Feb 10, 2021

What is Activated Sludge?

Activated sludge refers to a flocculent culture of organisms developed in aeration tanks under controlled conditions.

what is activated sludge?

Activated Sludge

Activated sludge refers to a flocculent culture of organisms developed in aeration tanks under controlled conditions, according to WEF

Activated sludge is typically brown in color. Activated sludge is also known as waste activated biosolids or waste activated solids. 

The sludge consists of a mixed blend of microorganisms, 95% of which are a variety of mostly aerobic species of bacteria, according to Science Direct. Activated sludge also contains populations of fungi, protozoa and higher forms of invertebrates.



According to Dokuz Eylül University, the basic function of the activated sludge process is to establish and maintain a viable population of microorganisms. There are several stages necessary for treatment. 

Aeration Tank

An aeration tank provides the required detention time and ensures that the activated sludge and the influent wastewater are thoroughly mixed.

Mechanical or Diffused Aeration

Mechanical aeration systems use agitators or mixers to mix air and mixed liquor. Diffused aeration systems use pressurized air released through diffusers near the bottom of the tank. 

Settling Tank

Activated sludge systems are equipped with plain settling tanks designed to provide approximately 2 to 4 hours of hydraulic detention time.

Return Sludge & Activated Sludge

The return sludge system includes pumps, a timer or variable speed drive, and a flow measurement device. An activated sludge system consists of the same components but in some cases, the waste activated sludge withdrawal is accomplished by adjusting valves on the return system. 


The Activated Sludge Process

The activated sludge process is designed to speed up the rate of decomposition of waste material in water and was first developed in 1914, according to the International Water Association.

The activated sludge process was first tested at the Manchester - Davyhulme Wastewater Treatment Plant, in a "mobile" pilot-plant. Most parts of the pilot-plant unit were made of wood,  but the installation already exhibited most characteristic features of the activated sludge process used today. This includes the continuous-flow arrangements with separate clarifiers and activated sludge recycle and the fill-and-draw arrangement, also known as a sequencing batch reactor (SBR), according to the International Water Association.

The activated sludge process is a means of treating both municipal and industrial wastewater

The activated sludge process is a multi-chamber reactor unit that uses highly concentrated microorganisms to degrade organics and remove nutrients from wastewater, producing quality effluent.

The goal is to maintain aerobic conditions and to keep the activated sludge suspended. To achieve this, a continuous and well-timed supply of oxygen is required. 

Flocs of bacteria are suspended and mixed with wastewater in an aerated tank, and the bacteria use the organic pollutants to grow and transform it to energy, water, CO2 and new cell material. The flocs can be removed in the secondary clarifier by gravity settling and some of this sludge is recycled from the clarifier back to the reactor. The effluent can then be discharged to final polishing.


Activated sludge sewage treatment is ideally completed in a centralized treatment facility and the treatment of the sludge can occur in most climates. Colder environments reduce the treatment capacity, however.

According to the Pennsylvania Department of Environmental Protection (DEP), the conventional activated sludge process is susceptible to failure from shock loads. Due to its relatively low mixed liquor suspended solids (MLSS) concentration and head-end loading, the conventional activated sludge process is best for low-strength, domestic wastes with minimal peak load considerations.

About the author

Cristina Tuser is associate editor for Water & Wastes Digest. Tuser can be reached at ctuser@sgcmail.com.