A test instrument proves its worth: 80 GHz solids level radar solves a difficult ash silo level measurement.

The sewage treatment plant Steinhäule of the ZVK (Sewage Treatment Plant Association) in Neu-Ulm is one of the most modern in Baden-Württemberg, and every plant component is inter-connected. Care is taken to ensure that waste heat is reused, raw materials saved and residual materials recycled. The plant cleans the wastewater generated by about 440,000 inhabitants and population equivalents, the other half of it is industrial wastewater. It is also one of only a few sewage treatment plants to have a fourth cleaning and treatment stage, in which mainly non-degradable substances such as hormones, drug residues, biocides, etc. are removed from the wastewater. These pollutants are filtered out with the help of activated carbon particles.

About one million cubic meters of sewage sludge are produced annually in the sewage treatment process. How can this be disposed of at low cost? The sludge is first dewatered. Then, the approx. 10,000 tonnes of dried sewage sludge that are produced annually are burned in a fluidized bed furnace at a temperature of approx. 720° C which reduces it to around 2,500 tonnes of ash. The resulting steam is used to dry sludge as well as generate electricity via a steam turbine. The ash itself is stored in four steel silos with a height of about 10 m. The silos are connected to one another via screw conveyors and are filled one after the other.

And this is exactly where the problematic measuring situation existed. Level in the silos is the basis for logistics planning for the trucks that transport the ash to other locations. The ash and filter dust are used as an aggregate substitute and resold to the building materials and  mining industries. Other use: since 99% of the phosphorus is eliminated via the absorption and filtration system, this fraction remains in the ash after thermal utilization (so what is it used for?). The fine, powdery ash, leads to extreme dust generation, making life extremely difficult for the level sensor previously used. Especially during the filling and emptying of the silos, the very intense dust generation caused jumps or fluctuations in the readings. What made it particularly difficult was that the measurement error was sporadic, so it could not be relied upon. Once because a sensor had erroneously indicated that the silo was full, a truck had to leave the facility only half filled with ash.

In June 2014, the service technician suggested replacing the old sensor with a VEGAPULS 69, which was then installed in the ash silo with the highest throughput. Thanks to the excellent focusing and wide dynamic range of VEGAPULS 69, an absolutely reliable measurement is now possible during all phases of operation. The level sensor has now proven its reliability and accuracy in applications with extreme dust generation. In order to evaluate the performance of the new sensor, the measured values from all four ash silos were recorded in the process control system, then compared and analysed. The evaluation of the trend record of the comparison between the old and the new measurements was so positive that the test instrument quickly became part of the plant’s permanent instrumentation. The measured value jumps and fluctuations have disappeared and the silo level has always been correctly indicated since the change. The ash transport vehicles can now be filled as required, without errors or delays. No more half-filled trucks have left the facility.