Introduction to Activated Sludge-process Flow and Performance Indicators

Learn about the Activated Sludge Process, a vital biological wastewater treatment method used in municipal and industrial sewage systems. This guide covers the process flow, system components, and key performance indicators such as MLSS, SVI, and oxygen levels, which define the efficiency and stability of wastewater purification. Gain insights into sludge control, aeration, and microbial characteristics to improve treatment performance and ensure long-term operational stability.

The Activated Sludge Process is one of the most widely applied biological wastewater treatment technologies worldwide. It relies on a complex microbial community — known as activated sludge — that breaks down and removes organic pollutants from wastewater through aerobic biological degradation.

Activated sludge can exist in two primary forms:

• Aerobic activated sludge, which operates with sufficient dissolved oxygen.
• Anaerobic granular activated sludge, which functions in oxygen-free environments for specific treatment applications.

Both types consist of a flocculent microbial symbiosis, combining bacteria, protozoa, organic colloids, and inorganic particles. These flocs exhibit strong adsorption, oxidation, and degradation capabilities, enabling the removal of suspended solids, organic compounds, and nutrients from sewage.

The activated sludge process typically follows preliminary sedimentation, aeration, biological reaction, sludge separation, and recirculation, maintaining a stable microbial ecosystem for continuous purification.

The basic process flow of an activated sludge system involves several key stages and components:

The aeration tank is the heart of the system. Here, oxygen is supplied (by aeration or mechanical stirring) to support microbial activity.
The microorganisms metabolize organic pollutants, converting them into carbon dioxide, water, and new cell material (biomass).

This tank serves two functions:

• Separation of sludge and treated effluent, ensuring clear water output.
• Sludge return, maintaining an optimal concentration of active microorganisms in the aeration tank.

A portion of the settled sludge is returned to the aeration tank to maintain a consistent biomass concentration.
The recirculation ratio directly affects process stability, oxygen consumption, and treatment efficiency.

Excess sludge — the portion not recirculated — is treated separately through anaerobic digestion, landfilling, or drying.
The dried sludge can often be used as fertilizer, providing a sustainable byproduct of the treatment process.

Adequate dissolved oxygen (DO) is critical. Proper aeration ensures:

• Efficient organic matter degradation.
• Prevention of filamentous bacterial overgrowth that causes sludge bulking.
• A stable microbial ecosystem for long-term operation.

To maintain efficient operation of the activated sludge process, the following conditions must be met:

1. Wastewater contains sufficient soluble and biodegradable organic matter.
   Easily degradable organics provide the necessary carbon source for microbial metabolism.
2. Adequate dissolved oxygen concentration.
   Oxygen is required for aerobic decomposition. DO levels are typically maintained between 2–4 mg/L.
3. Activated sludge remains suspended in the aeration tank.
   Proper mixing ensures full contact between microorganisms and wastewater.
4. Continuous sludge recycling and timely removal of excess sludge.
   This keeps the system in balance and prevents overgrowth or starvation.
5. Absence of toxic substances.
   Heavy metals, acids, or industrial toxins can disrupt microbial activity and impair treatment efficiency.

Activated sludge exhibits distinct physical and biochemical properties that determine its performance.

(1) Physical Properties

• Appearance: Brown to yellow or reddish color.
• Odor: Earthy or musty, typical of municipal sewage.
• Specific Gravity: Slightly above water (1.002–1.006).
• Particle Size: 0.02–0.2 mm.
• Specific Surface Area: 20–100 cm²/mL.

These characteristics promote strong adsorption and flocculation, which are crucial for pollutant capture and sedimentation.

(2) Biochemical Properties

• Moisture Content: 99.2–99.8%.
• Solid Composition:
  • Living cells (Ma) – active microorganisms.
  • Endogenous residues (Me) – metabolic byproducts.
  • Adsorbed organics (Mi) – partially degraded compounds.
  • Inorganic matter (Mii) – minerals and non-biodegradable solids.

Microorganisms are the functional core of the activated sludge process.
They can be categorized into bacteria, protozoa, and metazoa.

Key Bacterial Genera:

• Pseudomonas
• Bacillus
• Micrococcus
• Flavobacterium
• Alcaligenes
• Achromobacter
• Zooglea (important for forming bacterial flocs)

Core functions:

• Decompose organic carbon compounds.
• Oxidize ammonia into nitrate (nitrification).
• Form “bioflocs” through extracellular polymeric substances (EPS).

Other microorganisms such as protozoa and metazoa (about 10³ per mL) contribute to predation and biological balance, preventing bacterial overpopulation.

Represents the total concentration of suspended solids (microbial mass + organics + inorganics) in the aeration tank.
Unit: mg/L or g/m³.
An optimal MLSS level ensures sufficient microbial activity without causing sludge overload.

Indicates the biologically active fraction of MLSS — the living microbial biomass.
For urban sewage, the MLVSS/MLSS ratio typically ranges between 0.75–0.85.

Defined as the percentage of sludge volume that settles after 30 minutes in a 1-liter graduated cylinder.
Normal SV: 20–30%.
This parameter reflects sludge concentration and settling performance.

The volume (in mL) occupied by 1 g of dry sludge after 30 minutes of settling.

• Indicates sludge floc structure and settling ability.
• Low SVI → dense sludge, good settling.
• High SVI → poor settling, risk of sludge bulking.

For municipal wastewater, the typical SVI range is 50–150 mL/g.

Sludge bulking is a major operational challenge in the Activated Sludge Process.
It occurs when filamentous bacteria proliferate, forming light, fluffy flocs that settle poorly.

Causes:

• High C/N (carbon-nitrogen) or C/P (carbon-phosphorus) ratios.
• Low dissolved oxygen levels.
• Presence of inhibitory compounds.

Control Strategies:

• Maintain adequate aeration and DO levels (>2 mg/L).
• Regulate nutrient ratios (C:N:P ≈ 100:5:1).
• Use selective oxidation or chlorination to suppress filamentous growth.

The Activated Sludge Process is continuous and well-suited to automation and computer-based control.
Modern systems employ real-time sensors to monitor:

• Dissolved oxygen (DO)
• pH and temperature
• MLSS and SVI
• Ammonia and nitrate concentrations

Automation improves process stability, energy efficiency, and adaptability to varying wastewater flow and composition, making it ideal for municipal and industrial wastewater treatment plants.

The Activated Sludge Process remains the backbone of biological wastewater treatment, combining microbial metabolism with modern engineering to achieve high removal rates of organic matter, suspended solids, and nutrients.

By understanding its process flow, key indicators (MLSS, SVI, DO), and microbial dynamics, engineers can design and operate treatment plants that are efficient, stable, and environmentally sustainable.

When optimized and automated, the activated sludge system ensures reliable purification even under fluctuating wastewater conditions, reinforcing its importance in global sewage management and water reuse.

To biologically remove organic pollutants and nutrients from wastewater using microorganisms under controlled aerobic conditions.

MLSS measures the total suspended solids in the aeration tank, while MLVSS indicates the biologically active portion — the living microbes.

Maintain sufficient dissolved oxygen, balanced C/N/P ratios, and proper sludge age. Avoid excessive organic loading and toxic inflows.

The Sludge Volume Index (SVI) reflects sludge settleability. Abnormal SVI values can indicate poor floc formation or bulking.

Yes. Advanced systems use sensors and AI-based control to optimize oxygen supply, monitor sludge characteristics, and maintain stable operation automatically.

Xi’an CHIWATEC Water Treatment Technology is a high-tech enterprise specialized in various water processing devices. Aside from these individual products, which cover a number of types and series, we can also help with related comprehensive engineering projects. Thanks to our hard work and dedication upon our founding, we are now one of the fastest-developing water treatment equipment manufacturers in Western China.

Further reading:

• Type and structure of activated sludge aeration tank
• Comparison of various activated sludge processes
• Application and process parameters of activated sludge
• Introduction to activated sludge-process flow and performance indicators
• Introduction to Activated Sludge Process–Sewage Treatment