Over the course of the previous 30 to 40 years, there has been strong correlation to various filamentous bacteria morphotypes and the conditions in which these filament types predominate. One unique filament that can cause and contribute to bulking events is filament type 1851.

Historically in credible references such as the "Third Edition Manual on the Causes and Control of Activated Sludge Bulking, Foaming, and Other Solids Separations Problems" (2004, Jenkins, Richard, Daigger) the recognized growth cause for this filament is low food to microorganism (F/M) ratio.

Before going into further detail about filament type 1851, it may be beneficial to think of F/M in terms of availability of carbonaceous material for the bacteria as the water travels through a plug flow aeration basin reactor. As the carbonaceous material is depleted, there is less available food and bugs that have lower metabolic needs and slower growth rates begin to gain competitive advantages.

The term F/M ratio may be complicated at times depending on the individual application as we tend to view F/M as an entire value for a system, rather than taking into considerations the bioavailability of the carbonaceous material and how the F/M ratio changes as the wastewater is treated down the basin. For example, in some systems (such as dual sludge systems) the growth conditions in the first 10-15 minutes of aeration as well as the growth/endogeny/maintenance conditions on the back end of the aeration basin both may contribute to the selection of which bacteria predominate.

About filament type 1851

In our experience, we like to relate filament type 1851 to higher sludge retention time and/or lower F/M values with an emphasis on the higher SRT cause. From a genetic standpoint, type 1851 has been correlated to the Kouleothrix genus. The MIDAS field guide recognizes seven currently unnamed species within this genus. These filaments are believed to specialize in the metabolism of sugars and likely able to store sugars under anaerobic conditions and then use these as substrate at higher SRT values or lower F/M values.

Kouleothrix is a member of the Chloroflexi phylum which in general has these same properties and capabilities. It is likely there is further genetic diversity within the type 1851 filamentous morphotype with other candidates belonging to the genus Herpetosiphon, the Anaerolineaceae family of Chloroflexi, the Ca. Saccharibacteria phylum (formerly TM7- believed to be polysaccharide degraders), and several unclassified tax ID reads. In our ongoing database project (now more than 70 samples), Aster Bio tax ID numbers 292826, 292625, 475964, 292628, 200795, 32064 and 1508635 are all possible candidates. These tax ID reads end at various taxometry levels, with some stuck higher up the taxometry tree than others.

That said, all type 1851 filament types may not be identical from a genetic basis and it’s likely that some fall into low F/M category while in others storage and higher SRT values may be the more important growth factor consideration. It is fairly common that type 1851 filament types emerge prior to the other high-SRT filament type (type 0675/0041). We have had success in some plants monitoring this filament type, and if its abundance gets too high, decreasing the SRT value or vice versa.

From an identification purpose, “bundles” or “twisted ropes” are common within this filament type and the diameter is typically around 0.8 µm. Cells are rectangular, however, it is often difficult to see the individual septa (cell walls). Attached growth of perpendicular bacteria are common within this filament type, although it does not always have attached growth (depending on how quickly it is growing and how long it has been in the system).

Why you need the whole picture

As with everything when you’re using the microscope for process control, the “whole picture” scenario is needed for the best available interpretation of results. This includes the floc structure, the mix of higher life form organisms, and the rank/abundance of the other filamentous bacteria morphotypes or other indicator organism phylotypes that may be present. Knowing what is normal or a baseline is beneficial as many well-running plants (particularly oxidation ditches) may have common abundance of filaments such as type 1851. 

However, the floc structure may remain strong and the sludge volume index may be in good ranges. Just because a cause or condition may be present does not always equate to a problem that requires any operational adjustments. Each plant is different, and learning when a condition becomes a problem is often somewhat plant specific. Just because you have type 1851 doesn’t mean you always need to add carbon.

Control of filament type 1851 is typically accomplished by either reducing the SRT/MLSS concentration or in some instances chlorination of the return activated sludge or increasing the amount of available substrate the bugs receive. Note that each situation is specific, and proper identification of the problem and logistical aspects of respective choices must be taken into consideration for choosing the most appropriate, if any, operational actions to be taken. 

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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.