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Aeration tank

Activated sludge process is a common method of aerobic wastewater treatment. The purpose of the process is to reduce amount of dissolved organic matter from wastewater, using microorganisms growing in aeration tanks. Microorganisms convert dissolved organic matter into their own biomass, oxidizing carbonaceous matter, oxidizing nitrogenous matter and removing phosphates. The formed semi-liquid material (a community of microorganisms grouped in flocs) is than separated from treated supernatant.

Secondary clarifier

The treated wastewater runs over the edges of secondary clarifiers. A part of the settled sludge (RAS) is being returned into aeration tanks, where is mixed with "fresh" primary treated wastewater and biooxidation process goes on.

The settled sludge goes to further treatment - anaerobic decomposition in controlled conditions with biogas (methane) production.

In order to enable efficient treatment processes and avoid problems in technology, knowledge about hydroecology, microbiological communities and biology of microorganisms that take part in wastewater treatment process is necessary.


Under The Microscope: What is the role of microorganisms in wastewater treatment?
(Please wait for YouTube previews)


1. How activated sludge is formed? In order to show the activated sludge formation process, we performed an experiment. We aerated synthetic sewage (a medium similar to domestic waste waters) overnight in lab in non-sterile conditions. The medium became turbid. What happened? The bacteria present in medium developed, feeding on medium organic components. Free bacterial cells are shown in the clip....

Free bacteria in "synthetic sewage" medium aerated overnight



2. How flocs are formed? Feeding on organic matter dissolved in water in the presence of oxygen, bacteria decompose organic matter to carbon dioxide and water, transforming a part of organic matter into it's own biomass. Separating individual bacterial cells from water would be a very difficult process if they didn't have a very important property - a predisposition to gather together. Microorganisms aggregate in flocs - flake-like structures that consist of alive and dead cells of microorganisms and products of their metabolism. Such structures are easy settled, enabling us to separate treated effluent from sludge. In this video clip, the flocs formed during the 1st day in laboratory conditions, in the experiment with "synthetic sewage" mentioned above are shown.


Film: Bakterije formiraju flokule - pahuljičaste strukture koje se talože


3. Bacteria are not the only inhabitants in activated sludge. Other organisms - fungi, protozoans and metazoans also take part in treatment process. Free-sweemming ciliates feed on bacterial cells, reducing number of free cells in liquid phase.

In this video clip, free swimming ciliates feeding on bacterial cells in 24h aerated "synthetic sewage" medium are shown.

Free-swimming ciliates in activated sludge


4. Crawling ciliates "crop" bacteria from floc surface and contribute forming of regular-shaped flocs. Firm and regular-shaped flocs are settled better. Aspidisca costata, a common crawling ciliate in activated sludge is shown in the video clip. Activated sludge sample was taken from Subotica Wastewater Treatment Plant aeration tank


Aspidisca costata "crops" on the surface of a floc


5. Water is filtered also by stalked ciliates. Sessile ciliates, attached to flocs with stalks, feed on small particles using their cilia to filter water. Vorticella species are common inhabitants of activated sludge community.

Vorticella sp. in Subotica WWTP activated sludge


6. Multicellular animals (Metazoa) are also present in activated sludge. Rotifers are sometimes found in activated sludge. Like other activated sludge organisms, their contribute floc stabilization.

Rotifers in Subotica WWTP activated sludge


7. Some activated sludge organisms can be used as bioindicators, giving us useful information about system conditions. These data can help us in wastewater treatment process management.

For example, if amoeba are numerous in activated sludge, it indicates high organic load.

Amoeba in Subotica WWTP activated sludge
      Spirilla and spirochetes (spiral-shaped bacteria) are also high organic load indicators and point out to lack of oxygen.

Spirilla in Subotica WWTP activated sludge

Spirochaete in Subotica WWTP activated sludge


High diversity in community, especially diversity of stalked ciliates, usually points out to good system conditions. Some ciliates live in colonies, like Carchesium shown in this video clip.





Carchesium - colonial stalked ciliates in Subotica WWTP activated sludge  

8. Not all microorganisms are "welcome" in wastewater treatment plants. Some of them can complicate treatment processes by changing properties of activated sludge.

Filamentous bacteria, for example, can increase sludge volume index (SVI) of activated sludge. Protruding from the flocs, filaments slow down or obstruct their proper settling. As a consequence, sludge bulking occurs


Some branched filaments contribute creation of foam in aeration tanks.







©2008 Žika Rech - All images and videos were made in Subotica Wastewater Treatment Plant Biological Laboratory


These Web pages are part of the project "Popularizacion of Wastewater Treatment in Vojvodina", funded by The Fund for development of the nonprofit sector in the Autonomous Province of Vojvodina