Fermentative Bacteria

Organotrophic bacteria degrade organic compounds in order to obtain carbon and energy for cellular synthesis and activity. The degradation of organic compounds occurs intracellularly and results in the release of electrons from the hydrogen atoms in the organic compounds (Figure 16.1). The electrons provide the cell with energy and are removed from the cell by an electron transport molecule. The electron transport molecule may be free molecular oxygen (O2), nitrate (NO3-), sulfate (SO42-), carbon dioxide (CO2), or an organic molecule (Table 16.1).

Bacterial cells can use only one electron transport molecule at any time, and the choice of the molecule always will be for the molecule that provides the cell with the most energy, provided that the following conditions are satisfied:

• The molecule is available for bacterial use.

• The bacterial cell has the enzymatic ability to use the molecule.

The degradation of organic molecules can occur with free molecular oxygen (aerobic degradation) or without free molecular oxygen (anaerobic degradation). If the degradation of an organic molecule—for example, glucose (C6H12O6)—occurs with free molecular oxygen, the degradation is referred to as aerobic respiration (Equation 16.1). Aerobic respiration occurs in the aeration tank of an activated sludge process. Aerobic respiration results in the production of bacterial cells (sludge), carbon dioxide, and water.

There are four significant forms of anaerobic degradation of organic molecules that occur at wastewater treatment plants. These four are nitrate reduction soluble cBOD O2 NO3- SO42- CO2

soluble cBOD O2 NO3- SO42- CO2

acids, alcohols

FIGURE 16.1 Molecules available for the removal of electrons released from the degradation of carbonaceous substrates. Depending upon the molecule used (O2, NO3~, SO42-, CO2, or soluble cBOD) by bacteria to remove electrons (E) from the cell, a variety of substrates are produced.

acids, alcohols

FIGURE 16.1 Molecules available for the removal of electrons released from the degradation of carbonaceous substrates. Depending upon the molecule used (O2, NO3~, SO42-, CO2, or soluble cBOD) by bacteria to remove electrons (E) from the cell, a variety of substrates are produced.

TABLE 16.1 Electron Transport Molecules Used in the Degradation of Soluble Substrate

Operational Condition

Transport Molecule

Biological Process

Aerobic

Anoxic

Anaerobic

Anaerobic

Anaerobic

O2 Aerobic degradation of substrate

NOjf, NO2- Anoxic degradation of substrate

SO42- Sulfate reduction and degradation of substrate

CO2 Methanogenesis

Organic molecule Fermentation and degradation of substrate

(denitrification), sulfate reduction, methanogenesis (methane production), and fermentation.

Facultative anaerobic bacteria perform nitrate reduction. Nitrate reduction or denitrification commonly occurs in a denitrification tank, an anoxic selector, and a secondary clarifier (Equation 16.2). The occurrence of denitrification as a secondary clarifier is commonly referred to as "clumping." Nitrate reduction results in the production of bacterial cells (sludge), carbon dioxide, water, and molecular nitrogen (N2).

Obligatory anaerobic bacteria perform sulfate reduction. Sulfate reduction typically occurs in an anaerobic digester. Sulfate reduction does occur in sewer systems. Sulfate reduction often occurs in a secondary clarifier and a thickener, if the settled solids remain too long in these treatment units and free molecular oxygen and nitrate are not present. Sulfate reduction results in the production of bacterial cells (sludge), carbon dioxide, water, sulfide (HS-), and a variety of short chain organic compounds, mostly acids and alcohols. For example, from the anaerobic degradation of lactate (CH3CHOHCOOH), acetate (CH3COOH) is produced (Equation 16.3).

2CH3CHOHCOOH + SO42- + H + ^ 2CH3COO- + 2CO2 + 2H2O + HS- (16.3)

Oxygen-intolerant, obligatory anaerobic bacteria perform methanogenesis.There are two major routes of methanogenesis that occur in an anaerobic digester. The major route of methane production is the "splitting" of acetate by aceticlastic, methane-forming bacteria (Equation 16.4). Splitting of acetate results in the production of bacterial cells (sludge), methane (CH4), and water. The minor route of methane production is the reduction of carbon dioxide by hydrogen-oxidizing, methane-forming bacteria (Equation 16.5). The reduction of carbon dioxide results in the production of bacterial cells, methane, and water.

Organic molecules undergo glycolysis as they are degraded. At the end of glycolysis two, three-carbon molecules of pyruvate (CH3COCOOH) are produced. Pyruvate then is degraded further through aerobic respiration, nitrate reduction, sulfate reduction, methanogenesis, or fermentation.

Anaerobic degradation of organic compounds without free molecular oxygen, nitrate, sulfate, or carbon dioxide is fermentation. Fermentation results in the production of a variety of simplistic, soluble organic compounds, and the bacteria that perform fermentation are referred to as fermentative bacteria. Fermentation requires the use of an organic molecule to remove the electrons from the degrading compound. Fermentation is an inefficient process, and it releases little energy to the cell. Most of the energy released by the degraded compound remains in the fermented products. Fermentation typically occurs in an anaerobic digester, but it may also occur in sewer systems, a secondary clarifier, or a thickener.

Facultative anaerobic bacteria and strict anaerobic bacteria perform fermentation. Fermentation can occur by many different pathways (Figure 16.2). Fermentation results in the production of bacterial cells (sludge), water, and a large variety of organic compounds including lactic acid (Equation 16.6) and ethanol (Equation 16.7). Some fermentative pathways do produce carbon dioxide, while other pathways do not (Table 16.2). The types of organic compounds produced through fermentation are dependent upon the bacteria involved and the existing operational condition (e.g., pH and temperature). Fermentation results in the production of a

Substrate (glucose) degraded to pyruvate; pyruvate fermented through several possible pathways,

Pathway #1, Acetone-butanol fermentation: sugar -- bacteria --- > products Bacteria: Clostridium Products: acetone, butanol, ethanol

Pathway #2, Butanediol fermentation: sugar -- bacteria --- > products

Bacteria: Aerobacter, Enterobacter, Erwinia, Klebsiella, Serratia Products: acetate, 2,3-butanediol, butylene glycol, ethanol, carbon dioxide, hydrogen Pathway #3, Butyric-butylic fermentation: sugar -- bacteria --- > products Bacteria: Clostridium

Products: acetate, butyrate, ethanol, isopropanol, carbon dioxide, hydrogen Pathway #4, Homolactic fermentation: sugar -- bacteria --- > products

Bacteria: Bacillus, Lactobacillus, Streptococcus Products: lactate

Pathway #5, Mixed acid fermentation: sugar -- bacteria --- > products

Bacteria: Escherichia, Proteus, Providencia, Salmonella, Shigella, Yersinia Products: acetate, ethanol, formate, lactate, succinate, carbon dioxide, hydrogen Pathway #6, Propionic fermentation: sugar -- bacteria --- > products

Bacteria: Propionobacterium

Products: acetate, propionate, carbon dioxide

FIGURE 16.2 Some major bacterial fermentative pathways. When soluble cBOD is degraded under anaerobic/fermentative conditions, a variety of products can be produced depending upon the bacteria involved, operational conditions present, and fermentative pathway used by the bacteria.

variety of acids and alcohols, and it often is called "mixed acid and mixed alcohol" production. Due to the production of acids, fermentative bacteria also are known as acid-forming bacteria. Some acid-forming bacteria produce copious quantities of acetate and are known as acetogenic bacteria. Acetate is the major substrate for methane production in an anaerobic digester.

C6H12O6 ^ 2CH3CHOCOOH

TABLE 16.2 Significant Bacterial Fermentative Pathways

Fermentative Pathway

Products

Representative Bacterial Genus

Acetone-butanol Acetone, butanol, ethanol, Clostridium

Butanediol Acetate,2,3-butanediol, butylene, ethanol, gylcol, Enterobacter lactate, CO2, H2

Butyrate Acetate, butyrate, CO2, H2 Clostridium

Lactate Lactate Lactobacillus

Mixed acid Acetate, ethanol, lactate, CO2, H2 Escherichia

Propionate Propionate Propionibacterium

TABLE 16.3 Significant Genera of Fermentative Bacteria

Aeromonas Lactobacillus

Bacteroides Pasteurella

Bifidobacteria Propionobacterium

Citrobacter Proteus

Clostridium Providencia

Enterobacter Salmonella

Erwinia Serratia

Escherichia Shigella Klebsiella

In many of the fermentative pathways, hydrogen (H2) is produced. The production of hydrogen gas is important in anaerobic digesters. First, hydrogen is a significant substrate for the production of methane (CH4). Second, if hydrogen is not reduced to a low pressure in an anaerobic digester, the hydrogen pressure inhibits acetogenic bacteria.

Although facultative anaerobic bacteria and anaerobic bacteria (Table 16.3) are capable of fermentation, the more important bacteria are the strict anaerobic bacteria such as Bacteroides,Bifidobacteria, and Clostridium.These three genera of bacteria enter wastewater treatment plants through inflow and infiltration as soil and water organisms. Other fermentative bacteria enter wastewater treatment plants through inflow and infiltration and fecal waste.

There are two important groups of fermentative bacteria: the acidogenic bacteria and the acetogenic bacteria. The acidogenic bacteria or acid-formers such as Clostridium convert simple sugars, amino acids, and fatty acids to (1) organic acids such as acetate, butyrate, formate, lactate, and succinate, (2) alcohols such as ethanol and methanol, (3) acetone, and (4) carbon dioxide, hydrogen, and water. Several of these compounds are volatile and malodorous. Several of these compounds can be used directly by methane-forming bacteria, while other compounds can be converted to compounds that can be used by methane-forming bacteria.

Acetogenic bacteria produce acetate and hydrogen that can be used directly by methane-forming bacteria. Acetogenic bacteria convert several of the fatty acids [such as butyrate (Equation 16.8) and propionate (Equation 16.9)] and alcohols

[such as ethanol (Equation 16.10)] that are produced by the acidogenic bacteria to acetate, hydrogen, and carbon dioxide.

Fermentation in wastewater treatment systems is involved in several significant operational conditions. These conditions are

• Production of malodorous organic compounds

• Production of PHB granules that are necessary for biological phosphorus removal

• Production of substrate for methane-forming bacteria in anaerobic digesters

• Rapid and undesired growth of septic-loving filamentous organisms including type 021N, type 0041, Beggiatoa spp., Nosticoda limicola, and Thiothrix spp.

• Rising sludge in secondary clarifiers and thickeners

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