Method to control a process variable

A system for controlling a process variable representing an air mass flow fed into a liquid volume inside a container including a fluid intake and a fluid discharge in a process for treating the liquid while the liquid is flowing from the fluid intake through the container to the fluid discharge, said process being conducted in the container, comprising:

• a measuring unit configured to measure values of a first measurand representing a load of one or several substances found in a volume unit of the liquid in the fluid intake, wherein the volume unit is different than the liquid volume;
• a first control unit in communication with the measuring unit and configured to receive sequentially determined values of a reference variable based on the values of the first measurand, said reference variable representing a nominal value of the air mass flow to be fed into the liquid volume;
• and
• a first FIFO memory, wherein a current value for the reference variable is determined by sequentially saving values of the first measurand, or values derived therefrom, in the first FIFO memory having a number K of logically consecutive memory locations (j=i) for saving one value, respectively, in such a way that the oldest value saved in the first FIFO memory is saved in a first memory location (j=1) and the value saved last in the first FIFO memory is saved in a final memory location (j=k);
• wherein the current value is also determined by using only the n oldest values saved in the first FIFO memory, with n being a number of values that is smaller than the number K of the values saved in the memory locations of the first FIFO memory and greater than or equal to 1;
• wherein the first control unit is also configured to sequentially determine values of a regulating variable using both values for the reference variable and sequentially determined values of the process variable, said regulating variable being a variable for an aerator that serves to supply air into the liquid volume in the container and said variable influencing the air mass flow fed into the liquid volume by means of the aerator;
• wherein the first control unit is also configured to generate a control signal for controlling the aerator to supply air into the liquid volume in the container using the values of the regulating variable.