Spirochaetes are a phylotype commonly recognized in wastewater biological processes that are associated with high organic acid concentrations. Spirochaetes are recognized by their corkscrew-like motility and thin, tightly coiled morphological traits. 

From an academic standpoint Spirochaetes is actually a phylum that includes high diversity with over 30 recognized species as referenced on the MIDAS field guide. Not all members of the Spirochaetes phylum possess the Spirochaetes phylotype and morphological traits. From our split-sample microscopy DNA database project underway at this point in time, the strongest correlation to the Spirochaetes phylotype appears to be the genus Leptospira. 

Leptospira possess a diameter of approximately 0.1 µm and length of typically between 6-12 µm with a helical coiling pattern. Leptospira are aerobic and can also be microaerophoilic (survive in extremely low amounts of dissolved oxygen). Not a high amount of in situ data is available on the genus Leptospira at this time, however at lab scale it takes up long-chain fatty acids and fatty alcohols. It is also highly likely that many genuses (including many currently unnamed genuses) may also be able to possess the Spirochaetes phylotype. 

Due to the small physical size of Spirochaetes, they are not visible with brightfield microscopy (need phase contrast to see them). The majority of the Spirochaetes phylotypes appear to have gram negative staining characteristics. Spirochaetes appear to settle fairly well and therefore are not often viewed in the supernatant above settled mixed liquor in our experience. Because Spirochaetes are not believed to pose operational problems, they may be more easily thought of as a sign of conditions present (high organic acid concentrations) than anything else. Spirochaetes are commonly observed and even often present at high abundance in well-operating plants, in particular often in plants that are achieving enhanced biological phosphorus removal.  

In summary, seeing Spirochaetes is not necessarily cause for concern, however their presence does tell us in general that the incoming wastewater being treated likely has higher concentrations of organic acids. In some instances, such as biological nutrient removal processes, higher organic acid/volatile acid concentrations may be beneficial for treatment goals. Spirochaetes can be thought of as an “indicator organism” for practical wastewater microscopy purposes. Note that if organic acid concentrations become too high and/or overload designated selector zones, problems such as filamentous bulking, dispersed growth and overgrowth of zoogloea bacteria types may occur. 

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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.