Bug of the Month: Gordonia and Nocardioform Foaming

In this month's wastewater microbiology spotlight, we take a look at Nocardioform foaming and strategies for short- and long-term control

Bug of the Month: Gordonia and Nocardioform Foaming


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In this month’s edition, we’re going to take an in-depth look at Nocardioform foaming. Please note that the “bug” listed is a genus (containing at the time of writing 52 recognized species) rather than a specific morphotype. 

Actinomycetes-Mycolata (also known as Nocardioforms) are recognized for their true branching and Gram-positive staining characteristics. The Actinomycetes-Mycolata morphotype is generally 0.8-1.2 µm in diameter, with septa (cell walls) commonly visible, and a cell length of 1.0-2.0 µm in dimension. The morphotype recognized as Actinomycetes-Mycolata has vast genetic potential diversity and may include thousands of actual species. 

Sludge bulking and foaming

Actinomycetes-Mycolata filament types may contribute to sludge bulking as well as foaming events in biological wastewater treatment processes. Sludge bulking may occur with Actinomycetes-Mycolata filaments directly impacting the floc structure or they may also interfere with settling when located within the floc due to their hydrophobic nature (wanting to float versus the density of the floc wanting to settle). Mycolic acid within the cell walls of these filaments are believed to be the origin of their hydrophobic nature. 

When Actinomycetes-Mycolata filament abundance is high enough in relation to other bacteria (or commonly when Actinomycetes-Mycolata filaments grow dispersed) it is common for them to create foam in various areas of the treatment plant such as the surface of the aeration basin.

Actinomycetes-Mycolata foam is diagnosed through microscopy by viewing a significantly higher abundance of these filaments within the foam than the underlying mixed liquor. Actinomycetes-Mycolata filaments may contribute to foaming regardless if they are alive or dead. When these filaments die, it is common for them to lose their Gram-positive staining reaction, as often seen in anaerobic digester foaming incidents. 

Actinomycetes-Mycolata foam on the surface of the aeration basin can cause aesthetic issues as well as operational issues. Operational issues occur when a significant portion of the necessary biomass for treatment becomes entrapped within the foam, or when there is significant risk of the foam volcanoing over the sides of the aeration basin. Foam-over events are most commonly correlated with industrial contribution and often the presence of surfactants to assist in stabilizing the foam. Aesthetic issues are subjective and involve risk assessment, plant history and other factors in determining if and when troubleshooting is necessary. 

Gordonia is often recognized as a foaming culprit based upon DNA testing, however, it is only one of many genera capable of causing foam that look and behave nearly identically. Many of these genera are members or closely related to the Nocardiaceae family within the Actinobacteriota phylum. Gordonia are aerobic heterotrophs capable of utilizing long chain fatty acids, lipids, acetate, propionate and sometimes glucose. In municipal treatment systems, Gordonia, as well as other Actinomycetes-Mycolata morphotypes in general are believed to most commonly compete due to fats, oils and grease.

Control strategies

Long-term control of Actinomycetes-Mycolata foaming generally involves reduction of fats, oils and grease into the treatment process which may be achieved through primary treatment or other methods such as pretreatment ordinances and grease trap inspection/enforcement. Actinomycetes-Mycolata tend to gain competitive advantage when fats, oils and grease become septic and form long-chain fatty acids. 

Short-term control strategies for Actinomycetes-Mycolata foaming have varying degrees of success and may include reduction of the sludge retention time, physical removal of foam via vac truck, elimination of foam trapping, chlorination of the RAS (often more successful of Actinomycetes-Mycolata filaments are dispersed in solution), and spraying of foams with chlorine solution.

About the author: Ryan Hennessy is the principal scientist at Ryan Hennessy Wastewater MicrobiologyHe 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 ryan@rhwastewatermicrobiology.com. Hennessy's new book Wastewater Microbiology: Filamentous Bacteria Morphotype Identification Techniques, and Process Control Troubleshooting Strategies is now available on Amazon.


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