Aeration, Mixing & Dissolved Oxygen

Aeration and mixing are central to activated-sludge treatment. Aeration supplies oxygen for microorganisms, while mixing keeps biological solids suspended and brings microorganisms into contact with wastewater. Operators must provide enough oxygen and mixing for stable treatment without wasting energy through unnecessary aeration.

Dissolved oxygen, or DO, is one of the most useful activated-sludge process indicators, but it should never be interpreted by itself. Good control requires operators to connect DO with influent loading, ammonia, MLSS, aeration equipment, basin location, temperature, and effluent performance.

Why Activated Sludge Needs Oxygen

Aerobic microorganisms use oxygen while oxidizing biodegradable material.

Oxygen is required for processes such as:

• carbonaceous BOD removal;
• nitrification;
• endogenous respiration.

Aeration Has Two Main Jobs

Aeration commonly provides:

• oxygen transfer;
• mixing.

These functions are related but not identical.

Oxygen Transfer

Oxygen transfer is the movement of oxygen from the air or another oxygen source into the wastewater.

Transfer efficiency depends on factors such as:

• aeration equipment;
• airflow;
• bubble size;
• water depth;
• wastewater characteristics;
• temperature;
• existing DO concentration.

Mixing

Mixing keeps mixed liquor moving throughout the aeration basin.

Good mixing helps:

• keep solids suspended;
• distribute oxygen;
• distribute influent load;
• prevent excessive deposition.

Poor Mixing

Insufficient mixing can create:

• solids deposition;
• septic zones;
• uneven DO;
• localized odors;
• reduced effective basin volume.

Dissolved Oxygen

Dissolved oxygen is oxygen present in water and available for biological activity.

It is commonly measured in mg/L.

DO Is a Balance

Aeration-basin DO reflects the balance between:

• oxygen supplied;
• oxygen consumed.

If oxygen demand rises while air supply remains constant, DO usually falls.

Oxygen Demand

Major sources of oxygen demand include:

• biodegradable organic matter;
• ammonia oxidation;
• biomass respiration.

Carbonaceous Oxygen Demand

Heterotrophic microorganisms consume oxygen while removing biodegradable carbonaceous material.

Higher BOD loading generally increases oxygen demand.

Nitrification Oxygen Demand

Nitrifying organisms consume oxygen while converting ammonia to more oxidized nitrogen forms.

A plant performing nitrification therefore has additional oxygen demand beyond ordinary carbonaceous BOD removal.

Biomass Respiration

Even when readily available food is limited, microorganisms continue using oxygen for maintenance and endogenous respiration.

DO Changes with Influent Loading

If influent organic loading suddenly increases:

• oxygen demand can rise;
• DO can fall;
• blower output may need to increase.

DO Changes with Ammonia Loading

Increasing ammonia load can increase oxygen demand where nitrification is occurring.

DO Changes with MLSS

A higher biomass concentration can increase total respiration and oxygen demand.

This is one reason the same airflow may produce different DO at different MLSS concentrations.

Temperature Effects

Temperature affects both:

• biological reaction rate;
• oxygen solubility.

Warmer wastewater generally supports faster biological activity while holding less dissolved oxygen at saturation.

Seasonal DO Behavior

Operators may need different aeration settings in summer and winter because:

• loading can change;
• biological activity changes;
• oxygen-transfer conditions change.

Low DO

Low DO can contribute to:

• poor BOD removal;
• reduced nitrification;
• undesirable biological growth;
• septic conditions;
• odor.

Very Low DO and Filamentous Growth

Some filamentous organisms can gain an advantage under chronically low-oxygen conditions.

Poor DO control can therefore contribute to settling problems in some activated-sludge systems.

High DO

High DO is not automatically better.

Excessive aeration can:

• waste electricity;
• increase blower wear;
• create unnecessary turbulence;
• provide little additional biological benefit.

Control to Process Need

The goal is to provide enough oxygen for treatment while avoiding unnecessary excess.

DO Profile Through the Basin

DO can vary from one end of an aeration basin to another.

Near the influent end, oxygen demand may be highest because:

• fresh wastewater enters;
• food concentration is higher;
• ammonia concentration may be higher.

Downstream DO

As biodegradable material is consumed, oxygen demand often decreases.

DO may therefore rise toward the end of the basin if air distribution is uniform.

One DO Reading May Not Represent the Basin

Operators should understand where the DO sample or probe is located.

A single probe near the basin outlet may not reveal low DO near the influent zone.

Multiple DO Measurements

A DO profile can help identify:

• high-demand zones;
• poor air distribution;
• dead zones;
• over-aerated zones.

Diffused Aeration

Diffused aeration sends air through submerged diffusers.

Typical components include:

• blowers;
• air headers;
• air control valves;
• diffusers.

Fine-Bubble Diffusers

Fine-bubble diffusers create many small bubbles.

Small bubbles provide relatively high gas-water contact area and can improve oxygen-transfer efficiency.

Coarse-Bubble Diffusers

Coarse-bubble systems generally provide stronger mixing but lower oxygen-transfer efficiency than fine-bubble systems.

Diffuser Fouling

Diffusers can become fouled by:

• biological material;
• mineral deposits;
• debris.

Fouling can increase required air pressure and reduce oxygen transfer.

Signs of Diffuser Problems

Possible indicators include:

• uneven bubble pattern;
• increasing blower discharge pressure;
• lower DO at the same airflow;
• higher energy use.

Blowers

Blowers supply air to diffused-aeration systems.

Operators should monitor:

• airflow;
• discharge pressure;
• motor current;
• temperature;
• vibration;
• alarms.

Blower Capacity

Available blower capacity must be sufficient for peak oxygen demand and mixing requirements.

Blower Discharge Pressure

Blower pressure must overcome:

• water depth;
• pipe resistance;
• valve losses;
• diffuser resistance.

Rising Blower Pressure

If airflow remains similar but blower pressure rises, investigate:

• diffuser fouling;
• closed air valves;
• header restrictions;
• instrument error.

Low Airflow

Low airflow can result from:

• blower problem;
• control-valve problem;
• air leak;
• blocked header;
• incorrect control signal.

Air Distribution

Total airflow is important, but its distribution through the basin is also critical.

Poor air distribution can create:

• low-DO zones;
• over-aerated zones;
• uneven treatment.

Air Control Valves

Air valves can balance flow among:

• basins;
• zones;
• diffuser grids.

Do Not Balance Air by Appearance Alone

Bubble appearance can provide clues, but operators should also review:

• airflow measurements;
• DO profile;
• blower pressure;
• process performance.

Mechanical Aeration

Some activated-sludge systems use mechanical aerators rather than submerged diffusers.

Mechanical aerators can provide:

• oxygen transfer;
• mixing.

Mechanical Aerator Problems

Possible problems include:

• motor failure;
• gearbox problems;
• damaged impeller;
• reduced mixing;
• reduced oxygen transfer.

Mixers

Some processes use separate mixers where mixing is needed without substantial aeration.

This is common in process zones intended to remain:

• anoxic;
• anaerobic.

Aerobic, Anoxic, and Anaerobic Conditions

Aerobic means dissolved oxygen is available.

Anoxic conditions have little or no free DO but may contain nitrate that microorganisms can use.

Anaerobic conditions lack both free dissolved oxygen and significant oxidized nitrogen for respiration.

Do Not Aerate Every Process Zone Automatically

Plants designed for biological nutrient removal may intentionally maintain zones with different oxygen conditions.

DO Measurement

DO can be measured using:

• portable meters;
• fixed online probes.

Probe Location

A fixed DO probe should be located where its reading represents the process condition the control system is intended to manage.

DO Probe Fouling

Activated sludge can coat DO sensors.

Fouling can cause:

• slow response;
• incorrect readings;
• poor automatic aeration control.

Verify Unexpected DO Readings

If DO suddenly changes without an obvious process cause:

• inspect the probe;
• verify calibration;
• compare with a portable meter;
• review blower and airflow data.

Automatic DO Control

Automatic aeration systems may adjust:

• blower speed;
• blower staging;
• air-control valves.

The goal is to maintain a selected DO setpoint.

DO Setpoint

The appropriate setpoint depends on:

• process design;
• loading;
• nitrification requirements;
• basin configuration;
• facility operating goals.

Operators should not assume that one DO value is correct for every activated-sludge plant.

Setpoint Too Low

A setpoint that is too low can contribute to:

• poor carbonaceous treatment;
• loss of nitrification;
• settling problems.

Setpoint Too High

A setpoint that is unnecessarily high can increase:

• blower power;
• energy cost;
• equipment wear.

Energy Use

Aeration is commonly one of the largest energy users in an activated-sludge facility.

Small improvements in aeration control can therefore produce meaningful energy savings.

Do Not Save Energy by Sacrificing Treatment

Energy optimization should maintain:

• required effluent quality;
• process stability;
• adequate mixing;
• nitrification where required.

Airflow per Basin

Comparing airflow among similar basins can help identify:

• control imbalance;
• diffuser fouling;
• uneven loading.

DO and Organic Load

Consider a basin where influent BOD load rises while airflow remains constant.

Expected response:

• oxygen demand increases;
• DO tends to decrease.

DO and Ammonia

If effluent ammonia rises while DO is low, insufficient aeration may be contributing to loss of nitrification.

DO Normal but Ammonia High

If DO is adequate but ammonia remains high, investigate other causes such as:

• low SRT;
• low temperature;
• low alkalinity;
• toxicity;
• high ammonia loading.

DO and MLSS

If MLSS rises significantly without a change in airflow, total oxygen demand may increase.

DO and Wasting

Changes in wasting can affect biomass concentration and therefore alter long-term oxygen demand.

DO and RAS

RAS changes solids distribution between the clarifier and aeration basin.

Large RAS changes can therefore influence aeration-basin solids concentration and oxygen demand.

DO and Hydraulic Loading

High flow can change:

• detention time;
• load distribution;
• oxygen demand pattern.

Wet-Weather Conditions

During wet weather, operators may see:

• higher flow;
• lower influent BOD concentration;
• different oxygen-demand pattern;
• greater clarifier hydraulic loading.

Morning Load Increase

Some plants experience daily loading cycles.

A morning load increase may cause:

• falling DO;
• increased blower demand.

Trend DO with Airflow

DO becomes much more useful when trended with:

• airflow;
• blower speed;
• influent flow;
• organic load;
• ammonia;
• MLSS.

Example: DO Falls but Airflow Is Unchanged

Possible causes include:

• higher organic load;
• higher ammonia load;
• higher MLSS;
• reduced oxygen-transfer efficiency;
• DO sensor error.

Example: DO Falls and Airflow Falls

Review:

• blower status;
• VFD speed;
• air valve position;
• control signal;
• air header pressure.

Example: Airflow Increases but DO Does Not Improve

Possible causes include:

• rapid increase in oxygen demand;
• diffuser fouling;
• poor air distribution;
• DO sensor problem.

Example: DO Very High at Basin Outlet

Review whether:

• airflow can be reduced;
• upstream DO remains adequate;
• nitrification remains stable;
• mixing requirements are still met.

Example: One Basin Has Low DO

If parallel basins receive similar loading, review:

• air valve position;
• diffuser condition;
• airflow measurement;
• basin solids concentration;
• probe accuracy.

Example: Blower Pressure Is Increasing

Review:

• diffuser fouling;
• air-header restrictions;
• closed valves;
• pressure transmitter accuracy.

Example: Visible Solids Deposits in Aeration Basin

Possible causes include:

• insufficient mixing;
• failed diffuser zone;
• mechanical mixer failure;
• poor basin hydraulics.

Example: DO Probe Reads Zero but Basin Looks Well Aerated

Verify:

• probe condition;
• calibration;
• cable and transmitter;
• portable DO measurement.

Example: DO Oscillates Rapidly

Possible causes include:

• poor control tuning;
• unstable airflow;
• probe fouling;
• rapid loading changes.

Common Aeration and DO Mistakes

• Assuming more air always improves treatment.
• Using one DO reading to represent an entire basin.
• Ignoring influent loading when DO changes.
• Ignoring ammonia loading in nitrifying systems.
• Failing to distinguish oxygen transfer from mixing.
• Ignoring diffuser fouling.
• Ignoring blower discharge pressure trends.
• Trusting a dirty DO probe.
• Using the same aeration setting in all seasons without reviewing process demand.
• Reducing aeration for energy savings without confirming process performance.

A Practical Low-DO Review

1. Verify the DO measurement.
2. Review blower operation.
3. Review airflow and air-header pressure.
4. Review influent organic and ammonia loading.
5. Review MLSS.
6. Inspect air distribution.
7. Review diffuser condition.
8. Check for mixing problems.
9. Adjust aeration according to process needs and facility procedures.

A Practical High-DO Review

1. Verify the DO probe.
2. Review influent loading.
3. Review basin location of the high reading.
4. Review blower speed and airflow.
5. Confirm nitrification and effluent quality are stable.
6. Confirm minimum mixing needs.
7. Reduce unnecessary aeration according to approved process-control procedures.
8. Trend the response.

A Practical Aeration-Efficiency Review

1. Review DO profile.
2. Review total and basin airflow.
3. Review blower discharge pressure.
4. Review blower energy use.
5. Inspect diffuser bubble patterns.
6. Compare parallel basins.
7. Review treatment performance.
8. Identify over-aerated and under-aerated zones.

A Practical DO-Control Review

1. Confirm the process objective.
2. Verify probe location and calibration.
3. Review DO setpoint.
4. Review blower and valve control logic.
5. Compare DO with airflow and loading.
6. Review seasonal trends.
7. Check for control oscillation.
8. Document adjustments and process response.

What to Remember for the Exam

• Aeration provides oxygen and often provides mixing in activated-sludge systems.
• Mixing keeps biological solids suspended and distributes oxygen and food.
• DO reflects the balance between oxygen supplied and oxygen consumed.
• Major oxygen demands include carbonaceous BOD removal, nitrification, and biomass respiration.
• Higher organic or ammonia loading can lower DO if air supply does not increase.
• Higher MLSS can increase total oxygen demand.
• Low DO can reduce BOD removal, interfere with nitrification, and contribute to undesirable biological conditions.
• Excessive DO can waste energy without improving treatment.
• DO can vary significantly through an aeration basin, so one reading may not represent the entire process.
• Fine-bubble diffusers can provide efficient oxygen transfer but can lose performance when fouled.
• Increasing blower pressure at similar airflow can indicate diffuser or air-system restriction.
• Blowers should be monitored for airflow, pressure, current, temperature, vibration, and alarms.
• Air distribution among basins and zones is as important as total airflow.
• Mechanical aerators can provide both oxygen transfer and mixing.
• DO probes require cleaning, calibration, and verification.
• Automatic aeration can adjust blower speed, staging, or air valves to maintain a DO target.
• No single DO setpoint is correct for every activated-sludge system.
• Aeration is a major energy user, so excess air should be avoided while maintaining process performance.
• DO troubleshooting should compare airflow, blower pressure, loading, MLSS, ammonia, temperature, and instrument condition.
• Good aeration control provides enough oxygen and mixing for stable treatment while avoiding unnecessary energy use.