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An extension of PID control is cascade control. Cascade control uses two control loops to control a process: an inner “slave” control loop that has a physical controlled variable and an outer “master” control loop that does not. The controlled variable of the master controller is the setpoint of the slave controller. The slave controller must have a response time that is faster than the master controller so that the setpoint of the slave control loop does not change too fast and cause instability.

A common use of cascade control in wastewater treatment is when analytical instruments are used to control a process. In general, the analytical signal has a slower response time than a flow or pressure signal. In this instance, the flow or pressure can be used as the process variable of the slave loop for controlling those disturbances quickly. The analytical reading can be used as the process variable of the master to control the setpoint of the flow or pressure slave loop.