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Activated Sludge Process and Types for Waste Water Treatment

Updated: Dec 12, 2022

The Activated Sludge (AS) process is currently the most widely used biological wastewater treatment process in the world.

Since its conception in the late nineteenth century and subsequent development into a full-scale process in 1913 by Arden & Lockett at the Davyhulme sewage treatment works in Manchester, the basic process has been widely adopted and further developed giving it a unique flexibility of operation.

What Is the Activated Sludge Process in Waste Water Treatment?

In the Activated Sludge Process for waste water treatment, microorganisms, including bacteria, fungi, and protozoa, are used to speed up the decomposition of organic matter requiring oxygen for treatment.

In this activated sludge process, microorganisms are thoroughly mixed with organics under conditions that stimulate their growth and waste materials are removed. As the microorganisms grow and are mixed by the agitation of the air, the individual microorganisms’ clump and flocculate together to form a mass of microbes called activated sludge.

Activated Sludge Meaning

The Activated Sludge name and meaning originates from the process it involves: the production of an activated mass of microorganisms capable of aerobically stabilizing the organic content of wastewater, thus it is “activated”.

Conventional Activated Sludge System in Water & Sewage Treatment

A conventional activated sludge system for sewage treatment includes:

  • The aeration tank where the biological reactions occur;

  • An aeration source that provides oxygen and mixing;

  • A tank, known as the clarifier, where solids settle and are separated from treated wastewater;

  • A collecting means for the solids either to return them to the aeration tank, returned activated sludge, or to remove them from the process (waste-activated sludge).

A complete system for sewage treatment includes chlorinators, holding tanks, pumps, and control system that monitors the treatment efficiency.

Conventional Activated Sludge Process Plant Setup
Conventional Activated Sludge Plant Setup

Activated Sludge Plant Components and Design

The Activated Sludge Plant (ASP) consists of an equalization basin, PST, an aeration tank, a secondary settling tank (SST) and a sludge recycle line.

Feed waste water is homogenized in an equalization basin to reduce variations in the feed, which may cause process upsets of the microorganisms and diminish water treatment efficiency.

In the aeration tank, aerobic bacterial population is maintained in suspension in the mixed liquor and oxygen, as well as nutrients are provided. Oxygen is supplied either by mechanical or diffused aeration, which also aids in keeping the microbial population in suspension. The mixed liquor is continuously discharged from the aeration tank into a secondary clarifier in a wastewater and sewage treatment system.

Efficient & Effective Activated Sludge Process Design

Activated Sludge process offers efficient and effective removal of BOD, COD and nutrients when designed professionally and operated properly. The process itself has flexibility and numerous modifications can be tailored to meet specific requirements (e.g. for nitrogen removal).

Activated sludge is a complex mix of microbiology and biochemistry involving many different sorts of microbes. In the Activated Sludge Plant (ASP) bacteria secrete sticky substances that coat the minute particles carried in sewage. The particles stick together to form flocs of gel-like material, creating a support on, and in which, microbes exist. This is the brown colored activated sludge.

The activated sludge is aerated to dissolve oxygen which allows the organic matter (BOD) to be utilized by the bacteria. The organic matter, or food, sticks to the activated sludge. The oxygen dissolved in the water allows the bacteria to use the food (BOD) and also to change the ammonia to nitrate. The activated sludge tank should be big enough to allow sufficient contact time (retention time) between the sewage and the activated sludge for all the chemical changes to take place.

Return Activated Sludge (RAS)

The settled biomass, called Return Activated Sludge (RAS), is then returned to the beginning of the aeration process where it will absorb fresh sewage to start the process again. This enables the process to operate as a continuous cycle.

When the Activated Sludge reaches the end of the process it is still a highly active biomass but is now mixed with purified effluent. It is transferred to Settlement Tanks (Secondary Clarifiers) to allow separation from the purified effluent which may be discharged to the river or to some form of tertiary treatment.

Surplus Activated Sludge (SAS)

The Surplus Activated Sludge (SAS) is wasted by continuously withdrawing some of the RAS for sludge disposal.

In fact, as the RAS mixing with the fresh sewage will produce a gradual growth in the activated sludge present it is necessary to waste a certain quantity each day.

Activated Sludge Laboratory Tests
Activated Sludge Laboratory Tests

Activated Sludge Types

There are nine types of activated sludge processes:

1. Tapered Aeration

2. Step Aeration

3. High-Rate Treatment or Modified Aeration

4. Two Stage Aeration

5. Activated Aeration

6. Reaeration

7. Contact Stabilization

8. Complete Mix

9. Extended Aeration Method

1. Tapered Aeration or Controlled Aereation

Tapered aeration is also called controlled aeration. If samples are collected at various points in the aeration units, it will be found that the BOD of the mixed liquor is not uniform throughout; it is maximum near the inlet and minimum near the outlet. Due to this reason if the diffused air is applied at uniform rate, it cannot be utilized properly.

2. Step Aeration

If the sewage is added to the returned sludge at more than two points along the aeration channel, the load coming on the returned sludge is changing. In the previous article, it has been stated that the BOD of the mixed liquor is different at different points.

3. High-Rate Treatment or Modified Aeration

In the high-rate treatment (or Modified Areation) method, less quantity of returned sludge is used, therefore, it is called high-rate treatment. In high-rate treatment shorter detention period (2 hours), lesser amount of compressed air and small quantity of returned sludge is used.

4. Two-Stage Aeration

In two-stage aeration the sewage is allowed to flow in a pair of aeration and sedimentation units. The activated sludge is either returned for seeding purposes or excess sludge from the second stage is again sent to the first cycle from which both of it and excess of first stage are mixed with influent.

5. Activated Aeration

It is a combination of two tapered activated sludge plants in parallel using the same settled sewage as influent. The excessive activated sludge of one plant is used as seeding for another plant, which is called activated aeration.

6. Reaeration

In this method, the minimum aeration of sewage is done after adding returned sludge to it. To satisfy the BOD reaeration of returned sludge is done before mixing it with the sewage. If the reaeration is done in advance of trickling filter, it is called bio aeration.

7. Contact Stabilization

The contact stabilization is also known as biosorption. In this process, sewage may be treated either after primary settling or without primary settling. The sewage is aerated along with return sludge for a comparatively short period of 0.5 to 1.5 hrs., when the sludge absorbs the organic matter in the sewage.

The mixed liquor is then settled in a secondary settling tank. The return fraction of the sludge withdrawn from the settling tank is reaerated in a separate sludge reaeration tank for a period of 3-6 hrs., before it is fed back into the contact aeration tank. During the reaeration of the sludge, the absorbed organics are stabilized restoring the adsorptive capacity of the sludge.

8. Complete Mix

The complete mix activated sludge process employs a completely mixed flow regime. In a rectangular tank, complete mixing is achieved by distributing the sewage and the return sludge uniformly along one side of the tank and withdrawing the aerated sewage, uniformly along the opposite side. In case of circular or square tank, complete mixing is achieved by mechanical aerator with adequate mixing capacity installed at the center of the tank.

9. Extended Aeration Method

The flow scheme of the extended aeration process and its mixing regime are similar to that of the complete mix process. The oxidation ditch also conforms to the extended aeration principle and relies on aerating the mixed liquor in an endless ditch. Primary settling is omitted in the extended aeration method, but commination is often provided for screenings.


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