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nitrite, sulfate)

Source: Adapted from L.D. Benefield and C.W. Randall, 1980, Biological process design for wastewater treatment (Englewood Cliffs, N.J.: Prentice-Hall).

Source: Adapted from L.D. Benefield and C.W. Randall, 1980, Biological process design for wastewater treatment (Englewood Cliffs, N.J.: Prentice-Hall).

The nutritional requirements of microorganisms provide a basis for classification. Microorganisms are classified on the basis of the form of carbon they require:

Autotrophic: These microorganisms use carbon dioxide or bicarbonate as their sole source of carbon, from which they construct all their carbon-containing biomolecules. Heterotrophic: These microorganisms require carbon in the form of complex, reduced organic compounds, such as glucose.

Microorganisms are also classified on the basis of their required energy source:

Phototrophs: These microorganisms use light as their energy source.

Chemotrophs: These microorganisms use oxidation-reduction reactions to provide their energy.

Chemotrophic microorganisms can be further classified on the basis of the type of chemical compounds that they oxidize, i.e., on the basis of their electron donor. For example, chemoorganotrophs use complex organic molecules as their electron donors, while chemoautotrophs use simple inorganic molecules such as hydrogen sulfide (H2S) or ammonia (NH33). Table 7.22.2 summarizes microorganism classification by sources of energy and cell carbon.

In addition to energy and carbon sources, microorganisms require principal inorganic nutrients such as nitrogen, sulfur, phosphorus, potassium, magnesium, calcium, iron, sodium, and chloride. Minor nutrients of importance include zinc, manganese, molybdenum, selenium, cobalt, copper, nickel, and tungsten (Metcalf and Eddy, Inc. 1991). Microorganisms also require organic nutrients, known as growth factors, as precursors or constituents of organic cell material that cannot be synthesized from other carbon sources. These growth factors differ from one or-

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