Aeration Control

Why aeration control matters

Blowers supplying air to activated sludge basins consume half or more of the electricity at a typical wastewater plant. The air is needed: the bacteria that oxidize organic matter and ammonia need dissolved oxygen, and too little means poor treatment. But the demand varies through the day and the year with the load, and a plant that runs its blowers at a fixed output supplies the peak demand all the time. Aeration control matches the air to the demand, which typically cuts blower energy by a quarter to a half, and it does so with a chain of loops and instruments that has to be designed as a whole.

The cascade

1. 1

   DO control per zone

   A DO sensor in each aeration zone feeds a slow PID controller whose output is an airflow setpoint for that zone. The loop is slow, minutes, because the basin responds slowly and the sensor has a lag; the dissolved oxygen page covers the sensor placement and cleaning.

2. 2

   Airflow control per zone

   An airflow meter on the drop pipe to each zone feeds a faster PID that positions the zone control valve to hold the airflow setpoint. This inner loop removes the nonlinearity of the valve and the effect of header pressure changes from the DO loop.

3. 3

   Header pressure control

   A pressure transmitter on the main air header feeds a controller that sets the blower output, by guide vane, inlet valve, or speed, to hold a header pressure setpoint. The blowers deliver whatever the valves are taking.

4. 4

   Most-open-valve

   The header pressure setpoint is not fixed. A supervisory routine watches the zone valve positions and lowers the pressure setpoint until the most-open valve is near a target, typically 80 to 90 percent open, and raises it when a valve reaches fully open. Lower header pressure is less blower work, and the routine keeps the pressure at the minimum that still lets every zone get its air.

5. 5

   Blower staging

   When the running blowers reach their maximum, another is started; when they fall to their minimum turndown, one is stopped, with delays and a minimum time between staging events. The staging also respects which blowers are available and rotates them.

Blowers

Centrifugal blowers surge when the flow falls below a limit at a given pressure: the flow reverses momentarily, the machine shudders, and repeated surge damages it. The blower package controller enforces a surge line and will open a blow-off valve or shut down rather than cross it, which is the right behavior and which the plant control must anticipate. Staging down a blower before the running ones reach their turndown floor, and never asking the header pressure loop for a pressure the blowers cannot make at low flow, keeps the plant out of the surge protection.

Ammonia-based aeration control

Holding a fixed DO setpoint aerates for the design load at all times. The actual oxygen demand follows the ammonia and organic load, which varies through the day, and a plant that nitrifies needs enough DO to finish ammonia oxidation and no more. Ammonia-based aeration control puts an ammonia analyzer at the end of the aerobic zone, or at a point along it, and uses it to trim the DO setpoint: when ammonia is low, the load is met and the DO setpoint drops toward a floor; when ammonia rises, the DO setpoint rises toward a ceiling. The ammonia loop is slower still, tens of minutes to hours, and it sits above the DO loop as another cascade level. The savings are real, often another 10 to 20 percent of blower energy, and the cost is an ammonia analyzer that needs the same care as any wet chemistry instrument.

Instruments the scheme depends on

• DO sensors per zone, optical, with automatic cleaning, placed to represent the zone and checked against a portable meter.
• Airflow meters per zone, thermal mass or differential pressure, on straight pipe runs with the drop pipe geometry the meter needs, calibrated for the air temperature and pressure.
• Header pressure transmitter, with a range that resolves the fractions of a psi the most-open-valve logic moves it by.
• Zone valve position feedback, actual, from the actuator, not the command.
• Blower package status: running, available, output, inlet vane or speed position, surge state, from the package controller over a network.
• Ammonia analyzer where used, with its sample system and calibration schedule.
• Basin level and temperature, for the DO saturation correction and for the diffuser depth.

Failure handling

DO sensor invalid

The zone drops to a fixed airflow setpoint, the last good value or a configured default, and alarms. A DO reading stuck low would otherwise drive the zone to full air.

Airflow meter invalid

The zone valve holds position, or goes to a default, and the DO loop is suspended for the zone.

Header pressure invalid

The blowers hold their output, the staging is suspended, and the plant runs on the last known state until someone looks.

Blower fault

The staging logic removes it, starts the next available, and alarms. A plant with one blower short in summer is a plant that will fall behind on DO in the afternoon; the loss of standby is its own alarm.

Communication loss to the blower package

The blower keeps running on its own controller at its last setpoint; the plant alarms.