In wastewater treatment systems, dissolved oxygen (DO) in the aeration tank is a critical operating parameter.
Proper DO control directly affects treatment efficiency, effluent quality, and system stability.
If DO is not controlled well, problems will appear quickly.
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Mild cases: treatment efficiency decreases
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Severe cases: effluent quality fails to meet discharge standards ⚠️
Why Is Dissolved Oxygen Control So Important?
DO levels directly influence microbial activity and sludge performance.
When DO Is Too Low
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Microbial activity drops
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COD and ammonia removal efficiency decreases
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Risk of denitrification in the aeration tank
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Possible anaerobic sludge decay
When DO Is Too High
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Sludge ages too fast and becomes fragile
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Flocs break easily
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Sludge-water separation becomes poor
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Nitrogen and phosphorus removal conditions are damaged
Three Key Factors That Determine DO Setpoints
DO setpoints should never be fixed.
They must be adjusted based on real operating conditions.
1. Influent Water Quality
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High COD concentration → higher oxygen demand
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Fluctuating influent quality → DO must be adjusted dynamically
Stable DO control requires close attention to incoming wastewater changes.
2. Sludge Concentration (MLSS)
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Higher MLSS → higher oxygen consumption
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Recommended sludge concentration: 3000–5000 mg/L
Keeping MLSS within a reasonable range helps maintain stable DO control and efficient aeration.
3. Hydraulic Load
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Large flow rate or high velocity
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Shorter hydraulic retention time
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DO level tends to drop quickly
In this case, stronger aeration is required to maintain sufficient dissolved oxygen. 💧
Common Dissolved Oxygen Control Methods
Different plants use different DO control strategies based on size and automation level.
Constant Aeration
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Simple operation
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Common in small wastewater treatment plants
Limitations:
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Cannot respond to water quality changes
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Low energy efficiency
Constant DO Control
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Recommended DO in aeration tanks: 2–4 mg/L
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DO at the end of the tank: 1.5–2 mg/L
This helps prevent anaerobic conditions in the secondary clarifier.
Dynamic DO Adjustment (Recommended)
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Use real-time monitoring data
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Influent COD
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Effluent ammonia
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Automatically adjust aeration intensity
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Achieve better energy efficiency and process stability ✅
Conclusion
Proper dissolved oxygen control is essential for stable and efficient wastewater treatment.
DO levels should always be adjusted based on:
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Influent water quality
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Sludge concentration
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Hydraulic load
Avoid using one fixed DO setting for all conditions.
Flexible and dynamic DO control delivers better nitrogen and phosphorus removal, lower energy consumption, and more reliable effluent quality.
