The most important characteristic of the environment in which microorganisms grow is the terminal acceptor of the electrons they remove as they oxidize chemicals to obtain energy. There are three major types of electron acceptors: oxygen, inorganic compounds, and organic compounds. If dissolved oxygen is present or supplied in sufficient quantity so as to not be rate limiting, the environment is considered to be aerobic. Growth is generally most efficient in this environment and the amount of biomass formed per unit of waste destroyed is high. Strictly speaking, any environment that is not aerobic is anaerobic. Within the wastewater treatment field, however, the term anaerobic is normally reserved for the situation in which organic compounds, carbon dioxide, and sulfate serve as the major terminal electron acceptor and in which the electrode potential is very negative. Growth is less efficient under this condition. When nitrate and/or nitrite are present and serve as the primary electron acceptor in the absence of oxygen, the environment is called anoxic. The presence of nitrate and/or nitrite causes the electrode potential to be higher and growth to be more efficient than under anaerobic conditions, although not as high or as efficient as when oxygen is present.
The biochemical environment has a profound effect on the ecology of the microbial community. Aerobic operations tend to support complete food chains from bacteria at the bottom to rotifers at the top. Anoxic environments are more limited, and anaerobic are most limited, being predominantly bacterial. The biochemical environment influences the outcome of the treatment process because the microorganisms growing in the three environments may have very different metabolic pathways. This becomes important during the treatment of industrial wastewaters because some transformations can be carried out aerobically but not anaerobically, and vice versa.
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