Treatment Plant Operator’s Bug of the Month is an ongoing series that spotlights the organisms present in wastewater microbiology. Each month a new organism is featured, giving readers a profile of the species and how it functions in a wastewater treatment setting. 

In this peek under the microscope, take an up-close look at flagellates.

Flagellates and wastewater treatment systems

Flagellates are small (5 to 20 µm) and move using one or more long, whip-like flagellae. Flagellates consist of over 8,500 known different species. Many flagellate species found in activated sludge feed on soluble organic matter and often correlate with higher soluble biochemical oxygen demand (BOD) levels.

Some flagellates function colonially while others exist as a single cell. Flagellates multiply through binary fission with some species containing cyst stages. In general, flagellates prefer soluble nutrients or dead or decaying organic matter. Flagellates can compete with bacteria but can’t keep up with their logarithmic growth rate so only predominate when organic material is high.

Stress-event indicators

Flagellates often proliferate during start-up conditions or are the first protozoa to emerge after a toxic or stressful event. Seeing low amounts of flagellates is typical in systems that are performing well. When flagellates are predominant and/or present in high numbers, further investigation is often warranted.

It is important to keep in mind that flagellates and higher life-form organisms are only one of the pieces to the puzzle in properly accessing wastewater microbiology. The mix of higher life-form organisms can change quickly depending on a snapshot of current conditions, while looking at filamentous bacteria and other indicator organisms helps to provide a better understanding of conditions experienced during the previous few sludge cycles.

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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.