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

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 have been referred to as “sewage fungi”.

Protozoa - Amoeba

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 wastewater through predation of the bacteria.

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

There are examples of Amoeba without shells. The body shape changes with movement. The don’t have cilia.

There are also common Amoeba without a shell that are very tiny in comparison to other Amoeba types.

Protozoa – Stalked Ciliates

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

Sheathed Stalked Ciliate secrete a loric which is a an outer membrane that protects the ciliate.

Several types of Colonial Stalked Ciliate commonly occur in wastewater treatment.

Protozoa – Crawler Ciliates

There are two different Crawler Ciliates. These are called crawlers because they “crawl” over surfaces such as the 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 the plant is operating as it should.

Protozoa – Free Swimmers

Two examples of 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

These are one of the largest Flagellates found in activated sludge... so you have 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 flood 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.

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

Aquatic 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.

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.