The Les Grésillons sewage treatment plant (STP), located on the northern outskirts of Paris, currently serves some 300,000 people and processes 100,000m3 per day. This figure is set to triple by the time expansion is complete in 2015; it will purify the sewage produced by one million people in the greater Paris area and return it to the river Seine. Les Grésillons differs from conventional sewage plants with biological purification stages in that it uses special filter technology, rather than bacterial activation tanks. After the mechanical purification stage using sand filters and screening units, the pre-treated sewage passes through three levels containing biofilters, where the nitrogen is removed from the sewage. The supply to the 28 biofilters, as well as the backwashing process, is controlled by process gates (shut-off or butterfly valves) using automated pneumatic actuators. Festo developed a special Copac DLP linear actuator with a through-piston rod for the 28 Fontaine shut-off valves, which are not housed in order to facilitate the use of a handwheel in emergencies. Each shut-off valve is equipped with two proximity sensors for sensing the mechanical end stops and transmitting the information to the controller. Decentralised CPX/MPA valve terminals actuate the drives and record all data relating to the filtration tanks. Each tank communicates with the centralised Allen Bradley controller via the DeviceNet fieldbus interface of its CPX terminal. The CPX terminals also include Festo PLCs for managing the control circuits locally and ensuring safety positions. Diagnostic functionality enables the CPX terminals to quickly detect any faults in the plant. A further 70 filter systems are due to be installed at the Les Grésillons plant by 2012.
