The two principal genera of bacteria of importance in biological nitrification processes are Nitrosomonas and Nitrobacter. But Nitrosospira, Nitrosolobus and Nitrosovibrio are also nitrifying bacteria.

These groups are classified as autotrophic organisms. They are distinguished from heterotrophic bacteria in deriving energy from oxidation of inorganic nitrogen compounds, rather than from the oxidation of organic compounds. These organisms are also special because carbon dioxide is used for the synthesis of biomass rather than organic carbon. Each group is limited to the oxidation of certain species of nitrogen compounds. Nitrosomonas, Nitrosospira, Nitrosolubus and Nitrosovibrio can all oxidize ammonia into nitrite, but cannot complete the oxidation into nitrate. On the other hand, Nitrobacter is limited to the oxidation of nitrite into nitrate. The apparent inability of these organic developments has been investigated and there is evidence that Nitrobacter can also utilize organic carbon as an energy source. This bacterial species is therefore classified as a facultative autotroph. As complete nitrification is a sequential reaction, treatment processes must be designed to provide an environment suitable for the growth of both groups of nitrifying bacteria.

In contrast to many heterotrophs, the growth of nitrifiers is very slow, and the yield of cells per unit of energy oxidized is low. Like other micro-organisms, nitrifiers can grow at their maximum growth rate when optimum environmental factors can be obtained in an environment without any toxic substances.

Two conditions, therefore, must be fulfilled in order to obtain nitrification in a treatment plant. First, the sludge age has to be sufficiently high to prevent the wash out of the slow-growing nitrifiers applying active sludge design systems. Second, the contact time between the bacterial mass and the ammonia must be long enough to oxidize the ammonia. Table 3.1 compares some characteristics of Nitrosomonas and Nitrobacter.

In Chapters 5 and 6, different plant designs will be outlined. The different mass balance equations for different nitrification plants will also be discussed, showing the relationship between the biomass content and the nitrification efficiency of these plants.

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