Floc Forming Bacteria

Floc formation does not occur in an anaerobic digester. Septicity in the digester destroys floc formation. Floc formation does occur in the activated sludge process and is essential for its success. Floc formation permits the "packaging" of a large and diverse population of bacteria in numerous floc particles that (1) can be separated from the waste stream in the secondary clarifier and (2) can be recycled (Figure 14.1) as needed to achieve the following treatment objectives:

• Degrade carbonaceous BOD.

• Degrade nitrogenous BOD.

• Remove fine solids (colloids, dispersed cells, and particulate material).

• Remove phosphorus.

• Develop a diverse population of higher life forms (ciliated protozoa, rotifers, and free-living nematodes) that improve treatment efficiency.

Floc formation occurs naturally with increasing MCRT and is initiated by floc-forming bacteria (Table 14.1). These bacteria are able to produce three necessary cellular components that enable them to "stick" together or agglutinate. These cellular components are (1) pili or fibrils, (2) sticky polysaccharides, and (3) poly-P-hydroxybutyrate (PHB) or starch granules (Figure 14.2).

The pili or fibrils are extensions of the cell membrane that protrude through the cell wall into the bulk solution. The fibrils contain key functional groups such as the carboxyl group (-COOH) and the hydroxyl group (-OH) that become ionized with the lost of the hydrogen atoms. The ionized fibrils or bacterial cells are joined together by bivalent cations such as calcium (Ca2+) that are in solution (Figure 14.3). The joining of fibrils from different bacterial cells initiates floc formation. Ionized

FIGURE 14.1 Essential activities and structural features of the floc particle in activated sludge. Essential activities performed by the floc particle in the aeration tank include (1) removal of cBOD, (2) removal of nBOD, (3) removal of phosphorus, (4) removal of fine solids, and (5) removal of heavy metals. Necessary structural features of the floc particle that are important in the aeration tank and secondary clarifier include (1) firm structure that is resistant to shearing action or turbulence and (2) dense structure that contributes to acceptable settling.

FIGURE 14.1 Essential activities and structural features of the floc particle in activated sludge. Essential activities performed by the floc particle in the aeration tank include (1) removal of cBOD, (2) removal of nBOD, (3) removal of phosphorus, (4) removal of fine solids, and (5) removal of heavy metals. Necessary structural features of the floc particle that are important in the aeration tank and secondary clarifier include (1) firm structure that is resistant to shearing action or turbulence and (2) dense structure that contributes to acceptable settling.

TABLE 14.1 Significant Genera of Floc-Forming Bacteria

Achromobacter Citromonas

Aerobacter Escherichia

Alcaligenes Flavobacterium

Arthrobacter Pseudomonas

Bacillus Zoogloea fibrils that are not joined together remain exposed to the bulk solution and act as the "wisps of a broom" as they sweep and remove fine solids and heavy metals from the bulk solution.

There are several polysaccharides that make up the glycocalyx or sticky coating outside the bacterial cell wall and contribute to floc formation by "sticking" cells together (Figure 14.4). Some polysaccharides, such as those produced by young bacterial cells, are weak-bonding and produced in large quantities. Other polysaccharides, such as those produced by old bacterial cells, are strong-bonding and produced in small quantities. The differences in bonding strength and quantity of polysac-charides produced result in the development of (a) weak and buoyant floc particles at a young sludge age and (b) firm and dense floc particles at an old sludge age.

Poly-P-hydroxybutyrate (PHB) is a starch and serves two purposes. First, it is stored inside and outside the bacterial cell where it serves as a food reserve (Figure 14.5). Second, when stored outside the cell, PHB helps to anchor bacterial cells more

Polysaccharide Fibril

Polysaccharide Fibril

FIGURE 14.2 Necessary cellular components for the initiation of floc formation. There are three necessary cellular components for floc formation. These components include bacterial fibrils, "sticky" polysaccharides, and PHB or starch granules.

Heavy metals adsorbed to fibrils Fine solids adsorbed to fibrils

Heavy metals adsorbed to fibrils Fine solids adsorbed to fibrils

FIGURE 14.3 Joining of bacterial fibrils. The ionized fibrils on bacterial cells are joined together by bivalent cations such as calcium (Ca2+) that are in solution.

tightly together; that is, it improves floc formation. When present at the perimeter of the floc particle, PHB also helps to anchor particulate material to floc particles.

PHB production and deposition outside bacterial cells is slow. Therefore, rapidly growing, young floc particles have PHB mostly in the core of the floc particle, while slowly growing, old floc particles have PHB in large quantities in the core and

FIGURE 14.4 Joining of polysaccharides.

^^ PHB granule deposited outside the cell 14.5 Polyhydroxybutyrate (PHB) deposition.

FIGURE

^^ PHB granule deposited outside the cell 14.5 Polyhydroxybutyrate (PHB) deposition.

PHB granule
FIGURE 14.6 Deposition of PHB granules in young and old floc particles.

perimeter of the floc particles (Figure 14.6). Due to the differences in quantity and location of PHB, old floc particles or sludge is stronger and denser than young floc particles or sludge.

Filamentous organisms also perform a significant role in floc formation. They provide an internal "backbone" or network of strength for the floc particle that enables the particle to resist shearing action and increase in size. The increase in size provides for a larger number and diversity of bacteria and improved treatment efficiency.

The increase in size of the floc particles occurs as bacteria grow along the length of the filamentous organisms. Because floc formation is initiated before filamentous organisms can increase in length and extend beyond the perimeter of the floc particle, young floc particles have little or no filamentous organisms as compared to old floc particles. Due to the absence of filamentous organisms, young floc particles are spherical in shape (Figure 14.7), while old floc particles that contain filamentous organisms are irregular in shape (Figure 14.8).

There are significant differences between young floc particles or sludge and old floc particles or sludge (Table 14.2). These differences are responsible for the production of weak and buoyant floc particles at a young sludge age and firm and dense floc particles at an old sludge age. The size of a floc particle is determined by (1) the agglutinating strength of the floc-forming bacteria, (2) absence or presence of

FIGURE 14.7 Spherical floc particles.
FIGURE 14.8 Irregular floc particles.

TABLE 14.2 Significant Differences between Young Floc Particles and Old Floc Particles

Feature Young Floc Particles Old Floc Particles

Shape Spherical Irregular

Size Small (<150 |m) Medium (150-500 |m) and Large (>500 |m)

Color Light Golden-brown

Filamentous organisms Insignificant Significant

Strength Weak Firm

Settleability Poor Good

TABLE 14.3 Operational Conditions Associated with the Interruption of Floc Formation

Operational Condition

Description or Example

Cell bursting agent

Colloidal floc

Elevated temperature

Foam production and accumulation

High pH

Increase in percent MLVSS Lack of ciliated protozoa Low dissolved oxygen concentration Low pH

Nutrient deficiency Salinity

Scum production and accumulation

Septicity

Shearing action

Slug discharge of soluble cBOD

Sulfates

Surfactants

Total dissolved solids

Toxicity

Undesired filamentous organism growth Viscous floc or Zoogloeal growth Young sludge age

Lauryl sulfate

Nondegrading or slowly degrading colloids >32°C

Foam-producing filamentous organisms >8.5

Accumulation of fats, oils, and grease <100 per milliliter

<1 mg/liter for 10+ consecutive hours <6.5

Usually nitrogen or phosphorus

Excess manganese, potassium, or sodium

Toxicity and die-off of bacteria

Surface aeration

3X normal quantity of soluble cBOD >500mg/liter

Excess anionic detergents >5000mg/liter RAS chlorination >5 filaments per floc particle Rapid floc-forming bacterial growth <3 days MCRT

filamentous organisms, and (3) turbulence level in the treatment process.The change in color of the floc particles from white to golden-brown is due in large part to the accumulation of secreted oils by bacteria as they age.

Floc formation can be interrupted by several operational conditions (Table 14.3). Interruption of floc formation results in the loss of settleability, the loss of solids, and an increase in operational costs and, possibly, permit violations.

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