As distinct from nitrification, a relatively broad range of bacteria can accomplish denitrification. Genera of bacteria that are known to contain denitrifying bacteria include Pseudomonas, Micrococus, Archromobacter, Thiobacillus, and Bacillus (see Table 4.1). These bacteria are biochemically and taxonomically very diverse. Most are he-terotrophs and some utilize one-carbon compounds, whereas others grow auto-trophically on H2 and C02, or on reduced sulphur compounds. Most of the mentioned bacteria possess the enzyme reductase necessary to reduce nitrate to gaseous nitrogen. But some lack the nitrate reductase enzyme and are termed nitrite dependent; and others lack N20 reductase and thus yield N20 as the terminal product. Still other organisms possess N20 reductase but cannot produce NsO from nitrate or nitrite. These different groups of bacteria also accomplish nitrate reduction by what is known as a process of nitrate dissimilation, whereby nitrate or nitrite replaces oxygen in the respiratory process of the organism under anoxic conditions. Because of the ability of these organisms to use either nitrate or oxygen as the terminal electron acceptor while oxidizing organic matter, these organisms are termed facultative heterotrophic bacteria.
Surprisingly, most of the organisms known to denitrify are not strict anaerobes, but rather facultative organisms, which under anoxic conditions use nitrate as a final electron acceptor. The sludge in combined nitrification and denitrification design processes is alternatively exposed to aerobic and anaerobic conditions, and because the denitrifying bacteria are facultative, the change of an oxic environment will provoke only minor adaptation problems.
Table 4.1 Genera of bacteria which are abundant in sewage and capable of performing denitrification.
Abundant in sewage
Species within the genera are denitrifiers N03N2
Achromobacter Van Gils (1964)
Harris etal. (1927)
Van Gils (1964), Jasewicz and Porges (1956)
Smith etal. (1972)
Micrococcus Jasewicz and Porges (1956) Payne (1973), Porra and Lascelles (1965)
Proteus Harris etal. (1927)
Pseudomonas Jasewicz and Porges (1956) Best and Payne (1965), Fewson and Nicholas (1961),
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