The Role of
Nitrate Nitrogen in Bio-Oxidation
E. D. SCHROEDER, Assistant Professor
Civil Engineering Department
University of California
Davis, California
A. W. BUSCH, Professor
Environmental Engineering Department
Rice University
Houston, Texas
INTRODUCTION
Bacterial reduction of nitrate has been a subject of importance to engineers
since the early part of the century. Reduction of nitrate ion has been known to be
related to substrate oxidation, and nitrate has been referred to as a "chemical
source of oxygen." Nitrate has been added to oxygen-starved treatment systems
and streams with the purpose of keeping them aerobic. Results of nitrate addition
have been unpredictable and generally less than expected. The objectives of this
paper are the explanation of reported results and the discussion of the role of nitrate metabolism in bio-oxidation.
INORGANIC NITRATE METABOLISM
Nitrate reduction by micro-organisms may be divided into two general categories, assimilatory and dissimilatory (1). The term, denitrification, has often
been used to describe the nitrate reduction process, however, the term will be
used here only in cases where the gaseous end products NO, N2O and N2 are
formed.
Assimilatory nitrate reduction occurs in bacterial systems in which nitrate is
a source of nitrogen for synthesis. Micro-organisms in general, and also many
higher plants, have the ability to reduce nitrate (N+5) to ammonia (N-3). Nitrogen must be in the ammonia state for incorporation into cellular material (2,3).
Thus, microbial systems would be expected to utilize ammonia nitrogen preferentially over more oxidized forms such as nitrite or nitrate. Inhibition of nitrate
reduction by ammonia has been demonstrated with several microorganisms (2,4,
5). Examples have also been found in which nitrate assimilation was not inhibited
by ammonia, however (2,6).
Dissimilatory nitrate reduction has been defined as reduction of nitrate in
which nitrate serves as the essential hydrogen acceptor which enables an organism
to grow(l). A slightly broader definition will be used here which defines dissimilatory nitrate reduction as occurring where nitrate serves as the terminal exogenous hydrogen acceptor for the oxidation of a substrate. The difference between the two definitions may be explained by a brief discussion of bacterial
metabolism.
Terminal oxidation of organic compounds by bacteria centers around the
Krebs cycle, a bio-chemical reaction system with carbon dioxide and reduced
pyrimidine nucleotides as primary products.    A schematic diagram of the Krebs
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