Nostocoida limicola filament types are commonly associated with filamentous bulking in biological wastewater treatment systems throughout the world. 

Literature such as the 3rd Edition Manual on the Causes and Control of Activated Sludge Bulking, Foaming, and Other Solids Separation Problems (Jenkins, 2004) recognizes morphotypes Nostocoida limicola I, Nostocoida limicola II, and Nostocoida limicola III. Each morphotype is distinguished by its oval, discoid or hockey-puck-like cell shape and differentiated by the cell diameter, with with Nostocoida limicola I being the thinnest (0.8-1.0 µm) to Nostocoida limicola III the thickest (2 µm in diameter).

Originally (and generally in municipal treatment systems), Nostocoida limicola filament types were believed to be exclusively Gram and Neisser positive. However, over time, many exceptions to this have been noted in staining reactions depending on factors such as the substrate utilized, the wastewater chemistry, and the actual genetic properties of the filament. Nostocoida limicola filament types are recognized to proliferate at elevated concentrations of low molecular weight organic acids. Organic acids are a fraction of the soluble carbonaceous BOD which may be naturally occurring in many industrial wastes and septage, or formed in areas of septicity/fermentation such as collection systems, lift stations, equalization basins, primary clarifiers, and sludge handling return side-streams. Organic acid (volatile acid) concentrations of greater than 85 mg/L are generally recognized to promote filamentous bulking from organic acid filament types such as Nostocoida limicola. 

From a practical standpoint, speaking with the late Dr. Jenkins and the late Dr. Richard (authors of the 2004 manual), and from what we have learned about genetic sequencing in the previous 20 years, it appears most feasible to recognize Nostocoida limicola I and II as a singular morphotype and Nostocoida limicola III as a separate morphotype, since Nostocoida limicola III filament types may also be correlated with low phosphorus availability in addition to elevated organic acid concentrations. Combining Nostocoida limicola I and II simply changes the diameter range to 0.8-2.0 µm (same cause). 

From a genetic standpoint, Nostocoida limicola filament types have high potential diversity from a wide range of phyla, including Proteobacteria, Actinobacteriota, Firmicutes, and Planctomycetota. Morphology of the majority of genera capable of possessing Nostocoida limicola morphology is generally variable, with most genera also capable of growing in other forms (tetrads, clusters of cells, single cell form).

Genera capable and suspected to possess Nostocoida limicola morphology include, but are not limited to, Defluviicoccus (51 recognized species, GAOs), Tetrasphaera (33 recognized species,  PAOs), Streptococcus (31 recognized species), Lactococcus (12 recognized species), Trichococcus, Ca Alysiosphaera (13 recognized species) and Ca Nostocoida limicola.

In general, Nostocoida limicola I and II morphotypes tend to occur over a fairly broad range of SRT values, while Nostocoida limicola III morphotypes tend to favor high F/M ratio in the initial contact zone (first 15-30 minutes) of the aeration basin. 

From an operational standpoint, traditional troubleshooting strategies for organic acid filaments (such as reducing organic acid concentrations, selective RAS chlorination, settling aids, or step feed) have a proven track record of success regardless of the genetic diversity within the filamentous morphotype viewed under the microscope. DNA testing in combination with microscopy does have the ability to distinguish capabilities in addition to an affinity for organic acids or readily available soluble carbonaceous BOD — such as the type of substrate they are able to utilize if they are capable of fermentation, PHA storage, etc. —  provided that the read percentage of a genera associated with Nostocoida limicola morphology may be reasonably distinguished as a percentage of the overall DNA (rRNA) reads using 16SrRNA community analysis in comparison to microscopy.

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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.