This chapter aims to give a broad overview of the biological nitrification and denitrification systems and to compare the different unit processes explained in detail in later chapters. This should facilitate the understanding of the following chapters 3-6, dealing with the biological unit processes.

The contents of this chapter may be summarized as follows:

1) Classification of the different nitrification and denitrification unit processes (section 2.2).

2) The terminology used in the basic waste water treatment (section 2.3).

3) Comparison of the biofilm (attached-growth) and the activated sludge (suspended-

growth) unit processes (section 2.4).

4) Comparison of the nitrification rate for the unit processes described in later chapters (section 2.5).

The removal of nitrogen by biological nitrification and denitrification is a two-step process. In the first step (nitrification) ammonia is converted aerobically to nitrate (N03"). In the second step (denitrification) nitrates are converted to N20 or nitrogen gas (N2) under anoxic conditions.

Nitrification is an autotrophic process which means that the energy for bacterial growth is derived from the oxidation of nitrogen compounds, primarily ammonia. In contrast to heterotrophs, nitrifiers use carbon dioxide as a carbon source rather than organic carbon for the synthesis of new cells. Nitrifier cell-yield per unit of substrate metabolized is many times smaller than the cell yield for heterotrophs and denitrifier, see Table 2.3.

As will be described in Chapter 3 the nitrification process is a two-step process involving two genera of microorganisms, Nitrosomonas and Nitrobacter. In the first step, ammonium is converted to nitrite; in the second step, nitrite is converted to nitrate. The conversion processes are outlined in Section 3.4.

Chapter 4 describes how the denitrification can be accomplished biologically under anoxic conditions. Two types of enzyme systems are involved in the reduction of nitrate: assimilatory and dissimilatory. In the assimilatory nitrate reduction process, N03" -N is converted to ammonia nitrogen for the use by the cells in biosynthesis. It occurs when N03" -N is the only form of nitrogen available. In the dissimilatory nitrate reduction process, nitrogen gas is formed from nitrate. This latter process is normally called denitrification of waste water, and demands a carbon source to provide energy for the process. More than 2000 species of bacteria can perform the dissimilatory denitrification process.

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