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Education: Microbes

Wastewater Microbes and Organisms

In nature and in a wastewater treatment plant, organisms are the key players in keeping our water clean. The following photographs provide a general sampling of the organisms involved in the cleansing process. Under a microscope, a variety of organisms can be observed having different sizes, shapes, life cycles, mobility, and roles. 

Click on a photo below to view an enlargement in a new window.

Bacterial Filaments


Using a phase contrast microscope, living bacteria can be visualized quite well. Some are motile and may swim across the field of view, while others may appear to vibrate or drift.

Rod-shaped, Filamentous Bacteria, shown here, have been referred to as "sewage fungi."


Protozoa - Amoeba

Ameoba Shell

Several groups of protozoa are represented in wastewater treatment plants. These include amoeba, stalked ciliates, crawler ciliates, free swimmers, and flagellates.

The primary role of protozoa is to clarify the effluent through predation on bacteria.

Amoeba come in two forms, shelled and without a shell. Here is a common shelled one, Arcella. The shell is composed of tiny sand grains.

Amoeba PodHere's a common Amoeba without a shell. The body shape changes with movement. They don't have cilia.


AmeobaThese are also common, don't have a shell, and are very tiny in comparison to the other types of Amoeba.




Protozoa - Stalked Ciliates

Single CiliateSingle Stalk Ciliate consume food via vorticellids, oral cilia that wind completely around the top of the cell. The stalk contains a contractile, slightly sinuous filament that can rapidly coil up like a spring, pulling the cell body down.




SheathedSheathed Stalked Ciliate secrete a lorica, an outer membrane that protects the ciliate.


Colonial CiliateSeveral types of Colonial Stalked Ciliate commonly occur in wastewater treatment plants.





Protozoa - Crawler Ciliates

CostataThese are two different Crawler Ciliates. They are called crawlers because they "crawl" over surfaces such activated sludge floc so they can find bacteria to eat. They don't free-swim very well.

The side view of Euplotes shows the "setae" which act as their legs. Both Euplotes and Aspidisca are common in activated sludge and their presence is desired as they indicate that a plant is operating as it should.


Protozoa - Free Swimmers

LitonotusThese are two common free swimmers in activated sludge. They have cilia all over the body which allows them to swim freely through the water. They feed on bacteria.


Protozoa - Flagellates

PeranemaThis is one of the largest Flagellates found in activated sludge...so you can imagine the difficulty of photographing the smaller ones! This one has a long extended flagella and another that lays along the body. The flagella enables movement as well as the ability to catch food by pulling bacteria down to its "mouth" where the flagella attaches to the body.

There are two primary groups of Flagellates. The Peranema belongs to the group which ingests its food. The other group of Flagellates is more like bacteria. They don't ingest whole food. They take in food that is already partially "digested."




These images are of the same genus of Rotifer and are the most common in activated sludge. Rotifers are multi-celled animals which draw in chunks of bacterial floc to feed on.



ColurellaThis type of Rotifer is also fairly common and has a shell around it. Some have shells, others do not. The presence of Rotifers in activated sludge generally means a good, stable sludge with plenty of oxygen.





Worms - Aquatic Earthworms

Aquatic earthworm

Aquatic Earthworms are very much like the earthworms in our yards, but these live in an aquatic environment. The earthworms in a yard can't live in that type of environment...as is obvious every time the ground becomes saturated from rains. Worms come out en masse...much to the delight of robins.

Aquatic wormAquatic Earthworms have setae along the body which allows them to tunnel through the floc particles, ingesting chunks of bacterial floc. They are quite common in old activated sludge.





Worms - Nematodes


Roundworms (Nematodes) are also common in activated sludge. Unlike aquatic, they are not segmented and their intestines are straight (earthworms have convoluted intestines, like us). Roundworms also feed on chunks of bacterial floc.

The Roundworms seen in wastewater treatment plants are "free living," that is, they are not parasites. Most people are familiar with the intestinal roundworms people and pets get, as well as those that are plant parasites. Roundworms do not have the setae like earthworms and move through the substrate by whipping their body back and forth.