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Sewage treatment plant modernization in difficult environments
Switzerland
PROFIBUS PA

Project
Expansion and modernization projects at sewage treatment plants often have to manage some very difficult project parameters including limited space, stringent environmental regulations, the local climate and the need to maintain uninterrupted operations. Reliable and highly flexible automation systems play a key role in meeting these objectives. Two projects in Switzerland show that PROFIBUS is a very good solution in this type of application.



Limmattal sewage treatment plant

The Limmattal sewage treatment plant is located in a nature reserve near Zürich. It is surrounded on three sides by a wildlife sanctuary and a highway forms the boundary on the fourth side. Hence, new systems have to be installed in the same place as the existing systems. Carrying out a construction project at a plant that stays in continuous operation is particularly demanding. The  modernization and expansion work involved replacement of two biological stages, a smaller bio-filtration system, a chamber filter for additional water clarification and a pre-treatment system to handle the sludge liquor.
The Arosa sewage treatment plant is located at an altitude of 1,800 meters above sea level. During peak season in the winter, the town provides accommodation for 10,000 guestsin in addition to the 2,300 residents. Given the climatic conditions at this altitude, the fluctuations in contamination levels during the winter (when conditions are already difficult for the biological processes) and the need to eliminate most of the nutrients at the process stage, the engineers had to overcome some significant challenges during the design and execution phase.

Solution
Given the nature of the project requirements and drawing on its many years of experience, the St. Gallen based electrical engineering firm B. Graf AG proposed the deployment of a Profibus/Ethernet process control system for both projects. The solution includes Profibus DP as well as Profibus PA. Digital and analogue signals from various combinations of switchgear, arrays and pilot valve control units are routed through distributed peripheral nodes to the controller. The same applies to data communications with the frequency converters. Profibus PA links field process instrumentation (level and flow meters, analysis systems, etc.) to the control system. Fiber-optic based Ethernet handles communications at the automation level.
   

Conclusion
Profibus definitely makes things easier for system designers and users. It significantly reduces the amount of effort which is needed during the engineering and project execution phase, and it makes it much simpler to expand the system and install new field devices.  With this approach, it was possible to progress step-by-step through the transition from the old control system to the new computer-based solution.