Stalked ciliates are protozoa (higher life-form organisms) that are characterized by their cilia, which are hair-like organelles. Ciliates are eukaryotic and contain two nuclei: a macronucleus, responsible for mainly cell functions, and a micronucleus which works together with the macronucleus to process genetic material to offspring through meiosis cell division.

Stalked ciliates eat through a process called phagocytosis in which the plasma membrane of the cell is used to engulf large food particles, creating vacuoles (basically the “stomach” mechanism). The food follows a path in which it is broken down by lysosomes, and when small enough, enters the membrane of the cell. Excess food that remains after the path of food metabolism is then excreted.

Most stalked ciliates are heterotrophs that feed on small bacteria and other debris. Many stalked ciliate types use their cilia to filter, or sweep in, their food. The cilia then direct this food through the mouth and into the gullet where the vacuoles are then formed for the metabolism process. Other types of stalked ciliates may have different feeding methods such as absorption, or cannibalistic methods where they feed on other ciliates and higher life-form organisms rather than bacteria.

Role in wastewater treatment

The higher life-form organisms (stalked ciliates, rotifers, etc.) are estimated to make up typically between 5-10% of the mixed liquor mass, however they are not directly responsible for the majority of biological treatment. Rather, it is often easier to think of them as “polishers” that mostly reduce populations of dispersed bacteria. For these reasons, well-running treatment processes often contain a predominance of stalked ciliates within the mix of higher life-form organisms. 

Stalked ciliates are generally one of the more sensitive higher life-form organisms found in activated sludge systems. There are many different types of stalked ciliates commonly found in wastewater with different distinct features. Some stalked ciliate types are colonial with multiple heads. In periods of stress, it is common for the heads to detach from the stalks so “headless” stalked ciliates may be an indicator of recent stress to these microorganisms.

It is possible for efficient wastewater treatment to occur with or without the presence of stalked ciliates and other higher life-form organisms. If stalked ciliates are commonly found within your plant and are suddenly absent, this may warrant further investigation with an emphasis of looking for other potential signs of stress, like broken up flocs, dispersed growth, or dead/damaged filamentous bacteria.

In our experience there does not appear to be a strong correlation to stalked ciliates and sludge age conditions, and it is suspected that the type of food and the prey density of this food are typically the most significant growth factors allowing stalked ciliates to compete. From a practical wastewater microbiology standpoint — despite the high amounts of different types of stalked ciliates (estimated to be over 10,000 or more individual species) — categorizing them as “stalked ciliates” is typically adequate.

Stalked ciliate scum

It is common to see stalked ciliates as the predominant higher life-form organism in well-running systems. However, as with just about everything wastewater microbiology related, there can be exceptions in certain instances, such as the notorious stalked ciliate scum which can accumulate on DO probes and other sensors or interfere with tertiary filtration should a massive outbreak of stalked ciliates emerge.

There is definitely value in keeping an eye on higher life-form organisms such as stalked ciliates. However, it is also important to remember that they are only one piece of the puzzle. Microscopy with emphasis on floc structure; amounts/type of dispersed growth; filamentous bacteria abundance; filamentous bacteria impact on floc structure; filamentous bacteria morphotype abundance/associated causes; zoogloea bacteria abundance/impact on floc structure; along with in-house testing, analytical data and an operator’s historical experience with a plant are the best barometers used to make operational decisions.

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About the author: Ryan Hennessy is the microbiology and operations specialist at Midwest Contract Operations Inc. He was trained and mentored by Dr. Michael Richard for over 10 years in wastewater microbiology, and serves as a microbiology services consultant. Hennessy is a licensed wastewater treatment and municipal waterworks operator in the state of Wisconsin and fills in as needed for operations at several facilities. He can be reached at rhennessy@mco-us.com.