The Activated Sludge Process (ASP) is one of the most widely used biological Wastewater Treatment Technologiesin municipal and Industrial Wastewater treatment plants worldwide.

The core principle of the activated sludge process is to use microorganisms to remove organic pollutants from wastewater. Under suitable oxygen conditions, microorganisms consume organic matter as their food source and convert pollutants into carbon dioxide, water, and new microbial cells.

In simple terms:

Wastewater pollutants → Microorganisms consume organic matter → Biological degradation → Treated water

Due to its high treatment efficiency, stable operation, and adaptability to different wastewater conditions, activated sludge technology has become the foundation of many modern wastewater treatment systems.

An activated sludge system mainly consists of:

• Aeration Tank
• Secondary Clarifier
• Return Activated Sludge (RAS) System
• Waste Activated Sludge (WAS) Removal System

The typical treatment flow is:

Treatment Stage	Main Function
Pretreatmen	Remove large solids, sand, and floating materials
Aeration Tan	Biological degradation of organic pollutants
Secondary Clarifi	Separate activated sludge from treated water
Sludge Return System	Maintain sufficient microorganism concentration
Excess Sludge Removal	Control sludge growth and system stability

The wastewater first enters the aeration tank, where oxygen is supplied through an aeration system. Microorganisms form biological flocs called activated sludge, which absorb and degrade pollutants.

After biological treatment, the mixed liquor flows into the secondary clarifier. The activated sludge settles, and part of it is returned to the aeration tank to maintain the required biomass concentration.

Biological Process Inside the Aeration Tank

The aeration tank is the core unit of the activated sludge process.

By supplying oxygen through blowers and diffusers, microorganisms obtain enough oxygen to break down organic pollutants.

The main biological reactions include:

Biological Reaction	Function
Organic matter oxidation	Removes COD and BOD
Nitrification	Converts ammonia nitrogen into nitrate
Denitrification	Removes total nitrogen under anoxic conditions
Biological phosphorus removal	Reduces phosphorus concentration

The activated sludge contains various microorganisms, including:

• Heterotrophic bacteria – responsible for COD and BOD removal
• Nitrifying bacteria – responsible for ammonia nitrogen removal
• Denitrifying bacteria – responsible for nitrogen removal
• Protozoa and microorganisms – maintain sludge ecosystem balance

A healthy activated sludge structure is essential for stable wastewater treatment performance.

Key Operating Parameters of Activated Sludge System

1. MLSS (Mixed Liquor Suspended Solids)

MLSS represents the concentration of suspended solids and microorganisms inside the aeration tank.

A suitable MLSS level ensures sufficient microbial activity and treatment efficiency.

Process Type	Typical MLSS Range
Conventional Activated Sludge	2,000–4,000 mg/L
A²/O Process	3,000–5,000 mg/L
MBR Process	6,000–12,000 mg/L

Higher MLSS can improve treatment capacity, but excessive concentration may increase oxygen demand and cause sludge settling problems.

2. DO (Dissolved Oxygen)

Dissolved oxygen is one of the most important control parameters in biological wastewater treatment.

Treatment Zone	Recommended DO Level
Aerobic Zone	1.5–3 mg/L
Nitrification Zone	2–4 mg/L
Anoxic Zone	<0.5 mg/L

Insufficient DO may lead to:

• Poor COD removal
• Increased ammonia nitrogen
• Sludge bulking problems

Excessive DO may result in:

• Higher energy consumption
• Increased operating costs

Modern wastewater treatment plants increasingly use online DO monitoring and intelligent aeration control systems to optimize blower operation and reduce energy consumption.

3. SRT (Sludge Retention Time)

Sludge Retention Time (SRT), also known as sludge age, refers to the average time microorganisms remain in the treatment system.

Low SRT	High SRT
Younger microorganisms	More mature microorganisms
Faster organic degradation	Better nitrification performance
Higher sludge production	Lower excess sludge production

Proper SRT control is critical for stable biological nitrogen removal.

4. F/M Ratio (Food to Microorganism Ratio)

The F/M ratio represents the balance between organic loading and microbial concentration.

Operating Condition	F/M Ratio
High Load Operation	0.5–1.0 kgCOD/kgMLSS·d
Low Load Operation	0.05–0.15 kgCOD/kgMLSS·d

An improper F/M ratio may cause:

• Poor sludge settling
• Low biological activity
• Increased operational difficulties

Main Types of Activated Sludge Processes

Conventional Activated Sludge Process

The conventional activated sludge process is one of the most established biological treatment technologies.

Advantages:

• Mature technology
• Reliable operation
• Suitable for large-scale wastewater treatment plants

Applications:

• Municipal wastewater treatment plants
• Industrial wastewater treatment facilities

A/O Process (Anoxic/Oxic Process)

The A/O process combines anoxic and aerobic zones.

Zone	Main Function
Anoxic Zone	Denitrification
Aerobic Zone	Organic removal and nitrification

Advantages:

• Simple process design
• Lower investment cost
• Effective nitrogen removal

A²/O Process (Anaerobic-Anoxic-Oxic Process)

The A²/O process is widely applied in municipal wastewater treatment.

Process flow:

Anaerobic Zone → Anoxic Zone → Aerobic Zone → Secondary Clarifier

Treatment Function	Removal Target
Organic degradation	COD, BOD
Nitrification	Ammonia nitrogen
Denitrification	Total nitrogen
Biological phosphorus removal	Total phosphorus

SBR (Sequencing Batch Reactor)

SBR is a batch-operated activated sludge technology.

One reactor completes:

Operation Stage	Function
Filling	Wastewater input
Aeration	Biological treatment
Settling	Sludge separation
Decanting	Treated water discharge

Advantages:

• Small footprint
• Flexible operation
• High automation level

MBR (Membrane Bioreactor)

MBR combines activated sludge treatment with membrane filtration technology.

Advantages	Challenges
Excellent effluent quality	Membrane fouling
Low suspended solids	Higher energy consumption
Suitable for water reuse	Higher maintenance requirements

MBR is widely used where high-quality reclaimed water is required.

Common Problems in Activated Sludge Operation

Sludge Bulking

Sludge bulking occurs when activated sludge has poor settling performance.

Typical symptoms:

• High sludge volume index (SVI)
• Slow settling
• Increased suspended solids in effluent

Common causes:

Cause	Solution
Low DO	Increase aeration capacity
Unbalanced F/M ratio	Adjust organic loading
Excessive filamentous bacteria	Optimize operating conditions

Excessive Foam Formation

Possible causes:

• Nocardia bacteria
• Surfactants
• Low organic loading conditions

Solutions:

• Optimize sludge age
• Adjust aeration conditions
• Control influent characteristics

High Ammonia Nitrogen in Effluent

Main reasons:

• Insufficient nitrifying bacteria
• Low DO concentration
• Short SRT

Possible solutions:

• Increase sludge retention time
• Improve aeration efficiency
• Optimize sludge return ratio

Energy Consumption in Activated Sludge Wastewater Treatment

Energy consumption is one of the major concerns in wastewater treatment plants.

Equipment	Approximate Energy Consumption
Aeration System	50–70%
Sludge Treatment System	10–20%
Pumping System	Around 10%

The aeration system usually represents the largest energy consumer.

Therefore, precise oxygen control has become a key strategy for reducing wastewater treatment operating costs.

Modern solutions include:

• Online DO monitoring
• OUR (Oxygen Uptake Rate) control
• AI-based aeration optimization
• Variable frequency blower control

Equipment Related to Activated Sludge Process

Activated sludge systems usually work together with various wastewater treatment equipment.

Treatment Stage	Common Equipment
Pretreatment	Screens, Grit Chamber
Biological Treatment	Aeration Tank, Aeration System, Mixers
Solid Separation	Secondary Clarifier, Lamella Clarifier
Advanced Treatment	Filtration System, Disinfection Equipment
Sludge Treatment	Screw Press Dewatering Machine, Filter Press

For wastewater treatment equipment manufacturers, understanding activated sludge operation is essential to provide complete solutions for municipal and industrial customers.

Future Development Trends of Activated Sludge Technology

The wastewater treatment industry is moving toward more efficient, intelligent, and sustainable solutions.

Low-Carbon Wastewater Treatment

Future wastewater plants focus on:

• Reducing aeration energy consumption
• Improving oxygen transfer efficiency
• Recovering energy from sludge

Intelligent Operation

Digital technologies are being introduced through:

• Online sensors
• Automated control systems
• AI-based process optimization

These technologies help operators achieve more stable treatment performance with lower operating costs.

Resource Recovery

Modern wastewater treatment is shifting from pollution control to resource recovery.

Examples include:

• Biogas production from sludge digestion
• Phosphorus recovery
• Wastewater reuse

Conclusion

The activated sludge process remains one of the most important biological wastewater treatment technologies worldwide. With proper control of parameters such as MLSS, DO, SRT, and F/M ratio, activated sludge systems can achieve efficient removal of organic pollutants, nitrogen, and phosphorus.

Combined with advanced equipment such as lamella clarifiers, sludge dewatering machines, filter presses, and intelligent aeration control systems, activated sludge technology continues to support sustainable wastewater management and water reuse development.

For wastewater treatment projects, selecting the right process design and reliable equipment is the key to achieving stable performance, lower operating costs, and long-term environmental benefits.