Blowers, Aerators & Mixing Equipment

Blowers, aerators, and mixers are widely used in water and wastewater treatment to move air, transfer oxygen, keep solids suspended, distribute chemicals, and maintain uniform process conditions. Their performance directly affects biological treatment, chemical reactions, tank mixing, odor control, and energy use.

Operators should understand the difference between airflow, pressure, oxygen transfer, and mixing intensity. A blower may be operating normally while the aeration basin receives poor oxygen transfer because of diffuser fouling or process conditions.

Purpose of Aeration

Aeration introduces air or oxygen into water or wastewater.

Common purposes include:

• supplying oxygen to microorganisms;
• removing dissolved gases;
• oxidizing certain compounds;
• mixing tanks and basins;
• preventing solids deposition;
• supporting odor control.

Purpose of Mixing

Mixing keeps tank contents more uniform.

Mixing may be used to:

• keep solids suspended;
• distribute chemicals;
• prevent settling;
• maintain contact between microorganisms and wastewater;
• equalize concentration and temperature.

Blowers

A blower moves air through piping and into a process.

Wastewater treatment plants commonly use blowers to supply air to diffusers in aeration basins.

Common blower types include:

• positive-displacement blowers;
• centrifugal blowers;
• high-speed turbo blowers.

Positive-Displacement Blowers

A positive-displacement blower moves a relatively fixed volume of air for each revolution.

As discharge resistance increases, the blower continues attempting to move air and discharge pressure rises.

This makes pressure protection important.

Positive-Displacement Blower Characteristics

Typical characteristics include:

• airflow strongly related to blower speed;
• pressure determined largely by system resistance;
• relatively stable airflow as pressure changes within the operating range.

Centrifugal Blowers

Centrifugal blowers use rotating impellers to accelerate air.

Airflow and pressure depend on:

• blower speed;
• blower characteristics;
• system resistance;
• inlet conditions.

Like centrifugal pumps, centrifugal blowers operate at an intersection between equipment performance and system resistance.

Turbo Blowers

High-speed turbo blowers are a type of centrifugal blower commonly used in modern aeration systems.

They may offer:

• high efficiency;
• variable-speed operation;
• integrated controls;
• compact equipment.

They can also be sensitive to inlet-air condition, cooling, and operating range.

Airflow

Blower output may be measured in units such as:

• standard cubic feet per minute, or scfm;
• actual cubic feet per minute, or acfm.

Operators should know which measurement their instrumentation uses.

Standard Versus Actual Airflow

Air volume changes with:

• temperature;
• pressure;
• humidity.

Standard airflow adjusts air volume to defined reference conditions, while actual airflow describes volume under actual operating conditions.

Blower Discharge Pressure

Blower discharge pressure reflects the resistance the air must overcome.

Resistance may come from:

• water depth;
• air piping;
• valves;
• diffusers;
• fouling;
• check valves.

Water Depth Creates Backpressure

Air entering submerged diffusers must overcome the hydrostatic pressure caused by water depth.

Deeper diffuser submergence generally requires greater blower discharge pressure.

Diffusers

Diffusers release air below the water surface.

Common diffuser types include:

• fine-bubble diffusers;
• coarse-bubble diffusers.

Fine-Bubble Diffusers

Fine-bubble diffusers create many small bubbles.

They generally provide higher oxygen-transfer efficiency because smaller bubbles provide greater gas-liquid contact area and longer contact time.

Coarse-Bubble Diffusers

Coarse-bubble diffusers produce larger bubbles.

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

They may be useful where:

• mixing is especially important;
• solids or fouling are concerns;
• higher resistance to plugging is desired.

Oxygen Transfer

Not all oxygen in the supplied air transfers into the liquid.

Oxygen transfer depends on factors such as:

• bubble size;
• water depth;
• airflow;
• wastewater characteristics;
• temperature;
• dissolved oxygen concentration;
• diffuser condition.

Dissolved Oxygen

Dissolved oxygen, or DO, is oxygen dissolved in water.

In activated sludge systems, DO is commonly monitored to determine whether enough oxygen is available for biological treatment.

More Air Does Not Always Mean Better Operation

Excessive aeration can waste energy and may affect process performance.

Too much air can result in:

• unnecessary power use;
• excessive mixing;
• process-control problems in some treatment zones.

Aeration should be matched to actual process demand.

Airflow Control

Aeration airflow may be controlled by:

• blower speed;
• inlet guide vanes;
• discharge valves;
• air-header control valves;
• individual basin valves.

The method depends on blower type and system design.

Variable-Frequency Drives

VFDs can adjust blower motor speed.

Reducing speed can reduce:

• airflow;
• power demand.

The blower must still remain within its approved operating range.

DO-Based Aeration Control

Automatic aeration systems may adjust airflow based on dissolved oxygen measurement.

A typical control sequence is:

1. DO falls below target.
2. Controller increases airflow.
3. Blower speed or valve position changes.
4. DO rises toward the target.

DO Sensor Problems Can Affect Aeration

If a DO sensor reads incorrectly, the control system may:

• over-aerate;
• under-aerate;
• change blower speed unnecessarily.

Questionable readings should be verified before major process adjustments.

Air Headers

An air header distributes blower discharge air to basins or process zones.

The system may include:

• isolation valves;
• control valves;
• flow meters;
• pressure instruments;
• check valves.

Uneven Air Distribution

Uneven airflow can result from:

• incorrect valve position;
• diffuser fouling;
• unequal piping resistance;
• blocked air laterals;
• control problems.

Diffuser Fouling

Diffusers can become fouled by:

• biological growth;
• scale;
• solids;
• chemical deposits.

Fouling increases resistance to airflow.

Signs of Diffuser Fouling

Possible signs include:

• higher blower discharge pressure;
• lower airflow at the same blower speed;
• uneven bubble patterns;
• higher energy use;
• difficulty maintaining DO.

Increasing Blower Pressure

If blower discharge pressure gradually increases at similar basin depth and airflow, investigate:

• diffuser fouling;
• partially closed air valves;
• blocked piping;
• check-valve problems.

Low Blower Pressure

Unexpected low discharge pressure may indicate:

• large air leak;
• open bypass;
• lower basin water level;
• blower performance problem;
• incorrect pressure indication.

Air Leaks

Leaks in blower piping waste energy and reduce air available to the process.

Possible leak locations include:

• flanges;
• flexible connections;
• valves;
• instrument fittings;
• damaged piping.

Blower Inlet Air

Blowers require adequate inlet airflow.

Inlet systems may include:

• filters;
• silencers;
• ductwork.

Restricted inlet airflow can reduce performance and increase equipment stress.

Dirty Inlet Filters

A dirty blower inlet filter can cause:

• reduced airflow;
• increased inlet restriction;
• higher energy use;
• equipment overheating in some systems.

Blower Temperature

Compressing air increases temperature.

Operators may monitor:

• discharge-air temperature;
• bearing temperature;
• motor temperature;
• lubricant temperature.

High Blower Temperature

Possible causes include:

• high discharge pressure;
• restricted inlet;
• poor ventilation;
• lubrication problems;
• mechanical problems;
• operation outside the approved range.

Blower Lubrication

Some blowers require oil or grease lubrication.

Operators should check:

• lubricant level;
• lubricant condition;
• leaks;
• manufacturer maintenance interval.

Blower Vibration

Increasing vibration may indicate:

• bearing wear;
• misalignment;
• imbalance;
• loose mounting;
• internal damage;
• surge or unstable operation in some centrifugal blowers.

Centrifugal Blower Surge

Some centrifugal blowers have a minimum stable airflow.

If airflow becomes too low for the pressure being developed, unstable flow called surge can occur.

Surge may cause:

• pressure fluctuations;
• noise;
• vibration;
• equipment stress.

Blower Surge Is Not Water Hammer

Blower surge is an unstable aerodynamic condition in a centrifugal blower.

Water hammer is a hydraulic pressure transient in a liquid system.

The two should not be confused.

Relief Valves

Positive-displacement blowers may require pressure-relief protection.

If discharge becomes blocked, pressure can rise rapidly.

A relief valve helps protect:

• blower;
• piping;
• connected equipment.

Do Not Operate Against a Closed Discharge

A positive-displacement blower should not operate against a closed discharge unless the system is specifically designed to handle that condition.

Pressure can rise quickly and damage equipment.

Blower Check Valves

Check valves can help prevent reverse airflow when a blower stops.

Problems can include:

• leakage;
• slam;
• sticking;
• increased resistance.

Multiple Blowers

Plants may use multiple blowers to match changing air demand.

A control strategy may use:

• lead blower;
• lag blower;
• standby blower.

Additional blowers may start as process demand increases.

Blower Sequencing

Starting too many blowers unnecessarily can:

• waste energy;
• reduce individual blower efficiency;
• create unstable control.

Blower sequencing should match process airflow demand.

Mechanical Surface Aerators

Mechanical aerators transfer oxygen by physically disturbing the water surface.

Examples include:

• surface aerators;
• brush aerators;
• rotor aerators.

They can provide both oxygen transfer and mixing.

Mechanical Aerator Problems

Possible problems include:

• motor overload;
• gearbox problems;
• bearing wear;
• damaged blades;
• incorrect submergence;
• excessive vibration.

Aerator Submergence

Mechanical aerator performance can change with water level.

Incorrect submergence may affect:

• oxygen transfer;
• mixing;
• motor load.

Mixers

Mixers provide mechanical energy to keep tank contents in motion.

Common applications include:

• rapid mixing;
• flocculation;
• activated-sludge basins;
• anoxic zones;
• chemical tanks;
• sludge storage.

Submersible Mixers

Submersible mixers use a submerged motor and propeller or impeller.

Operator concerns include:

• seal condition;
• motor temperature;
• vibration;
• ragging;
• cable condition;
• mounting system.

Top-Entry Mixers

Top-entry mixers use a motor and drive mounted above the tank.

They may include:

• gearbox;
• shaft;
• impeller;
• bearings;
• support structure.

Mixing Does Not Always Mean Aeration

Some mixers are intended to move liquid without introducing significant oxygen.

This is important in zones where low oxygen is required, such as some anoxic processes.

Mixing Intensity

Too little mixing can allow:

• solids settling;
• poor chemical distribution;
• short-circuiting;
• uneven process conditions.

Too much mixing can:

• waste energy;
• damage fragile floc;
• create undesirable process conditions.

Solids Deposition

If mixing is inadequate, solids may accumulate on:

• tank floors;
• corners;
• low-velocity zones.

Accumulated solids can cause:

• odor;
• reduced effective volume;
• process instability.

Mixing Equipment Vibration

Possible causes include:

• damaged impeller;
• ragging;
• shaft misalignment;
• bearing wear;
• loose mounting;
• imbalance.

Ragging

Fibrous material can wrap around:

• mixer propellers;
• shafts;
• aerator components.

Ragging can cause:

• higher motor current;
• lower mixing performance;
• vibration;
• overload trips.

Motor Current as an Indicator

Motor current can help evaluate blower, aerator, and mixer load.

Higher current may indicate:

• mechanical binding;
• increased process load;
• ragging;
• bearing problems.

Lower current may indicate reduced mechanical load or loss of normal performance.

Energy Use

Aeration is often one of the major electrical loads at a wastewater treatment plant.

Energy use can be influenced by:

• blower efficiency;
• diffuser condition;
• DO target;
• air distribution;
• process demand;
• control strategy.

Do Not Control Aeration by Airflow Alone

Airflow tells the operator how much air is supplied, not how much oxygen is actually transferred into the wastewater.

Operators should also consider:

• DO;
• process loading;
• ammonia performance;
• diffuser condition.

Uneven DO Between Basins

If similar basins have very different DO values, investigate:

• airflow distribution;
• diffuser fouling;
• process loading;
• DO sensor accuracy;
• mixing condition.

Low DO with High Airflow

Possible causes include:

• high biological oxygen demand;
• diffuser fouling;
• poor oxygen-transfer efficiency;
• DO sensor problem;
• poor mixing;
• high wastewater temperature.

High DO with High Airflow

This may indicate unnecessary aeration and energy use.

Operators should verify:

• DO target;
• sensor calibration;
• control settings;
• process requirements.

Low Airflow with High Blower Pressure

This pattern commonly suggests increased resistance.

Possible causes include:

• fouled diffusers;
• closed air valve;
• blocked piping;
• restricted discharge path.

Low Airflow with Low Blower Pressure

Possible causes include:

• blower capacity problem;
• low blower speed;
• large air leak;
• instrument error.

Blower Will Not Start

Possible causes include:

• loss of power;
• motor overload;
• VFD fault;
• failed permissive;
• control mode problem;
• high-temperature trip;
• mechanical binding.

Repeated Blower Trips

Do not repeatedly reset a blower trip without investigation.

Review:

• motor current;
• discharge pressure;
• temperature;
• vibration;
• airflow;
• VFD fault code;
• inlet restriction.

Standby Blowers

Standby blowers should be exercised and maintained.

A standby unit may fail when needed because of:

• stuck valves;
• lubrication problems;
• electrical faults;
• control problems;
• corrosion.

Switching Blowers

When changing from one blower to another, operators should follow the approved sequence to avoid:

• loss of airflow;
• pressure surge;
• reverse airflow;
• unstable control.

Routine Blower Inspection

A routine inspection may include:

• airflow;
• discharge pressure;
• inlet condition;
• motor current;
• temperature;
• vibration;
• noise;
• lubricant level;
• filter condition;
• alarms.

Routine Aeration-System Inspection

Operators may review:

• DO;
• airflow by basin;
• bubble pattern;
• blower pressure;
• air-valve positions;
• process performance.

Routine Mixer Inspection

Useful observations include:

• motor current;
• noise;
• vibration;
• tank mixing pattern;
• solids deposition;
• seal alarms;
• mounting condition.

Safe Maintenance

Blowers, aerators, and mixers involve hazards such as:

• electrical energy;
• rotating equipment;
• pressurized air;
• automatic startup;
• open tanks and basins.

Required lockout/tagout and other safety procedures must be followed before servicing equipment.

Stored Air Pressure

Air piping can remain pressurized after a blower stops.

Pressure should be safely relieved or isolated according to facility procedures before maintenance.

Automatic Startup

A blower or mixer in automatic mode may start because of:

• DO change;
• process demand;
• timer;
• PLC command;
• remote operator command.

A control stop is not a substitute for required energy isolation.

Common Blower and Aeration Mistakes

• Assuming high airflow always means good oxygen transfer.
• Ignoring rising blower discharge pressure.
• Ignoring dirty inlet filters.
• Running a positive-displacement blower against a closed discharge.
• Operating a centrifugal blower below its stable operating range.
• Ignoring uneven bubble patterns.
• Increasing airflow without verifying DO measurement.
• Ignoring diffuser fouling.
• Assuming more aeration is always better.
• Ignoring blower temperature or vibration trends.
• Allowing excessive solids deposition because mixing appears to be operating.
• Resetting repeated motor or VFD trips without investigation.

A Practical Low-DO Troubleshooting Sequence

1. Verify the DO reading.
2. Review process loading.
3. Check blower operation.
4. Review total airflow.
5. Review airflow distribution among basins.
6. Check blower discharge pressure.
7. Inspect bubble pattern.
8. Check diffuser fouling indicators.
9. Review mixer operation where applicable.
10. Adjust airflow according to approved process-control procedures.
11. Observe the process response.

A Practical Blower Troubleshooting Sequence

1. Define the symptom.
2. Review alarms and control mode.
3. Check airflow.
4. Check discharge pressure.
5. Check inlet filter and inlet restriction.
6. Review motor current and speed.
7. Check temperature.
8. Listen for abnormal noise.
9. Check vibration.
10. Review downstream valves and diffuser resistance.
11. Compare with normal historical operation.
12. Escalate maintenance when mechanical or electrical work is required.

What to Remember for the Exam

• Blowers move air through piping to aeration and other process systems.
• Positive-displacement blowers move a relatively fixed volume of air per revolution and can develop high pressure if discharge is restricted.
• Centrifugal blowers operate according to the interaction between the blower curve and system resistance.
• Fine-bubble diffusers generally provide higher oxygen-transfer efficiency than coarse-bubble diffusers.
• Coarse-bubble systems can provide strong mixing and may tolerate fouling better in some applications.
• Blower discharge pressure reflects resistance from water depth, piping, valves, and diffusers.
• Diffuser fouling generally increases airflow resistance and blower discharge pressure.
• Airflow is not the same as oxygen transfer.
• Dissolved oxygen should be used with airflow and process data when evaluating aeration performance.
• More aeration is not always better and can waste substantial energy.
• A dirty blower inlet filter can reduce performance and increase equipment stress.
• Positive-displacement blowers require protection against excessive discharge pressure.
• Centrifugal blower surge is an unstable airflow condition and is different from water hammer.
• Mixing keeps solids and chemicals distributed but does not always provide significant aeration.
• Inadequate mixing can allow solids deposition and uneven process conditions.
• Ragging can increase mixer load, vibration, and motor current.
• Low airflow with high discharge pressure suggests increased downstream resistance.
• Increasing blower temperature or vibration should be investigated.
• Standby blowers should be exercised and maintained.
• Required lockout/tagout and pressure isolation must be used before servicing blowers, aerators, and mixers.