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The city of Athens, Greece is constructing a biological sewage treatment plant for their metropolis of five million inhabitants – pneumaticprocess technology has provided a reliable automation system for thisplant on an island close to the Greek capital.
In Greek mythology, the citizens of Athens chose Athena, the goddess of wisdom, over Poseidon as their protector – in revenge, he flooded the Attica Plain,which surrounds Athens. In view of the fact that the sewage from Athens has up to now been discharged almost without filtration into the bayof Piraeus, it was high time to try to bring the god of the sea backinto a friendly mood. Greece therefore began the construction of Europe’s largest sewage treatment plant, which will be the third largest inthe world.
Psyttalia Wastewater Treatment Plant is the name ofthis ambitious project on the island of the same name in the SaronicGulf, the bay before Athens and its harbour area of Piraeus. Sewage will be pumped through pipelines 1.5km long from the mainland to the rocky island of Psyttalia. The technical data underline the gigantic scale of the plant: The biological treatment stage will comprise 12 bioreactors with a capacity of almost 300,000m³ and a daily throughput of 1,000,000m³. Seven turbo compressors work with a rating of 2.5 megawattsand a capacity of 80,000m³/h each.
Explosion protection as standard
In order to ensure that the plant will be able to operate reliably for many years, the construction companies in this joint venture, Themeliodomi, Aktor and Athena from Greece, Giovanni Putignano & Figli from Italy and Passavant Roediger Products from Germany, chose pneumatic automation technology. “Particularly in the area of the fermentation tanks, in which the high-temperature sludge fermentation processgenerates gases such as methane and carbon dioxide, it is vital that the drives used to open and close the shut-off valves offer explosion protection,” says George Loussis, the consortium’s mechanical engineerresponsible for the construction of the plant.
“But this is notthe only reason why we chose pneumatic drives for this large plant,”continues his colleague Nicholaos Zaminos. For Zaminos as a system planner, the long service life and overload tolerance of pneumatic drivesare equally forceful arguments in favour of their use to open and close flaps and process valves.
Maintenance-free and overload-tolerant
“Pneumatic drives are maintenance-free for their entire service life. With a life of more than a million operating cycles, pneumaticdrives will outlive any process valve,” notes Zaminos. What is more,as the shut-off valves in the sewage treatment plant are actuated at irregular intervals, tolerance to overload is a not inconsiderable factor. Accumulated or dried material on the valves may mean that their breakaway torque is much higher than normal. “Pneumatic drives are ‘overload-tolerant’, while electrical drives run up against their torque limiters if required to deliver exceptionally high torque or force,” Zaminos reports from experience. With pneumatic drives, it is easy to increase the operating pressure and thus the force and torque generated.
As most process valves in sewage plants are operated in a simple open/close mode or manually, the use of pneumatic technology has made it possible to achieve a significant rationalization effect. With electrical drives, in contrast to pneumatics, it is necessary for control system planners to include monitoring functions for values such as overtemperature, torque, switching frequency and servicing and maintenance intervals, which means a vast number of inputs and outputs to wireup. An equally important factor in harsh environments such as that ofa sewage treatment plant is that pneumatics is robust and its components consist of a relatively small number of corrosion-resistant parts.
Pneumatic cylinders provide reliable control
The flow of sludge into the fermentation tanks is controlled by Copac DLP pneumaticlinear drives supplied by Festo. The Copac is ideal for the control ofgate valves. It acts directly on the valve gate and makes it possibleto bring this precisely into various positions. In total, the consortium of five construction companies will install 50 Festo Copac pneumatic cylinders to transport and control the fermentation sludge.
The transport and control system for the biogas generated in the fermentation tanks utilises 40 of Festo’s pneumatic swivel drives Copar DRD/DRE. The drives are used to produce open/close functions and, in conjunction with commercially-available position controllers, to achieve precise positioning, particularly with ball valves, stopcocks, shut-offand flow control valves. The biogas produced during the fermentationprocess in the tanks is used as an energy supply for the sewage treatment plant. The Copars also control the distribution of hot water for the heat exchangers which are used to maintain a temperature of 36°C inthe fermentation tanks to dry the sludge. The energy for this is supplied by the fermentation gas which is generated.
Other items among the diverse pneumatic equipment of the plant are multifunctional Type 03 valve terminals with standard valves, control cabinets and LDFadsorption dryers, all from Festo. The adsorption dryers prevent corrosion and wear by efficiently drying the compressed air used to drive components.
Equipped with reliable technology in a highly modernsewage treatment plant, Athens will hopefully no longer need to fearthe wrath of the moody god Poseidon. Quite the opposite – the god of the sea is sure to be impressed with the cleanliness which this plant will create in his watery kingdom.
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