Anabolism Versus Catabolism

Substrate that is absorbed by bacterial cells is degraded to provide carbon and energy for cellular growth and cellular activity. When substrate is used for cellular synthesis, small molecules are joined together to form large molecules and cellular growth occurs. This is referred to anabolism, and sludge production increases. When

Bacterial Population (MLVSS)

Increasing

Bacterial Population (MLVSS)

Increasing

Increasing.

FIGURE 9.4 Bacterial growth curve, occurrence of anabolism and catabolism. Anabolic reactions favor increased sludge production, while catabolic reactions favor decreased sludge production. Operational conditions that promote anabolic reactions include (1) excess loss of solids from the secondary clarifiers, (2) overwasting or erratic wasting rates, (3) recovery from toxicity, (4) recovery from washout (excess I/I), and (5) slug discharge of soluble cBOD. Operational conditions that promote catabolic reactions include (1) maintenance of a low F/M and high MCRT, (2) decrease in cBOD loading, (3) rotating aeration tanks "on-line" and "off-line," and (4) increasing the HRT of the aeration tanks.

Increasing.

MCRT (days)

FIGURE 9.4 Bacterial growth curve, occurrence of anabolism and catabolism. Anabolic reactions favor increased sludge production, while catabolic reactions favor decreased sludge production. Operational conditions that promote anabolic reactions include (1) excess loss of solids from the secondary clarifiers, (2) overwasting or erratic wasting rates, (3) recovery from toxicity, (4) recovery from washout (excess I/I), and (5) slug discharge of soluble cBOD. Operational conditions that promote catabolic reactions include (1) maintenance of a low F/M and high MCRT, (2) decrease in cBOD loading, (3) rotating aeration tanks "on-line" and "off-line," and (4) increasing the HRT of the aeration tanks.

substrate is used for energy production, large molecules are degraded to smaller molecules and cellular energy is obtained. This is referred to as catabolism, and sludge production decreases.

On the bacterial growth curve (Figure 9.4), anabolism or increase in sludge production occurs before endogenous phase of growth. Catabolism occurs with the onset of endogenous phase of growth. Anabolism always occurs after the start-up of a wastewater treatment plant.

Because sludge thickening, dewatering, and disposal costs are a major operational expense for most wastewater treatment plants, these plants should be operated to favor catabolic conditions or a decrease in sludge production. Anabolism should occur once, immediately after start-up. However, there are operational conditions that do favor anabolism and an increase in sludge production. These operational or anabolic conditions including the following:

• Excess loss of solids from the secondary clarifiers in the activated sludge process

• Overwasting of solids or erratic wasting rates

• Recovery from inhibition or toxicity

• Recovery from washout

• Slug discharge of soluble cBOD

TABLE 9.4 Operational Parameters of the Activated Sludge Process that May Be Regulated to Decrease Sludge Production

Parameter

Regulation to Decrease Sludge Production cBOD loading

MCRT

Anoxic periods or zones Slug discharges of soluble cBOD TSS:BOD to aeration tank

Decrease

Decrease

Increase

Increase

Prevent Lower

Insoluble polysaccharide granule Slime or polysaccharide coating

c

\

f

^^ \

V

_J

V

J

FIGURE 9.5 Stored food. Large quantities of food often are stored by bacteria as starch granules or slime (polysaccharide coating). Starch granules may be located within the bacterial cell or outside the bacterial cell in the slime.

FIGURE 9.5 Stored food. Large quantities of food often are stored by bacteria as starch granules or slime (polysaccharide coating). Starch granules may be located within the bacterial cell or outside the bacterial cell in the slime.

During endogenous phase of growth, the activated sludge process can be monitored and regulated to decrease sludge production (Table 9.4). By decreasing the substrate (cBOD) loading to the wastewater treatment plant, less bacterial growth (sludge production) occurs and food stored by bacteria is solubilized and degraded (Figure 9.5). A significant decrease in cBOD loading may be obtained by reducing the quantity of fats, oils, and grease discharged to the sewer system. By using appropriate-sized grease traps on cafeteria, commercial, and industrial waste streams and capturing and disposing of fats, oils, and grease at approved landfills, decreased cBOD loading can be obtained. In addition to decreased cBOD loading, the following benefits are obtained for the sewer system and the wastewater treatment plant:

• Decreased occurrences of saponification of fats, oils, and grease in the sewer system, resulting in less interruption of sewer service and decreased maintenance of the sewer system

• Decrease in volatile content of the mixed liquor suspended solids (MLSS), resulting in improved settleability

• Decrease in the growth of foam-producing filamentous organisms (Microthrix parvicella and Nocardioforms) that use fats, oils, and grease as a substrate

By lowering the F/M of the activated sludge process, less substrate is available to all bacteria. Therefore, decreased cellular growth results, bacteria consume stored food, and bacteria die. When lowering the F/M, caution should be used to prevent the undesired growth of low F/M filamentous organisms such as Haliscomenobac-ter hydrossis, Microthrix parvicella, Nocardioforms, type 021N, and type 0041.

By increasing the hydraulic retention time (HRT) within the aeration tanks, solubilization of some particulate and colloidal cBOD may occur. Once solubilized, these substrates can be transformed to bacterial cells that represent less weight than the original particulate and colloidal cBOD. However, by increasing the HRT and solubilizing substrates, an increase in dissolved oxygen demand may occur, with the occurrence of nitrification.

By increasing the MCRT a large number of "older" bacteria are maintained in the activated sludge process. These bacteria favor catabolic events (decreased sludge production) over anabolic events (increased anabolic events). However, caution should be exercised when increasing the MCRT. An increase in MCRT may contribute to the following operational concerns:

• Significant endogenous respiration that consumes large quantities of dissolved oxygen

• Undesired growth of high MCRT filamentous organisms such as Microthrix parvicella, type 0041, type 0092, and type 1851

• Undesired growth of nitrifying bacteria and nitrification

The use of anoxic periods (use of nitrate) to degrade cBOD results in decreased sludge production. For example, based upon sludge yields, one pound of sugar degraded with free molecular oxygen results in the production of approximately 0.6 pound of sludge (bacterial cells), while one pound of sugar degraded with nitrate results in the production of approximately 0.4 pound of sludge (bacterial cells). When using anoxic periods, the presence of nitrate ions should never be exhausted. If nitrate is exhausted, septicity occurs.

Slug discharges of soluble cBOD should be prevented. These discharges usually are easily degradable and result in the rapid proliferation of bacterial cells. This rapid and young growth is accompanied with the production of a copious quantity of buoyant and insoluble polysaccharides. Discharges of highly soluble cBOD should be identified and prevented.

By lowering the TSS (total suspended solids) : BOD ratio to the aeration tank influent, decreased sludge production occurs. Less suspended solids to the aeration tank results in less solids (sludge) from the aeration tank. By improving primary clarifier efficiency, a reduction in the quantity of suspended solids discharged to the aeration tank can be reduced. For example, additional clarifiers can be placed "online," clarifiers can be pumped more frequently, and a polymer can be added to the primary clarifier influent to remove more solids.

Part III

Nitrogen, Phosphorus, and Sulfur Bacteria

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Responses

  • Edward Brown
    What type of wastewater treatment operates on the endogenous bacteria growth phase?
    11 months ago

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