Hydrolytic Bacteria

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A large and diverse population of bacteria and their enzymes are necessary to degrade the large quantity and variety of substrates that enter a biological treatment unit. Because different groups of bacteria reproduce at different rates, the mean cell residence time (MCRT) or solids retention time (SRT) of a treatment unit must be adjusted to grow the required bacterial population and their appropriate enzymes for appropriate biological activities. These activities include (1) the degradation of specific substrates, (2) floc formation, and (3) the removal of specific pollutants such as phosphorus (Tables 8.1 and 8.2). The MCRT also can be adjusted to prevent the degradation of a specific substrate. For example, the MCRT of an activated sludge process can be adjusted to promote biological phosphorus removal and prevent nitrification (Table 8.1).

The MCRT or SRT necessary for appropriate biological activity is influenced by temperature. With increasing temperature the activity of the bacteria or biomass in a biological treatment unit increases; therefore a smaller number of bacteria or quantity of biomass is required. This permits a decrease in MCRT or SRT. With decreasing temperature the activity of the bacteria or biomass in a biological treatment unit decreases, and therefore a larger number of bacteria or quantity of biomass is required. This requires an increase in MCRT or SRT. Bacteria in a biological treatment unit may be present mostly in the dispersed state (Figure 8.1) such as a suspended growth anaerobic digester or the flocculated state (Figure 8.2) such as the activated sludge process.

In biological treatment units, substrates are absorbed or adsorbed by bacteria (Figure 8.3). Absorbed substrates are those that are soluble, are simple in structure, and quickly enter bacterial cells. Examples of absorbed substrates or soluble cBOD include acetate (CH3COOH), ethanol (CH3CH2OH), and glucose ^H^^These substrates are quickly degraded by endoenzymes; as a result, the substrates present

TABLE 8.1 MCRT (Approximate Days) Required for the Initiation of Significant Biological Activities in the Activated Sludge Process

Biological Activity Approximate

MCRT Required

Degradation of soluble cBOD

0.3

Floc formation (domestic wastewater)

1

Solubilization of particulte and collodial cBOD

2

Biological phosphorus removal

2

Floc formation (industrial wastewater)

3

Degradation of xenobiotic cBOD

5

Maturation of floc particles

10

Nitrification (temperature-dependent)

3-15

TABLE 8.2 SRT (Approximate Days) Required for the Initiation of Significant Biological Activities in the Anaerobic Digester

Biological Activity

Approximate SRT Required

Fermentation (acidogenesis), acids and alcohol production Solubilization of particulate and colloidal cBOD Methane production (H2 utilizing methanogens) Methane production (acetate utilizing methanogens) Fermentation of volatile fatty acids Fermentation of long-chain fatty acids Solubiization of lipids

FIGURE 8.1 Dispersed bacterial growth.
FIGURE 8.2 Flocculated bacteria or floc particle.

Complex soluble cBOD

Complex soluble cBOD

FIGURE 8.3 Absorption and adsorption of substrate. Only simplistic soluble substrates, cBOD, nBOD (ionized ammonia and nitrite), and some lipids, are easily absorbed by bacterial cells. Other substrates are adsorbed to bacteria and are solublized through the action of exoenzymes or conveyed into the cell with specific transport molecules (proteins).

FIGURE 8.3 Absorption and adsorption of substrate. Only simplistic soluble substrates, cBOD, nBOD (ionized ammonia and nitrite), and some lipids, are easily absorbed by bacterial cells. Other substrates are adsorbed to bacteria and are solublized through the action of exoenzymes or conveyed into the cell with specific transport molecules (proteins).

an immediate demand for nutrients and an electron carrier molecule such as free molecular oxygen or nitrate (NO3-). Adsorbed substrates are those that are insoluble or poorly soluble, are complex in structure, and do not enter bacterial cells. Examples of these adsorbed substrates include starches such as cellulose, lipids (fats and oils), proteins, and even disaccharides such as lactose and maltose. These sub-

Particulate cBOD Colloidal cBOD

Particulate cBOD Colloidal cBOD

FIGURE 8.4 Substrate adsorption to fibrils and slime. Substrates that are not absorbed or carried into a bacterium by a transport molecule are adsorbed to bacterial fibrils or slime where they are degraded into soluble molecules through the action of exoenzymes.

strates must be hydrolyzed into simple soluble molecules in order to enter bacterial cells.

Poorly soluble and insoluble substrates as well as nondegradable pollutants are adsorbed to the surface of bacterial cells or floc particles (Figure 8.3). Adsorbed substrates and pollutants are removed from the waste stream directly by electrochemical process (compatible charge) and indirectly through the coating action of secretions of higher life forms, ciliated protozoa and metazoa, especially rotifers and free-living nematodes. If substrates and pollutants have compatible charge, they attach to the negatively charged fibrils of bacterial cells that extend into the bulk solution (Figure 8.4). If the substrates and pollutants do not have compatible charge for direct adsorption to fibrils, their charge is made compatible for adsorption by the coating action of these higher life forms.

If sufficient residence time exists, hydrolytic bacteria produce exoenzymes in the biological treatment unit. When the exoenzymes are released and come in contact with the adsorbed substrates, the substrates are hydrolyzed (solublized) into smaller and soluble substrates that then are absorbed by they hydrolytic bacteria and non-hydrolytic bacteria in the biological treatment unit (Figure 8.5). Once absorbed, the small and soluble substrates are degraded by endoenzymes.

Hydrolytic bacteria consist of a consortia of Gram-positive, rod-shaped, facultative anaerobic bacteria and anaerobic bacteria that break down poorly soluble and insoluble complex carbohydrates, lipids, and proteins into simple and soluble sugars, fatty acids and glycerine (CH2OHCHOHCH2OH), and amino acids, respectively. These soluble substrates are available for absorption and degradation by numerous bacteria.

Wastewater Organisms

FIGURE 8.5 Hydrolysis and absorption of substrate. Particulate and colloidal cBOD that is adsorbed to the surface of bacterial cells is hydrolyzed (solublized) by the production and release of exoen-zymes from hydrolytic (exoenzyme-producing) bacteria. Once particulate and colloidal cBOD have been hydrolyzed, the soluble substrates produced through hydrolysis are absorbed and degraded by not only the hydrolytic bacteria but also the nonhydrolytic bacteria. The hydrolysis and absorption of substrate occurs in a limited amount in the activated sludge process (flocculated bacteria) and in a large amount in the anaerobic digester due to the higher solids retention time and larger diversity of hydrolytic bacteria in the anaerobic digester.

FIGURE 8.5 Hydrolysis and absorption of substrate. Particulate and colloidal cBOD that is adsorbed to the surface of bacterial cells is hydrolyzed (solublized) by the production and release of exoen-zymes from hydrolytic (exoenzyme-producing) bacteria. Once particulate and colloidal cBOD have been hydrolyzed, the soluble substrates produced through hydrolysis are absorbed and degraded by not only the hydrolytic bacteria but also the nonhydrolytic bacteria. The hydrolysis and absorption of substrate occurs in a limited amount in the activated sludge process (flocculated bacteria) and in a large amount in the anaerobic digester due to the higher solids retention time and larger diversity of hydrolytic bacteria in the anaerobic digester.

TABLE 8.3 Examples of Exoenzymes

Exoenzyme (Specificity or Compounds)

Function

Amylase (carbohydrate) Caseinase (milk protein) Cellulase (carbohydrate) Gelatinase (gelatin) Lactase (disaccharide/sugar) Lipase (lipids)

Maltase (disaccharide/sugar) Proteinases (protein) Sucrase (disaccharide/sugar)

Converts starch to maltose Converts milk protein to peptides and amino acids Converts cellulose to cellobiose Converts gelatin to peptides and amino acids Converts lactose to glucose and galactose Converts lipids (fat) to glycerol and fatty acids Converts maltose to two glucose molecules Converts proteins to peptides and amino acids Converts sucrose to glucose and fructose

Hydrolysis is the addition of water ("hydro") to complex molecules by bacteria to split ("lysis") unique chemical bonds in the complex molecules, thereby permitting the production and release of simple and soluble molecules. The addition of water and breakage of chemical bonds is catalyzed by exoenzymes such as cellulase (hydrolyze starches or carbohydrates), lipases (hydrolyze lipids), and proteases (hydrolyze proteins) (Table 8.3). Hydrolysis occurs slowly. The more slowly a complex molecule is hydrolyzed, the more slowly soluble substrates are made avail-

able to the biomass and an immediate demand for nutrients and electron carrier molecules such as free molecular oxygen or nitrate is prevented.

Hydrolysis performs two important roles in biological treatment units. First, many units receive only complex substrates. Here, hydrolysis is essential to provide soluble substrates to the biomass. Only soluble substrates can be absorbed and degraded by bacteria. Second, in any biological treatment unit, bacteria die and hydrolysis permits the solublization and degradation of cellular components rather than their accumulation.

The ability of any biological treatment unit to degrade a substrate is dependent upon the presence of the following critical factors:

• A diverse population of bacteria and enzymes

• An adequate number of enzymes

• Acceptable operational conditions including MCRT

• The molecular structure of the substrate

Bacteria degrade first the highly soluble and simple substrates, then the insoluble and complex substrates. A substrate such as sugar is readily degradable because the following conditions occur in the biological treatment unit:

• All the necessary enzymes for the degradation of the substrate are initially present in adequate numbers.

• The rate of all biochemical reactions that are necessary for complete degradation of the substrate are optimum.

• An adequate residence time is available for the degradation of the substrate.

A substrate such as chitin or wax is poorly degradable because the following conditions occur in the biological treatment unit:

• All the necessary enzymes for the degradation of the substrate are not initially present in adequate numbers.

• The rate of all biochemical reactions that are necessary for complete degradation of the substrate are not optimum.

• An adequate residence time is not available for the degradation of the substrate.

Poorly degradable substrates have chemical structures for which no organism can produce an enzyme for its degradation. Poorly degradable substrates also are known as incompletely degradable substrates or nonbiodegradable substrates.

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  • Lobelia
    What bacteria can produce exoenzymes?
    8 months ago

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