Excessive DO and High Blower Demand

1Symptom

DO in some or all zones runs well above setpoint (for example 4 to 6 mg/L at the end of the basin) and blower power is high for the load. Pin floc or poor denitrification may show up as side effects.

2Check first

1. Compare DO by zone and time of day with the setpoints.
2. Check minimum airflow per diffuser: many systems must run a minimum air rate for mixing and diffuser health, which can push DO high at low load.
3. Check valve positions. With most open valve control, one valve should be near open; if all valves are pinched, header pressure is too high.
4. Check whether the blowers are running at a turndown limit.

3Possible causes

• Air needed for mixing exceeds air needed for oxygen at low load.
• Poor DO control tuning or DO setpoints set higher than needed.
• Too many blowers in service, or a blower that cannot turn down far enough.
• Low load periods (nights, weekends) with fixed air rates.

4Next investigation

• Calculate oxygen demand and compare the air rate with what the diffusers need for mixing.
• Trend kWh per pound of BOD or ammonia removed as a performance metric.
• Look at whether nitrate in recycled flows carries DO into anoxic zones.

5Corrective actions

1. Lower DO setpoints step by step where ammonia stays low (many plants run 1.0 to 2.0 mg/L).
2. Consider ammonia based aeration control so air follows load.
3. Tune or enable most open valve pressure control.
4. Take a zone off aeration and use intermittent aeration or mixers if the design allows.
5. Review blower sizing; a smaller blower for low flow seasons can save a lot.