Bacterial Growth

Biological, wastewater treatment plants are simply biological amplifiers. The plants permit organisms (biomass or sludge), primarily bacteria, to increase in number by using the pollutants (substrates and nutrients) in the wastewater and converting them to new organisms (biomass or sludge) and nonpolluting wastes and less polluting wastes (Table 9.1). Nonpolluting wastes do not contribute to operational or environmental problems. Less polluting wastes are not as harmful as the original pollutants but may contribute to operational and/or environmental problems.

The carbon and energy sources (wastes) in the wastewater for bacterial growth are referred to as substrates. Therefore, the degradation of substrates for bacterial growth is referred to as substrate utilization. The increase in the quantity of bacteria (sludge) per unit of substrate utilized is known as the growth yield or sludge yield (Table 9.2).

Bacterial cells do grow; but as soon as a cell or mother cell has approximately doubled in size, it divides into two offspring or daughter cells. This form of growth or reproduction is known as binary fission (Figure 9.1. In some bacteria, incomplete separation of the daughter cells produces different arrangements or patterns of cellular growth such as filaments and tetrads.

Because bacterial cells grow in size only to divide into two cells, microbial growth in the microbiology laboratory can be defined in terms of the number of cells produced through division. However, in the wastewater treatment plant the number of cells produced in a biological treatment unit such as the activated sludge process cannot be determined due to the following difficulties:

• There is no growth medium that would permit the growth and enumeration of all bacteria in any biological treatment unit.

TABLE 9.1 Examples of Nonpolluting Wastes and Less Polluting Wastes Produced from Aerobic9 and Anoxic6 Degradation of Proteins"

Wastes

TABLE 9.1 Examples of Nonpolluting Wastes and Less Polluting Wastes Produced from Aerobic9 and Anoxic6 Degradation of Proteins"

Wastes

Degradation

Nonpolluting

Less Polluting

Concern with Less Polluting Waste

Aerobic

CO2, H2O

H2PO4-

Algal blooms in receiving waters

NH4+

Nitrification and oxygen demand

SO42-

Reduction and H2S production

Anoxic

CO2, H2O, N2, N2O

H2PO4-

Algal blooms in receiving waters

HS-

Growth of filamentous organisms

NH4+

Nitrification and oxygen demand

a Aerboic degradation uses free molecular oxygen. b Anoxic degradation uses nitrate (NO3-). c Proteins contain C, H, O, N, S, and P.

a Aerboic degradation uses free molecular oxygen. b Anoxic degradation uses nitrate (NO3-). c Proteins contain C, H, O, N, S, and P.

TABLE 9.2 Examples of Sludge Yields (Pounds of Sludge per Pound of cBOD Removed)

Quantity of cBOD (lb) Type of cBOD Degradation Approximate Sludge Yield (lb)

1 Sugar Aerobic 0.6

1 Sugar Anoxic 0.4

1 Proteins Aerobic 0.4

• Due to the large diversity of bacteria in a biological treatment unit, there are many different generation (reproduction) times. Some generation times may be as short as 15 minutes (organotrophs) and as long as 15 days (nitrifying bacteria).

• Erratic wasting rates produce "pockets" of young growth and old growth with different generation times.

• It is time-consuming, labor intense, and expensive to use a variety of techniques to determine the number of different bacteria in a biological treatment unit.

Due to the difficulties encountered in enumerating bacteria in wastewater treatment plants, volatile solids concentrations are used to estimate the size or mass of bacterial populations. Because bacteria are organic in composition, they are volatilized in a muffle furnace at 550°C. Therefore, an increase in volatile solids concentration is considered to be an increase in the mass of the bacterial population, while a decrease in volatile solids concentration is considered to be a decrease in the mass of the bacterial population. The mixed liquor volatile suspended solids (MLVSS) concentration is used in the activated sludge process to estimate the mass of the bacterial population.

The use of volatile solids such as the MLVSS to determine the mass of the bacterial population provides only an estimate of the mass. There are several operational concerns that impact the composition of the volatile solids that influence an estimate of the bacterial population. These concerns are as follows:

• Volatile solids concentration can change significantly due to the accumulation or lost of compounds such as fats, oils, and grease, insoluble polysaccharides

Cell wall

Cell membrane

Nucleoid

Cell wall

Cell membrane

Nucleoid

CEJIIgp

FIGURE 9.1 Binary fission in a bacterium. During binary fission the nucleoid elongates (a) and then divides (b). Division of the nucleoid is accompanied by the formation of transverse septum by the cell wall (b). After the transverse septum is complete (c), two new cells or daughter cells are produced (d).

produced through slug discharges of soluble cBOD, and stored starches produced during a nutrient deficiency.

• Protozoa and metazoa may contribute to up to 5% of the volatile content of the solids.

• Volatile solids concentration does not indicate if the bacterial population has experienced inhibition or toxicity.

Bacterial cells in biological treatment units reproduce mainly by binary fission; that is, each mother cell produces two daughter cells. The growth of the bacterial population is defined as an increase in the mass of bacteria, and the growth of the bacteria can occur as a batch culture (closed system) or a continuous culture (open system). In a batch culture there is a limited quantity of substrate. An example of a batch culture occurs in an activated sludge process when an aeration tank is taken "off-line" but is not drained and continues to be aerated and mixed. An example of a continuous culture in an activated sludge process occurs when an aeration tank remains "on-line" and continuously receives substrates.

TABLE 9.3 Phases of Bacterial Growth

Phase

Key Events

Lag Log

Endogenous Death

Time for bacteria to "adjust" to their new environment

Enzymes produced for the degradation of substrate

No increase in bacterial mass

Substrate is adsorbed and absorbed

Substrate is degraded

Cellular synthesis occurs

Rapid increase in bacterial mass

Carrying capacity of the environment is reached

Growth rate equals death rate

Bacterial mass remains relatively constant

Substrate decreases

Accumulation of wastes occurs

Death rate greater than growth rate

Bacterial mass decreases

Bacterial Population (MLVSS)

Increasing

Bacterial Population (MLVSS)

Increasing

Growth Rate Bcteia Wastewater
_ Increasing _

MCRT (days)

FIGURE 9.2 Bacterial growth curve, batch culture or batch reactor. In a batch culture, substrates are provided only once [e.g., an aeration tank taken off-line or a sequential batch reactor (SBR)].

There are four distinct phases of bacterial growth in batch cultures and continuous cultures (Table 9.3).These phases are lag, log, endogenous, and death or decline. Only the death phase in the growth curves for batch cultures (Figure 9.2) and continuous cultures (Figure 9.3) differs.

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