system performance the aerobic phase is then completed and the air from the air blower 5 is turned off through signals from the analog output module 16 of plc 13 to the blower speed controller 17 the airflow rate e g volume of air introduced per time during the aerobic treatment phase is controlled as follows during the aerobic phase the do level is sampled by the plc 13 once per minute and a running average do value is calculated based on these samples at the end of the aeration phase the resulting average do along with the configured do set point value is input to a proportional integral derivative pid algorithm in the plc 13 that is configured to run a single iteration the pid algorithm is an industry standard formula that is built in to the plc 13 and many other control systems for closed loop control each pid implementation varies due to the tuning of the pid control parameters performed by the system engineer prior to system startup the pid algorithm calculates a new output value that is used to set the airflow rate for the next cycle of the aerobic treatment phase this process repeats once each aerobic phase the plc 13 sets the airflow rate by outputting a value from the analog output module 16 to the blower speed controller 17 the airflow rate is adjusted just once during each treatment cycle in between aeration phases this once per cycle or cyclic airflow rate control is essential to the real time control process as it allows airflow rate control to operate without interfering with the detection of the do curve bending point in contrast if the airflow rate were to be adjusted during the aerobic phase to achieve a do.