Low DO in the Aeration Basin

1Symptom

Dissolved oxygen in one or more aeration zones runs below setpoint (often below 1 mg/L) for hours at a time, usually at the afternoon or evening peak, or in the first zone of a plug flow basin. Blowers may be near full output.

2Check first

1. Verify the DO probe: clean it, compare to a calibrated handheld meter, and check the probe location.
2. Check blower output and header pressure. Are all blowers available? Is any valve stuck?
3. Look for diffuser problems: boils, coarse bubbling, or a rising header pressure over months (fouling).
4. Check the loading: influent BOD, ammonia, return streams such as centrate, and hauled waste.
5. Check MLSS. Higher MLSS increases oxygen demand from endogenous respiration.

3Possible causes

• Aeration capacity limited: blower out of service, fouled diffusers, broken header or leaking piping.
• Load higher than design, or a slug load.
• Centrate or filtrate return at the same time as the daily peak.
• Control problem: DO setpoint, valve tuning, most open valve logic or ammonia based control limits.
• High temperature (lower oxygen saturation).

4Next investigation

• Calculate the oxygen requirement and compare it with available air.
• Plot DO and airflow per grid on the same chart to see which grid limits.
• Schedule dewatering so centrate returns at low load, if possible.

5Corrective actions

1. Restore blower capacity and fix leaks. At White River WRRF, one blower out for a week dropped average DO to about 1.0 mg/L and started low DO filaments.
2. Shift air to the starved grid; check valve positions.
3. Reduce MLSS gradually if it is higher than needed.
4. Clean or replace fouled diffusers based on header pressure and a dynamic wet pressure test.
5. Equalize or meter return streams and hauled waste.