activated sludge and activated sludge treatment

Activated sludge and activated sludge treatment process- Definition

Activated sludge:

British researchers Edward Ardern and W.T. Lockett coined the phrase “activated sludge” in 1914, and they patented the process for utilizing activated culture or flocs from the aeration tank and hence called as activated sludge treatment process. This brownish culture, grown under controlled conditions, can aerobically stabilize the organic content of wastewater. Bacteria, fungus, protozoa, and other microscopic organisms such as rotifers and nematode worms are found in activated sludge. These bacteria are the most significant and are responsible for the floc’s functional and structural activities. The bacteria in activated sludge or floc vary depending on the type of organic waste. Fungi are less common in biological flocs because they develop in a filamentous form, inhibiting the formation of good flocs.

Activated sludge treatment process

The activated sludge treatment process have many designs but all of them have three common component; an aeration tank; a settling tank and a return activated sludge equipment.

The Activated Sludge Treatment Process. (Pic courtesy: Pawel Fracz)

Aeration Tank:

The primary treated effluent enters aeration tank. Where air is mixed to promote microbial growth. The flocs produced by microbes in the water feeding on organic material in the wastewater. This settles and removes the flocs. The aeration performed during the process serves three purposes: mixing returned sludge with primary treated effluent, keeping activated sludge suspended, and supplying oxygen to microbes for necessary biochemical reactions.

Settling Tank:

After the aeration tank, the secondary clarifier or settling tank separates biological flocs from the treated effluent. Flocculation happens when one type of microbial growth comes into contact with another. The condition of the clarifier should not be so turbulent that flocs break. The ability of the floc particles to settle and compact depends on their density, size, and shape, as well as the efficiency of the clarifier.

Return Activated Sludge:

Secondary treated effluent still contains flocs of highly active biomass. Some of this biomass is returned to the aeration tank after passing through the settling tank or secondary clarifier, allowing the process to run in a continuous cycle. This returned sludge forms the return activated sludge and remaining sludge which does not goes back into the aeration tank forms waste activated sludge.


  1. WEFNet Org.
  2. Dokuz Eylül Üniversitesi.
  3. Oregon State University.
  4. WEF Org.
  5. Mountain Empire Community College.
  6. Netsolwater.

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One Comment

  1. It was interesting to learn that we can find bacteria, fungi, protozoa, and other microscopic organisms in activated sludge which are all needed for the treatment process. I imagine if it’s your job to do wastewater treatment, it would help to have the right equipment to use for dredging before you proceed with the activated sludge process. I’ll keep this in mind in case I ever need to use dredging equipment for future needs.

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