44    FIXED-FILM NITRIFICATION SYSTEM
MODIFICATIONS FOR AQUATIC TOXICITY CONTROL
Enos L. Stover, President
Douglas J. Fort, Vice President
The Stover Group
Stillwater, Oklahoma 74076
INTRODUCTION
The various compounds of nitrogen are of great interest because of the importance of nitrogen
in the life processes of all plants and animals. Nitrogen chemistry is very complex because nitrogen can exist in several valance states. The valance state can be changed by living organisms,
and the valance changes created by bacteria can be either negative or positive, depending on the
prevailing environmental conditions and aerobic, anoxic, or anaerobic biological metabolism.
The nitrogen cycle best illustrates the relationships that exist between the various forms of nitrogen compounds and the changes that can occur in nature. The nitrogen reactions that are important relative to biological treatment of wastewaters follow:
N2
T
Organic Nitrogen <-* NH3 - N <-> N02- N «-» NO, - N
During conditions of excess ammonia-nitrogen beyond synthesis requirements, many times biological nitrification reactions occur. During these reactions ammonia-nitrogen is oxidized to nitrite-nitrogen and then to nitrate-nitrogen. Once in the form of nitrite-nitrogen or nitrate-nitrogen
the nitrogen cannot be used for biological synthesis reactions. Under anoxic conditions the nitrate-nitrogen can be reduced to nitrite-nitrogen and the nitrite-nitrogen can be reduced to nitrogen gas. The nitrogen gas is insoluble in water. Under aerobic conditions and the absence of ammonia-nitrogen, nitrate-nitrogen reduction to ammonia-nitrogen can be carried out by some
bacteria.
In aqueous solutions, the dissolved ammonia molecule exists in the hydrated form as ionized
ammonia (NH4+) and unionized ammonia (NH3). The unionized ammonia is in equilibrium with
the ammonium ion and the hydroxide ion. The equilibrium equation is expressed as:
NH3(g) + nH20(l) = NH3 • n (H20)(aq) = NH4+ + OH" + (n-1) H20
The form of ammonia in solution that is toxic to aquatic life forms is thought to be the unionized form (NH3), though there is still considerable debate on the subject. In order to predict the
toxicity of ammonia to the biomonitoring organisms, it is therefore important to know the concentration of NH3 (the toxic form) in solution. This is dependent on several factors, including
total ammonia concentration (NH3 + NH4+), pH, and temperature. The NH3 concentration increases as pH and/or temperature increases, but as ionic strength increases, NH3 concentration
decreases. In fresh water systems, however, this decrease is negligible. The equilibrium partitioning of total ammonia can typically be accurately calculated if pH and temperature are known.
In addition, the toxicity of NH3 is impacted by pH, such that increasing pH reduces the toxicity
of the NH3 species.
51st Purdue Industrial Waste Conference Proceedings, 1996, Ann Arbor Press, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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