The purpose of this chapter is to present a review of the chemistry and biochemistry of nitrification. An understanding of this subject is important for an understanding of the factors affecting the performance, design and operation of nitrification.

Biological processes for the control of nitrogenous residuals in effluents can be classified in two broad areas: the production of an effluent where nitrogen (ammonia and organic nitrogen) is converted into nitrate nitrogen: and the reduction of nitrate into nitrogen gas.

In the first stage, nitrification is carried out by bacteria oxidizing ammonia into nitrate with the intermediate formation of nitrite. Nitrification must conform to existing water standards, where reduction of the residual demand on nitrogenous oxygen due to the presence of ammonia is necessary, or where reduction of ammonia is required to conform with existing standards.

The second stage, denitrification (for details see Chapter 4), is used following the nitrification when the total nitrogenous content of the effluent must be reduced.

These conversions are of great importance because ammonia is a highly toxic metabolic waste of aquatic organisms. Nitrite is somewhat less toxic than ammonia (as NHa), although nitrite toxicity may occur at concentrations of less than 2.5 ppm for some species (Westin 1973). Nitrate is considered relatively non-toxic to most aquatic organisms. As an example, Knepp and Arkin (1985) showed that for channel catfish (a highly tolerant species) the LD50 value for ammonia was 37.5 ppm, but nitrate concentrations as high as 400 ppm did not affect feeding activities or growth rates.

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