At least 300 different species of bacteria are found in the feces of a single individual. Most of these bacteria are strict anaerobes. The majority of the remaining bacteria are facultative anaerobes. Escherichia coli is a common facultative anaerobe in feces.Bacteria from fecal wastes as well as hundreds of soil and water bacteria that
enter a conveyance system through inflow and infiltration (I/I) are found in the influent of municipal wastewater treatment processes. For the purpose of this text, bacteria that are commonly found in wastewater treatment processes are divided into groups according to 1) their response to free molecular oxygen (O2) and 2) their enzymatic ability to degrade substrate in the anaerobic digester.
RESPONSE TO FREE MOLECULAR OXYGEN
Bacteria may be divided further into three groups according to their response to free molecular oxygen (Table 2.1). These groups are 1) strict aerobes, 2) facultative anaerobes, and 3) anaerobes, including the methane-forming bacteria. Strict aerobes are active and degrade substrate only in the presence of free molecular oxygen. These organisms are present in relatively large numbers in aerobic fixed-film processes, for example, trickling filters, and aerobic suspended-growth processes, for example, activated sludge. In the presence of free molecular oxygen they perform significant roles in the degradation of wastes. However, strict aerobes die in an anaerobic digester in which free molecular oxygen is absent. Facultative anaerobes are active in the presence or absence of free molecular oxygen. If present, free molecular oxygen is used for enzymatic activity and thedegradation of wastes. If free molecular oxygen is absent, another molecule, for example, nitrate ion (NO3–), is used to degrade wastes such as methanol (CH3OH) (Equation 2.1). When nitrate ions are used, denitrification occurs and dinitrogen gas(N) is produced.
Most bacteria within fixed-film processes and suspended growth processes are facultative anaerobes, and these organisms also perform many significant roles in the degradation of wastes. Approximately 80% of the bacteria within these aerobic processes are facultative anaerobes. These organisms are found in relatively large numbers not only in aerobic processes but also in anaerobic processes. During the degradation of wastes within an anaerobic digester, facultative anaerobic bacteria, for example, Enterobacter spp., produce a variety of acids and alcohols, carbon dioxide (CO2), and hydrogen from carbohydrates, lipids, and pro-
teins. Some organisms, for example, Escherichia coli , produce malodorous compounds such as indole and skatole.Anaerobes are inactive in the presence of free molecular oxygen and may be divided into two subgroups: oxygen-tolerant species and oxygen-intolerant species or strict anaerobes (Table 2.2). Some anaerobes are strong acid producers, such as,Streptococcus spp., whereas other anaerobes, such as Desulfomarculum spp., reduce sulfate (SO42) to hydrogen sulfide (H2S) (Equation 2.2). Although oxygen tolerant anaerobes survive in the presence of free molecular oxygen, these organisms cannot perform normal cellular activities, including the degradation of substrate, in the presence of free molecular oxygen. Strict anaerobes, including methane-forming bacteria, die in the presence of free molecular oxygen.
These organisms include facultative anaerobes that ferment simple, soluble organic compounds and strict anaerobes that ferment complex proteins and carbohydrates.The products of fermentation vary greatly depending on the bacteria involved in the fermentative process. Therefore, changes in operational conditions that result in changes in dominant bacteria also result in changes in the concentrations of acids and alcohols that are produced during fermentation. Changes in the concentrations of acids and alcohols significantly change the substrates available for methane forming bacteria, their activity, and, consequently, digester performance. Most strict anaerobes are scavengers. These organisms are found where anaerobic conditions exist in lakes, river bottoms, human intestinal tracts, and anaerobic digesters. Anaerobes survive and degrade substrate most efficiently when the oxidation-reduction potential (ORP) of their environment is between –200 and –400 millivolts (mV). Any amount of dissolved oxygen in an anaerobic digester raises the ORP of the sludge and discourages anaerobic activity including hydrolysis, acetogenesis, and methanogenesis. Therefore, sludges and wastewaters fed to an anaeroic digester should have no molecular oxygen. Settled and thickened sludges usually do not have a residual dissolved oxygen concentration. These sludges typically have a low ORP (–100 to –300 mV).The ORP of a wastewater or sludge can be obtained by using an electrometric pH meter with a millivolt scale and an ORP probe. The ORP of a wastewater or sludge is measured on the millivolt scale of the pH meter. The ORP is a measurement of the relative amounts of oxidized materials, such
as nitrate ions (NO3) and sulfate ions (SO42), and reduced materials, such as ammonium ions (NH4+) (Table 2.3). At ORP values greater than +50 mV, free molecular oxygen is available in the wastewater or sludge and may be used by aerobes and facultative anaerobes for the degradation of organic compounds. This degradation occurs under an oxic condition. At ORP values between +50 and –50 mV, free molecular oxygen is not available but nitrate ions or nitrite ions (NO2) are available for the degradation of organic compounds. The degradation of organic compounds without free molecular oxygen is an anaerobic condition. The use of nitrate ions or nitrite ions occurs under an anoxic condition and is referred to as denitrification, clumping, and rising sludge in the secondary clarifier of an activated sludge process. At ORP values less than –50 mV, nitrate ions and nitrite ions are not available but sulfate ions are available for the degradation of organic compounds. This degradation also occurs without free molecular oxygen. When sulfate is used to degrade organic compounds, sulfate is reduced and hydrogen sulfide is formed along with a variety of acids and alcohols. At ORP values less than –100 mV, the degradation of organic compounds proceeds as one portion of the compound is reduced while another portion of the compound is oxidized. This form of anaerobic degradation of organic compounds is commonly known as mixed-acid fermentation because a mixture of acids, for example, acetate, butyrate, formate, and propionate, are produced. A mixture of
alcohols is also produced during fermentation. At ORP values less than –300 mV, anaerobic degradation of organic compounds and methane production occur. During methane production, simple organic compounds such as acetate are converted to methane, and carbon dioxide and hydrogen are combined to form methane.
ENZYMATIC ABILITY TO DEGRADE SUBSTRATE
Bacteria degrade substrate through the use of enzymes. Enzymes are proteinaceous molecules that catalyze biochemical reactions. Two types of enzymes are involved in substrate degradation—endoenzymes and exoenzymes (Figure 2.1). Endoenzymes are produced in the cell and degrade soluble substrate within the
cell. Exoenzymes also are produced in the cell but are released through the “slime”coating the cell to the insoluble substrate attached to the slime. Once in contact with the substrate the exoenzyme solubilizes particulate and colloidal substrates. Once solubilized, these substrates enter the cell and are degraded by endoenzymes. The
production of exoenzymes and solubilization of particulate and colloidal substrates
usually take several hours.
All bacteria produce endoenzymes, but not all bacteria produce exoenzymes. No
bacterium produces all the exoenzymes that are needed to degrade the large variety
of particulate and colloidal substrates that are found in sludges and wastewaters
(Table 2.4). Each exoenzyme as well as each endoenzyme degrades only a specific
substrate or group of substrates. Therefore, a large and diverse community of bac-
teria is needed to ensure that the proper types of exoenzymes and endoenzymes
are available for degradation of the substrates present.
The relative abundance of bacteria within an anaerobic digester often is greater
than 10
16
cells per milliliter. This population consists of saccharolytic bacteria
(~10
8
cells/ml), proteolytic bacteria (~10
6
cells/ml), lipolytic bacteria (~10
5
cells/ml),
and methane-forming bacteria (~10
8
cells/ml).
There are three important bacterial groups in anaerobic digesters with respect
to the substrates utilized by each group. These groups include the acetate-forming
(acetogenic) bacteria, the sulfate-reducing bacteria, and the methane-forming
bacteria. The acetate-forming bacteria and sulfate-reducing bacteria are reviewed
in this chapter, and the methane-forming bacteria are reviewed in Chapter 3.
enter a conveyance system through inflow and infiltration (I/I) are found in the influent of municipal wastewater treatment processes. For the purpose of this text, bacteria that are commonly found in wastewater treatment processes are divided into groups according to 1) their response to free molecular oxygen (O2) and 2) their enzymatic ability to degrade substrate in the anaerobic digester.
RESPONSE TO FREE MOLECULAR OXYGEN
Bacteria may be divided further into three groups according to their response to free molecular oxygen (Table 2.1). These groups are 1) strict aerobes, 2) facultative anaerobes, and 3) anaerobes, including the methane-forming bacteria. Strict aerobes are active and degrade substrate only in the presence of free molecular oxygen. These organisms are present in relatively large numbers in aerobic fixed-film processes, for example, trickling filters, and aerobic suspended-growth processes, for example, activated sludge. In the presence of free molecular oxygen they perform significant roles in the degradation of wastes. However, strict aerobes die in an anaerobic digester in which free molecular oxygen is absent. Facultative anaerobes are active in the presence or absence of free molecular oxygen. If present, free molecular oxygen is used for enzymatic activity and thedegradation of wastes. If free molecular oxygen is absent, another molecule, for example, nitrate ion (NO3–), is used to degrade wastes such as methanol (CH3OH) (Equation 2.1). When nitrate ions are used, denitrification occurs and dinitrogen gas(N) is produced.
 |
| groups of bacteria |
 |
| groups of anaerobic Bacteria |
teins. Some organisms, for example, Escherichia coli , produce malodorous compounds such as indole and skatole.Anaerobes are inactive in the presence of free molecular oxygen and may be divided into two subgroups: oxygen-tolerant species and oxygen-intolerant species or strict anaerobes (Table 2.2). Some anaerobes are strong acid producers, such as,Streptococcus spp., whereas other anaerobes, such as Desulfomarculum spp., reduce sulfate (SO42) to hydrogen sulfide (H2S) (Equation 2.2). Although oxygen tolerant anaerobes survive in the presence of free molecular oxygen, these organisms cannot perform normal cellular activities, including the degradation of substrate, in the presence of free molecular oxygen. Strict anaerobes, including methane-forming bacteria, die in the presence of free molecular oxygen.
These organisms include facultative anaerobes that ferment simple, soluble organic compounds and strict anaerobes that ferment complex proteins and carbohydrates.The products of fermentation vary greatly depending on the bacteria involved in the fermentative process. Therefore, changes in operational conditions that result in changes in dominant bacteria also result in changes in the concentrations of acids and alcohols that are produced during fermentation. Changes in the concentrations of acids and alcohols significantly change the substrates available for methane forming bacteria, their activity, and, consequently, digester performance. Most strict anaerobes are scavengers. These organisms are found where anaerobic conditions exist in lakes, river bottoms, human intestinal tracts, and anaerobic digesters. Anaerobes survive and degrade substrate most efficiently when the oxidation-reduction potential (ORP) of their environment is between –200 and –400 millivolts (mV). Any amount of dissolved oxygen in an anaerobic digester raises the ORP of the sludge and discourages anaerobic activity including hydrolysis, acetogenesis, and methanogenesis. Therefore, sludges and wastewaters fed to an anaeroic digester should have no molecular oxygen. Settled and thickened sludges usually do not have a residual dissolved oxygen concentration. These sludges typically have a low ORP (–100 to –300 mV).The ORP of a wastewater or sludge can be obtained by using an electrometric pH meter with a millivolt scale and an ORP probe. The ORP of a wastewater or sludge is measured on the millivolt scale of the pH meter. The ORP is a measurement of the relative amounts of oxidized materials, such
as nitrate ions (NO3) and sulfate ions (SO42), and reduced materials, such as ammonium ions (NH4+) (Table 2.3). At ORP values greater than +50 mV, free molecular oxygen is available in the wastewater or sludge and may be used by aerobes and facultative anaerobes for the degradation of organic compounds. This degradation occurs under an oxic condition. At ORP values between +50 and –50 mV, free molecular oxygen is not available but nitrate ions or nitrite ions (NO2) are available for the degradation of organic compounds. The degradation of organic compounds without free molecular oxygen is an anaerobic condition. The use of nitrate ions or nitrite ions occurs under an anoxic condition and is referred to as denitrification, clumping, and rising sludge in the secondary clarifier of an activated sludge process. At ORP values less than –50 mV, nitrate ions and nitrite ions are not available but sulfate ions are available for the degradation of organic compounds. This degradation also occurs without free molecular oxygen. When sulfate is used to degrade organic compounds, sulfate is reduced and hydrogen sulfide is formed along with a variety of acids and alcohols. At ORP values less than –100 mV, the degradation of organic compounds proceeds as one portion of the compound is reduced while another portion of the compound is oxidized. This form of anaerobic degradation of organic compounds is commonly known as mixed-acid fermentation because a mixture of acids, for example, acetate, butyrate, formate, and propionate, are produced. A mixture of
alcohols is also produced during fermentation. At ORP values less than –300 mV, anaerobic degradation of organic compounds and methane production occur. During methane production, simple organic compounds such as acetate are converted to methane, and carbon dioxide and hydrogen are combined to form methane.
 |
| oxidation reduction potential and cellular activity |
 |
| figure 2.1 |
Bacteria degrade substrate through the use of enzymes. Enzymes are proteinaceous molecules that catalyze biochemical reactions. Two types of enzymes are involved in substrate degradation—endoenzymes and exoenzymes (Figure 2.1). Endoenzymes are produced in the cell and degrade soluble substrate within the
cell. Exoenzymes also are produced in the cell but are released through the “slime”coating the cell to the insoluble substrate attached to the slime. Once in contact with the substrate the exoenzyme solubilizes particulate and colloidal substrates. Once solubilized, these substrates enter the cell and are degraded by endoenzymes. The
production of exoenzymes and solubilization of particulate and colloidal substrates
usually take several hours.
All bacteria produce endoenzymes, but not all bacteria produce exoenzymes. No
bacterium produces all the exoenzymes that are needed to degrade the large variety
of particulate and colloidal substrates that are found in sludges and wastewaters
(Table 2.4). Each exoenzyme as well as each endoenzyme degrades only a specific
substrate or group of substrates. Therefore, a large and diverse community of bac-
teria is needed to ensure that the proper types of exoenzymes and endoenzymes
are available for degradation of the substrates present.
The relative abundance of bacteria within an anaerobic digester often is greater
than 10
16
cells per milliliter. This population consists of saccharolytic bacteria
(~10
8
cells/ml), proteolytic bacteria (~10
6
cells/ml), lipolytic bacteria (~10
5
cells/ml),
and methane-forming bacteria (~10
8
cells/ml).
There are three important bacterial groups in anaerobic digesters with respect
to the substrates utilized by each group. These groups include the acetate-forming
(acetogenic) bacteria, the sulfate-reducing bacteria, and the methane-forming
bacteria. The acetate-forming bacteria and sulfate-reducing bacteria are reviewed
in this chapter, and the methane-forming bacteria are reviewed in Chapter 3.
