controller in order to regulate the rate of recirculation most accurately reference is now made to fig3 and 4 of the appended drawings illustrating schematically the structure of the automatic logic tool used in the process and the apparatus forming the subject matter of the invention the automated recirculation management logic tool according to the invention is composed of three main modules served by a common signal input stage the module a is an interface between user and controller which allows manual information about measurements and desired settings verification of compatibility between process constraints and desired settings and display of the operating data and information coming from the controller the module b is the unit for controlling the rates of recirculation it is composed of two control logic tools b 1 and b 2 activated according to the availability of the information sources and of a common mode of carrying out the actions b 3 for controlling the recirculation equipment the control logic tool b 1 operates in a regulation mode its objective is to maintain the recirculation concentration c r to within u00b1 1 g l about the setpoint value so as to guarantee the retention time of the sludge undergoing clarification and to facilitate the management of the extractions the measurement of the sludge concentration in the recirculation line is used here to control and regulate the rate of recirculation q r this regulation.