Most expert systems exhibit adaptive control in which there is a slow improvement in process control which is associated with the system's learning. Waste water treatment typically undergoes embedded control. The control that is needed uses proportional, integral, and derivative functions, and thus is called a PID controller. Proportional control avoids delay in the system. Integral control eliminates offset which can be invoked by using proportional control.
Derivative control is necessary because large derivatives cause noise and thereby distort signals.
Often a mixture of PID control is necessary and the weighing for each type of control may vary during different parts of the process. Thus, using an absolute control would not adequately address this type of process because it would not give the desired steady-state output concentration. While invoking PID control, the system has the opportunity to correct itself and adjust it weighing factors with time. There are several other factors to consider when making rule based decisions. First, expert systems work locally because rules and data are applied locally.