Bacteria grow in the presence or absence of free molecular oxygen and can be placed in three groups according to their need for or response to free molecular oxygen.These groups are aerobes, anaerobes, and facultative anaerobes (Figure 3.8).
Aerobes require oxygen for the degradation of substrate. Examples of aerobic bacteria in activated sludge process include the filamentous organisms Halis-comenobacter hydrossis and Sphaerotilus natans, the floc former Zoogloea ramigera, and the nitrifying bacteria Nitrosomonas and Nitrobacter. Active aerobes are not found in anaerobic digesters. Anaerobic bacteria do not use free molecular oxygen
for the degradation of substrates. These organisms include sulfate-reducing bacteria and methane-forming bacteria that use sulfate (SO42) and carbon dioxide, respectively.
Between the aerobes and anaerobes are the facultative anaerobes.The term "facultative" implies the ability to live under different conditions. Facultative anaerobic bacteria have the ability to use free molecular oxygen or another molecule to degrade substrate. Facultative anaerobic bacteria have the most complex enzyme systems with respect to degradation of substrate. They have one enzyme system for the use of free molecular oxygen and have another enzyme system for the use of an alternate molecule to degrade substrate when oxygen is not available. However, they prefer oxygen to other molecules such as nitrate (NO3-) to degrade substrate, because they produce more offspring (sludge) from the same quantity of substrate with the use of free molecular oxygen than another molecule. Denitrifying bacteria including Bacillus, Escherichia, and Pseudomonas are facultative anaerobic bacteria.
There are two important groups of anaerobic bacteria. These groups are the oxygen-tolerant (aerotolerant) anaerobic bacteria and the oxygen-intolerant (obligate) anaerobic bacteria. The oxygen tolerant anaerobes can survive in the presence of free molecular oxygen. Oxygen tolerant anaerobes may or may not be active in the presence of free molecular oxygen. Obligate anaerobes such as the methane-forming bacteria die in the presence of free molecular oxygen.
Obligate anaerobes are not killed by free molecular oxygen. They are killed by superoxide (O2-) and hydrogen peroxide (H2O2). These products are formed when oxygen enters the bacterial cell. When oxygen enters the bacterial cell, it is converted to superoxide. Although superoxide is highly toxic, it is converted by the enzyme superoxide dismutase to oxygen and hydrogen peroxide (Equation 3.1). Although hydrogen peroxide also is highly toxic, it is converted by the enzyme cata-lase to oxygen and water (Equation 3.2). Most obligate anaerobic bacteria including the methane-forming bacteria lack the enzymes superoxide dismutase and catalase and succumb to the toxic effects of superoxide and hydrogen peroxide.
2O2-+ 2H+ superoxidedismutase,ifpresent > O2 + H2O2 (3.1)
TABLE 3.3 Concentration of Oxygen Needed in the Activated Sludge Process for Acceptable Biological Activity
Aerobic Biological Activity
Minimum Concentration (mg/liter)
Endogenous respiration Floc formation Nitrification
Control growth of low DO filaments
Correlated with chemical oxygen demand (COD) removed across aeration tank
With respect to the quantity of oxygen necessary in activated sludge process to ensure acceptable biological activity by aerobe and facultative anaerobes, there are four activities of concern (Table 3.3).
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