The Eickelboom filament morphotype commonly recognized as Haliscomenobacter hydrossis has a long and somewhat mysterious history. 

Its recognized growth cause has changed from earlier literature as a low F/M filament type to other causes such as low DO and low phosphorus availability. The truth about this filament type is that there is very high potential genetic diversity within the Haliscomenobacter filament type with genera most commonly attributed to either the Chloroflexi or the Bacteroidota phyla and is is likely that the above list of causes may either be relevant or not depending upon the genetic diversity of the actual filament, and/or the given treatment plant conditions. 

The genus Haliscomenobacter is recognized to contain 10 species (Haliscomenobacter hydrossis and nine others currently unnamed). The MiDAS field guide recognizes the Haliscomenobacter genus of being able to break down N-acetylglucosamine, allowing it to utilize decaying biomass and also to uptake glucose and propionate in aerobic conditions in situ.

In literature written by Per Neilson — in addition to running microscopy and DNA tests on concurrent activated sludge samples — it becomes clear that the actual potential genetic diversity within the Haliscomenobacter filament type is very high. In the author’s experience, Haliscomenobacter filament types often correlate not only to the Haliscomenobacter genus, but also to many closely related genera within the Saprospiraceae family (Bacteroidata phylum), as well as genera including but not limited to Longilinea, Flexilinea, and Leptolinea (all of the Chloroflexi phylum). Of note, most correlations within the Chloroflexi phylum are members of the Anaerolineaceae family (recognized to perform fermentative functions).

From personal observations analyzing thousands of wastewater microscopy samples, the Haliscomenobacter filament type must be treated with caution as far as assigning any specific growth causes to the recognized filament type. In using Dr. Jenkins’ method of ranking filamentous bacteria abundance, one to five filaments of the same morphotype must be present within most of the flocs to be ranked “common.” Typically, if Haliscomenobacter filament types are ranked less than common, it is good practice to note them but look more at other filament types and indicator organism phenotypes — along with their respective rank and abundance — when looking to assign an overall big-picture diagnosis to what is occurring within the plant.

If Haliscomenobacter filament types are responsible for sludge bulking, further investigation of any potential growth causes for Haliscomenobacter filament types is warranted. Instances of low DO and low phosphorus availability are both commonly recognized. In general, it is more common for phosphorus to be limited in industrial wastewater treatment plants that add supplemental phosphorus than in municipal wastewater treatment systems.

From a practical standpoint, if Haliscomenobacter-like filaments are present within the floc at high abundance, but do not appear to be impacting the floc structure (rather providing a backbone) it may often be best practice to note that Haliscomenobacter was observed but to look more specifically at what filament type or other indicator organism phenotype appears to be directly responsible for any issues with bulking of solids separation for troubleshooting purposes. 

Common short-term control methods such as RAS chlorination and the use of settling aids are generally applied when Haliscomenobacter filament types become problematic. Ideally, the root cause for the Haliscomenobacter filament type is discovered and remedied long term to prevent further issues.

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