achieve nitrification via ammonia removal and to ensure an environment for phosphorus accumulating organisms to absorb excessive phosphorus.
Oxygen is typically supplied to the aerobic reactor via air. Air contains about 21% oxygen. This can be accomplished by using mixers to entrain air into the reactor or by using compressed air that is diffused into the reactor. Use of compressed air is significantly more common. Blowers that supply this air are typically the largest source of energy use at a WRRF. There may be several blowers available to allow for variable demand of air. Most blowers are centrifugal and their output is typically varied by using guide vanes or inlet valves. Variable-frequency drives (VFDs), which are used to vary the speed of the motor, are gaining popularity for use in centrifugal blowers and are the most common method to vary the output of a positive-displacement blower.
Oxygen is typically distributed to several reactors and, often, to specific areas within each reactor. A valve is typically used at each addition point to control the amount of air feed at that location. As the valves’ positions modulate, the pressure within the overall air header extending from the blowers is affected. Blower output is typically modulated to maintain header pressure within an acceptable range. Pressure that is too high could cause the blower to surge. In addition, the higher the pressure maintained, the more energy use of the blower.
8.1.2 Process Variables
The following are typical process variables needed for control:
• Dissolved oxygen concentration,
• Air header pressure, and
• Airflow.
8.1.3 Controlled Variables
The following are typical controlled variables used for automatic control:
• Blowers’ start–stop,
• Blower speed control,
• Blower guide vane position control, and
• Individual air valve position control.
8.1.4 Control Strategies
The main control strategy for aeration control is to vary the individual air valve’s position in each reactor or reactor area to maintain the dissolved oxygen level in that area at a setpoint. As dissolved oxygen drops below the setpoint, the valve is opened, and, as dissolved oxygen rises above the setpoint, the valve closes. In some instances, airflow to each reactor is monitored and used to modulate the individual air valve’s position. Dissolved oxygen, in this instance, is used in a cascade loop to modify the airflow setpoint.
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