The Activated Sludge Process

By May 9, 2023 No Comments
Activated Sludge Tank at Wastewater Treatment Plant
The activated sludge process involves the use of microorganisms and oxygen to break down organic matter and remove pollutants from wastewater.

What is activated sludge, how does the process work, and when is it used?

The activated sludge process is a biological wastewater treatment method that is commonly utilized in both municipal and industrial treatment systems. The process involves the use of oxygen or air and specific microorganisms to break down organic matter and remove nutrients from the wastewater, resulting in the formation of a sludge containing the oxidized compounds.

How Does Activated Sludge Treatment Work?

The activated sludge process utilizes a multichambered reactor consisting of an aeration tank, where air or oxygen is continuously introduced to support the growth of microorganisms, and a settling tank or clarifier, where the biological material settles away from the treated water. The separated waste sludge can be further treated before being sent for disposal.

Various methods of aeration can be employed in the activated sludge process, such as diffused aeration, surface aerators, or the addition of pure oxygen. Oxygen or air plays a dual role: breaking up and suspending solids in the aeration tank, and supporting the growth and activity of aerobic bacteria that break down the organic waste.


During the activated sludge process, sludge particles and live bacteria settle to the bottom of the clarifier while the clear treated water rises to the surface. A portion of the sludge and bacteria is typically recycled back into the aeration tank to facilitate the treatment of the next batch of wastewater. Once the wastewater has been fully treated, the clean water at the surface of the settling tank can be discharged into the disinfection unit.

Pros and Cons of Activated Sludge Treatment

The activated sludge process offers several advantages over other treatment options. The chambers require less frequent emptying, the volume of sludge is reduced, and beneficial bacteria are recycled back into the system to reseed the treatment process. It is a straightforward, reliable, and cost-effective method of wastewater treatment.


Despite its benefits, the activated sludge process has some drawbacks. The main obstacle is the high initial capital cost and the high energy consumption required for aeration, which can result in higher operation costs. Additionally, the setup can be complex, requiring a high level of expertise for effective design, construction, operation, and maintenance of the wastewater treatment system. Moreover, the effluent and sludge produced may require further treatment before discharge, adding to the overall costs of the process.

Start Small to Stay Under Budget

While the activated sludge process may present some challenges, there are solutions available to overcome these problems. For instance, AUC Group provides modular packaged wastewater treatment solutions that enable clients to start with a smaller system and expand it as their needs grow, reducing initial costs. Additionally, AUC offers a Lease Plant Program that allows clients to obtain the necessary infrastructure without a high initial capital outlay. This program offers flexible short- and long-term lease agreements, as well as a lease-to-purchase option, providing clients with greater financial flexibility.

If you are looking for wastewater treatment, AUC is here to offer the best solution. Contact us to learn more about our offerings and flexible lease options.

Image Credit: dedmityay/123rf