41    PERFORMANCE CHARACTERISTICS OF AN ACTIVATED
SLUDGE SYSTEM WHEN NITRATE IS THE SOLE
SOURCE OF NITROGEN
Nanriette Corey, Engineer
James M. Montgomery, Consulting Engineer
Metarie, Louisiana 70002
Larry Benefield, Professor
Department of Civil Engineering
Auburn University, Alabama 36849
INTRODUCTION
On a weight basis most of the organic matter of a bacterial cell consists of macromolecules which
belong to four classes: nucleic acids, proteins, polysaccharides, and complex lipids. Although the
percentages will vary depending on the actual growth conditions, a rapidly growing cell will contain
approximately 60% protein, 19% nucleic acid, 15% lipid and 6% carbohydrate.
Organic removal in an activated sludge process is accomplished by aerobic heterotrophic microorganisms which utilize a portion of the organic material as a carbon source for synthesis of new
biomass and oxidize the remainder to provide energy for the synthesis function as well as for the other
nongrowth functions. In order for the synthesis function to proceed, the wastewater must contain an
adequate supply of all the elements which are found in the cytoplasmic material of a cell.
Nitrogen is a component of proteins, nucleic acids, amino sugars, etc., and is, therefore, essential
for microbial growth. Generally, the nitrogen requirement is easily met with municipal wastewaters,
but many industrial wastewaters do not contain sufficient nitrogen and require its addition as a
supplement.
When nitrogen addition is required, it is normally added in the ammonium form. For wastewaters
totally deficient in nitrogen, the cost for supplemental nitrogen can be quite high. Therefore, it is
desirable in such cases to minimize, to the greatest extent possible, the overall operational costs for the
activated sludge process. One way this might be accomplished is to use nitrate as a source of nitrogen.
It is possible that the use of this nitrogen source could alter certain system operating parameters such
as sludge production in such a manner that the overall effect of nitrogen supplementation on operating costs would be minimal. However, information in this area is very limited. The objective of this
research was to, therefore, determine how the operating characteristics of the activated sludge proces
are affected when nitrate is used as the sole source of nitrogen.
LITERATURE REVIEW
Ammonia is the only form of inorganic nitrogen that can be directly assimilated into amino acids.
The ability of an organism to utilize other forms of inorganic nitrogen depends on the presence of
enzymes or enzyme systems that are able to convert these compounds to ammonia. Some of the major
reactions which have been proposed to describe the interconversion of various inorganic nitrogen
compounds are presented in Figure 1.
Certain microorganisms have the ability to utilize nitrate as a terminal electron acceptor for anaerobic respiration through a process known as dissimilatory nitrate reduction. Nitrate can also be used by
some microorganisms as a source of nitrogen through a process known as assimilatory nitrate
reduction.
Two types of dissimilative nitrate reduction processes have been identified in microorganisms.2
These are fermentative nitrate reduction and respiratory nitrate reduction. Fermentative nitrate reduction is the term used by Broda3 to describe the process when no membrane-bound enzymes, cytochromes or electron transport phosphorylations are involved. In this case nitrate acts as an electron
45th Purdue Industrial Waste Conference Proceedings, © 1991 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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