The type PRATP is characterized by a sludge extraction system of suction type and is preferred if the waters have high sludge volumes. It consists of a mobile girder used as a walkway with a length which is 1 + 1/3 of the clarifier radius, a slewing ring unit, a central deflector, a traction trolley, a set of sludge scraper blades, a suitable sludge suction system with recovery tank and siphon, a scum box, a skimming blade, a peripheral deflector and a notch weir.
The slewing ring unit is placed in the middle of the tank, the traction trolley and the drive unit are instead located at the periphery. The sewage inlet is central, the flow is adequately deviated by the deflector in order to optimize the sedimentation of the sludge which precipitates to the bottom and is gathered by the sludge scraper blade, sucked by the specific system and conveyed to the recovery tank. From here the sludge is then extracted by a suction siphon activated by an electric aspirator.
The electric aspirator has the only function of putting the siphon into operation, after that, the activation is automatic.