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Facts at your Fingertips: Activated Sludge Process

By Scott Jenkins, Chemical Engineering magazine |

The activated sludge process (ASP) is designed to speed up the rate of decomposition of waste material in water. First developed in 1914, it remains an important means of treating both municipal and industrial wastewaters. This one-page reference provides information on the key process components and operating parameters of the ASP.

Role of microbes and enzymes

The ASP involves the creation and management of a human-engineered and managed ecosystem of desired microbial populations that convert biodegradable organic substances in the wastewater inlet stream into CO2 and new biomass. The term “activated sludge” refers to the particles produced in wastewater by the growth of organisms in aeration tanks. It differs from primary sludge in that the sludge particles contain many living organisms that feed on the incoming wastewater.

Activated sludge consists of a mixed community of microorganisms, 95% of which are a variety of (mostly aerobic) species of bacteria. Activated sludge also contains significant populations of fungi, protozoa and higher forms of invertebrates.

At the level of actual biodegradation reactions, two categories of enzymes are involved — namely extra- and intracellular enzymes. Extracellular enzymes are excreted at the cell surface by the microorganisms, and they help to break down the pollutants outside the cells into smaller metabolites that are small enough to permeate the cell walls. As the smaller fragments are transported into the cells, they are integrated into the microbe’s metabolic process, where they are further oxidized to provide cellular energy, or are converted into new “building blocks” for cellular synthesis by intracellular enzymes.


Figure 1. The diagram shows the basic processes found in activated sludge process (ASP) operations

Figure 1. The diagram shows the basic processes found in activated sludge process (ASP) operations

ASP process steps

In the ASP, activated sludge is added to wastewater, and the mixture is aerated and agitated. With sufficient food and oxygen, aerobic bacteria thrive in the aeration tank (see Figure 1).

Solids-retention time (SRT) is an important operational parameter that represents the average time spent by the sludge in the biodegradation basin. The SRT is controlled by the sludge-wasting rate.

As microorganisms grow, they form particles that clump together, and are allowed to settle to the bottom of a tank. The liquid above it is relatively free of organic material and suspended solids. When the waste reaches the end of the tank, the bacteria has used most of the organic matter to produce new cells.

After a certain period of time, the activated sludge is allowed to settle out by sedimentation, and is either disposed of (waste actived sludge) or returned to the aeration tank (return activated sludge). Flocculation refers to the process by which small, suspended particles bunch together into heavier particles (flocs) and settle out.


For a classic ASP wastewater-treatment plant to work effectively, the sludge flocs need to have good settling properties. Good floc formation allows for an efficient solid-liquid separation to occur, after the sludge has removed the organic contaminants from the wastewater in the biodegradation basin. Efficient solid-liquid separation between the sludge and the treated wastewater is typically carried out using simple gravity settling in clarifiers to yield a solids-poor effluent stream.


Aeration is a key aspect of the ASP because it fulfills two critical functions. It supplies the oxygen needed for the microorganisms to grow, and also promotes optimum contact between the dissolved and suspended organic matter in the wastewater and the microorganisms.

Aeration can be performed mechanically or by using a diffuser system. Mechanical aerators physically splash the wastewater into the atmosphere above the tank to create turbulence. Mechanical aerators include brushes, blades or propellers that introduce air from the atmosphere into the wastewater. Surface aerators float at the surface or are mounted on supports in or above the basin. Mechanical aerators tend to incur lower installation and maintenance costs.

A diffused-air system introduces compressed air through a perforated membrane into the wastewater. Diffusers are classified by the physical characteristics of the equipment, or by the size of the air bubbles.



1. Peeters, B. and Vernimmen, L., Challenges of Handling Filamentous and Viscous Wastewater Sludge, Chem. Eng., September 2016, pp. 52–58.

2. National Small Flows Clearinghouse, West Virginia University, Explaining the activated sludge process, Pipeline, 14(2), Spring 2003.

3. Pakzadeh, B. and Zbacnik, R., Treating Wastewater for Industrial Reuse, Chem. Eng., September 2015, pp. 56–61.

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