Eickelboom 021N filament types are commonly recognized to cause and contribute to sludge bulking in industrial and municipal wastewater treatment facilities. 

The morphological characteristics of type 021N include irregular cell shape (commonly barrel-like), visible septa (cross walls), indentations at the septa, and diameter ranges of 1.6 to approximately 2.8 µm. Type 021N filaments commonly extend from the floc structure, often causing inter-floc filamentous bridging. Due to the large diameter (and often long length) of these filaments, proliferation of type 021N filament types commonly correlates to significantly high sludge volume index (SVI) values.

Type 021N filaments are represented by a fairly diverse group of genera, most commonly of the Gammaproteobacteria class of the Proteobacteria phylum and are generally recognized as mixatrophs with the ability to utilize either organic acids or sulfide for available substrate. Often, but not always, 021N filament types contain intracellular sulfur granules. Should intracellular sulfur granules be viewed, the filament is utilizing sulfide, however in practice sulfide and organic acids are commonly simultaneously present, so only removing sulfide is often not a deterrent for type 021N morphotype growth. 

Organic acids are naturally present in many industrial wastewaters or may be formed in areas of septicity/fermentation like collection systems or primary clarifiers. Note that once organic acids are present, this “food” for type 021N filaments remains. Because type 021N filament types are a functionality of the type of substrate present, increasing dissolved oxygen set points in the aeration basin generally does not discourage growth of type 021N filaments unless the aeration basin itself was the source of fermentation or organic acid formation.

Type 021N filament types are notorious for their often high kinetic growth rates, and when these filaments are proliferating, it is not uncommon for the SVI value to increase rapidly in a very short amount of time. In addition to their fast growth rates, type 021N filaments also commonly store Polyhydroxybutyrates (PHB), allowing them to latch on to available substrate to help them store and outcompete other more favorable bacteria when high concentrations of organic acids or readily soluble BOD are present.

Long-term control of type 021N filaments often involves reducing the organic acid/volatile acid concentrations of the influent (if possible), slowing bleeding in high-strength waste, anaerobic pretreatment, step-feed configuration, occasional recirculation of treated effluent, and other methods such as enforcing discharge limits for industrial contributors to municipal systems. In many instances, it is most logistical to let type 021N filaments grow and selectively kill them through RAS chlorination tactics. Chlorination is often a good short-term control method because type 021N filaments most commonly bridge flocs together and are thus first targeted by the chlorine.

There is literature that also correlates type 021N morphology to low nitrogen availability, and while this at times may be true, it is generally considered good practice to investigate septicity/organic acids as the first potential growth cause. Should total inorganic nitrogen residuals in the filtered mixed liquor be less than 0.5 mg/L for chronic durations of time, investigation of low nitrogen as a growth cause may warrant further action. Anthrone test results of greater than 20% polysaccharide per dry weight of biomass are also strong indicators of low nutrient availability over the prior one to two sludge cycles.

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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.