Filamentous Bacteria

Although there are some filamentous algae and filamentous fungi in the activated sludge process, most filamentous organisms are bacteria (Table 15.1). Filamentous bacteria enter the activated sludge process through (1) inflow and infiltration as soil and water organisms, (2) their growth in biofilm in sanitary sewers, and (3) the effluent of industrial wastewater treatment plants that biologically pretreat their wastewater.

Filamentous bacteria perform positive roles and negative roles in the activated sludge process. Positive roles include (1) degradation of soluble cBOD, (2) improvement in floc formation, and (3) degradation of some complex forms of cBOD. Negative roles include (1) settleability problems, (2) loss of solids, and (3) foam production. Foam-producing filamentous organisms include Microthrix parvicella, Nocardioforms, type 0092, and type 1863.

Whether filamentous bacteria perform positive or negative roles in the activated sludge process is determined by their relative abundance. Positive roles are experienced when one to five filamentous bacteria are present in most floc particles. This level of filamentous growth is equal to a relative abundance rating of "3" or "common" on the table of Relative Abundance Ratings for Filamentous Organisms (Table 15.2). At relative abundance ratings of "0," "1," and "2," filamentous bacteria are not present in adequate numbers to provide significant positive roles. Relative abundance ratings < "3" are due to (1) young sludge age, (2) complex cBOD as the major substrates, and (3) toxicity.

At relative abundance ratings > "3" the negative roles of filamentous bacteria are experienced. The undesired growth of filamentous bacteria at ratings > "3" can be associated with specific operational conditions for each filamentous bacteria (Table 15.3). It may be necessary to perform a Gram stain on a smear of mixed liquor to

TABLE 15.1 Major Filamentous Bacteria in the Activated Sludge Process

Beggiatoa Type 0581

Haliscomenobacter hydrossis Type 0675

Microthrix parvicella Type 0803

Nocardioforms Type 0961

Nosticoda limicola Type 1701

Sphaerotilus natans Type 1702

Thiothrix Type 1851

Type 0041 Type 1863

Type 0092 Type 021N

TABLE 15.2 Relative Abundance Ratings for Filamentous Organisms




Undesired Settleability Experience





"Very common"

"Abundant" "Excessive"

Filaments not observed

Filaments present; in an occasional floc

Filaments present; frequently observed, but only in some floc Filaments present; 1-5 filaments in most floc

Filaments present; 6-20 filaments in most floc

Filaments present; >20 filaments in most floc Filaments present; more filaments than floc or filaments are excessive in bulk solution

No No No

No, unless significant interbloc bridging or open floc formation or foam-producing filaments in floc Yes

Yes Yes determine the relative abundance of the filamentous bacteria, if the filamentous bacteria are (1) translucent, (2) short in length, and (3) mostly within the floc particles. Of the settleability problems and loss of solids issues related to undesired growth of filamentous bacteria, most are due to 10 filamentous bacteria. These bacteria include Haliscomenobacter hydrossis, Microthrix parvicella, Nocardioforms, Sphaerotilus natans, Thiothrix spp., type 0041, type 0092, type 0675, type 1701, and type 021N.

In addition to undesired filamentous bacterial growth, there are two filamentous bacteria/floc particle structures that contribute to settleability problems and loss of solids. These forms are interfloc bridging (Figure 15.1) and open floc formation (Figure 15.2). Interfloc bridging is the joining in the bulk solution of the extended filamentous bacteria from the perimeter of two or more floc particles. Open floc formation is the scattering of the floc bacteria in small groups along the lengths of the filamentous bacteria in the floc particle.

There are four foam-producing filamentous bacteria. These organisms are Microthrix parvicella, Nocardioforms, type 0092, and type 1863. These organisms

TABLE 15.3 Operational Conditions Associated with the Undesired Growth of Filamentous Bacteria

Operational Condition High MCRT(>10 days)

Fats, oils, and grease

High F/M or slug discharge of soluble cBOD High pH (>8.0)

Low dissolved oxygen and high MCRT Low dissolved oxygen and low to moderate

Low nitrogen or phosphorus

Low pH (<6.5) Organic acids

Readily degradable substrates, e.g., alcohols, amino acids with sulfur, glucose, volatile fatty acids Septicity/sulfides (1-15mg/liter) Slowly degradable substrates Warm wastewater temperature Winter proliferation

Filamentous Bacteria

Microthrix parvicella 0092, Microthrix parvicella, Nocardioforms 1863

Microthrix parvicella Microthrix parvicella

Haliscomenobacter hydrossis, Sphaerotilus natans, 1701

Haliscomenobacter hydrossis, Microthrix parvicella, Nocardioforms, 0041, 0092, 0581, 0675, 0803, 0961, 021N Haliscomenobacter hydrossis, Nocardioforms, Sphaerotilus natans, Thiothrix, 0041, 0092, 0675, 1701, 021N Nocardioforms Beggiatoa, Thiothrix, 021N Haliscomenobacter hydrossis, Nosticoda limicola, Sphaerotilus natans, Thiothrix, 1851, 021N

Beggiatoa, Nosticoda limicola, Thiothrix, 0041, 021N Microthrix parvicella, Nocardioforms, 0041, 0092 Sphaerotilus natans, 1701 Microthrix parvicella

FIGURE 15.1 Interbloc bridging.
FIGURE 15.2 Open floc formation.

TABLE 15.4 Manuals Available for the Identification of Filamentous Organisms

D. H. Eikelboom and H. J. J. van Buijsen. 1989. Microscopic Sludge Investigation Manual. TNO Research Institute for Environmental Hygiene, Water and Soil Division, P.O. Box 214, 2600 AE Delft, The Netherlands.

D. Jenkins, M. Richards, and G. T. Daigger. 2004. Manual on the Causes and Control of Activated Sludge Bulking and Foaming, 3rd edition. CRC Lewis Publications, Boca Raton, FL.

produce viscous chocolate-brown foam. Control of the growth of these filamentous bacteria requires appropriate treatment measures for not only the mixed liquor but also the foam.

Filamentous organisms are identified to name or type number according to their morphology, response to staining techniques, and ability to oxidize sulfur. There are two manuals that are used for the identification of filamentous organisms (Table 15.4). By identifying the filamentous organisms that are responsible for operational problems, the operational conditions that permit the rapid and undesired growth of the filamentous organisms also can be identified (Table 15.3). Once the operational conditions have been identified, they may be regulated to control the undesired growth of the filamentous organisms.

The identification of filamentous bacteria and operational conditions that are responsible for their undesired growth is part of a slow, specific operational measure that often is used to control undesired growth of filamentous bacteria. In addition to this measure, there are other operational measures that can be used to control either the growth of the filamentous bacteria or the operational problems that they cause. These include (1) the use of rapid, nonspecific control measures and (2) use of selectors (Table 15.5).


TABLE 15.5 Operational Measures Available for the Control of Undesired Filamentous Bacteria or Their Operational Problems

Rapid, nonspecific control measures Increase the return activated sludge (RAS) rate Manipulate the substrate feed point to the aeration tank Add a coagulant to the secondary clarifier influent Add a polymer to the secondary clarifier influent Add a toxicant Slow, specific control measures Identify the problematic filamentous organisms

Identify the operational conditions responsible for undesired filamentous growth Regulate the operational conditions to prevent undesired filamentous growth Use of selectors Anoxic Anaerobic F/M

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