constructed like greenhouses, these halls are situated on around 61,000 square metres of consolidated ground, the former site of sewage sludge ponds. In the future, wheeled loaders will distribute the sludge to the glass halls via a central aisle according to a detailed material flow analysis. In the glass halls, the sludge dries for about 10 to 14 days under natural sunlight – regularly circulated by an automated process – setting an example in ecological terms. If there is not enough solar radiation, heating coils below the hall ceiling feed heat from the combined heat and power plant itself or from a four-stage cascading combined heat and power plant with a 10 megawatt heating capacity.

One figure in particular highlights the dimensions involved: every year, around 100.000 cubic metres of water will evaporate from the sewage sludge when the drying system is operating at full capacity. In purely arithmetical terms, this is roughly equivalent to the amount of drinking water consumed by the entire city of Frankfurt am Main every day!

Lightweight halls with the "urge to move"

"On this scale, sewage sludge drying is a major quantum leap, also compared to all previous applications, which achieved a maximum of ten percent of the output we need," explains the project manager of the chief planner Norbert Schepers from Emscher Wassertechnik GmbH. Including the detailed questions this raises for engineering, such as how the considerable thermal expansion of the lightweight constructions, and hence of course also those of the pipelines, can be absorbed with a hall length of more than 160 metres and even more than 200 metresin the central aisle. Especially if atmospheric influences in the microclimate might make these expansions "counteractive" or at least highly divergent.

In the meantime, however, more than 12 kilometres of steel pipeline have been successfully installed using the Viega "Megapress" system. On the long straight sections, of course, with appropriate compensators or remarkably wide expansion bends – at every outlet of the main distribution pipeline into the individual halls and at the connection of the heating coils, though, with a special feature – flexible "hose connections". The background: In addition to the thermal influences mentioned above, structural dynamic movements of the lightweight constructions also place the installations under strain. Fixed pipe connections would therefore be exposed to the most extreme loads. The flexible connections, however, allow structural decoupling that compensates for all shearing motions.

Pressing more economical than welding

The general contractor in Rödinghausen handling the work, IRB Industrie-Rohrbau GmbH, believes that pressing the pipe connections of the thick-walled steel pipes "cold" instead of welding them in the conventional way achieved another decisive advantage going beyond what was generally expected of the pipeline system. Especially given the quantity structures coupled with extremely high loads involved here, this approach was far more economical, says IRB construction manager Michael Puckrandt: "We spend up to 80 percent less time on each connection joint compared to welding, depending on the nominal width. Despite the higher material costs in the full-cost pricing, this makes press connecting technology a very interesting proposition for us as the company implementing the work".

And how does the customer react, especially with a project of such an unusual nature, when there is no experience to fall back on? Master heating installer Puckrandt said: "Initially, every customer expects to see the result that was ordered. No matter whether this result is achieved by welding or by pressing. Here in particular, however, we checked in advance in close consultation with all those involved, also including Viega as the manufacturer, how the zinc nickel-coated connectors would behave in this installation environment, for example, or whether the press connection would definitely withstand the sometimes immense mechanical loads". The result is more than convincing, because in addition to the robust material properties, the connectors are reliably tight, even on the steel pipes coated in accordance with AGI Q151, with comparatively little preliminary and finishing work. "Under harsher conditions too", emphasises Michael Puckrandt: "Because extremely high forces are still acting on the connections on the main line. Nonetheless, even here and under changing external influences, the Megapress system has satisfied all the requirements so far."

For the general contractor from the German region of Westphalia, this makes the installation environment, which is as extraordinary as it is difficult under operating conditions, a reference project object that ultimately enhances a skilled welder's performance with cost-efficient, safe press connecting technology, says Michael Puckrandt: "For us, this is a tangible step towards a sustainable future. Because the problem of skilled labour is already acute in the pipeline installation sector. Good and experienced welders are increasingly difficult to find".

Object Solar-thermal sewage sludge drying plant
Location Bottrop, Germany
Year 2021
Object Type New building
Area of use Industry