Chapter 15  Ammonia               return to Table of Contents                                                                      

 

                        Nitrogen

                        Ammonia

                        Total Kjeldahl Nitrogen (TKN)

                        Organic Nitrogen

                        Nitrite

                        Nitrate and Nitrate Nitrogen

 

                 Some of the information in Chapter 10 which covers Nitrogen, is duplicated in this chapter as a convenience for the reader.

                 The pyramidal ammonia molecule has a lone pair of electrons on the nitrogen atom and polar N--H bonds.  The water molecule has two polar bonds involving hydrogen and two lone pairs -- the right combination for optimum hydrogen bonding -- in contrast to the one lone pair and three polar bonds of the ammonia molecule.

                 Molecular nitrogen that is present in the air is of (almost) no concern as related to nitrates: the only way molecular nitrogen can become nitrite and nitrate is through fixation of the molecular nitrogen by bacteria such as rhizobium.  Without bugs (rhizobium, or some plants with nitrogen fixing nodules which most of the time are bacterial colonies) atmospheric nitrogen (N₂) does not get into the cycle.

                 The Nitrogen Cycle

                 Whereas carbon, hydrogen, and oxygen are actively cycled by microorganisms, plants, and animals, the biogeochemical cycling of nitrogen is largely dependent on the metabolic activities of microorganisms alone.  Aside from the chemical fixation of molecular nitrogen by human beings to form nitrogen fertilizers, the ability to fix atmospheric nitrogen (the conversion of  N₂ to ammonia or organic nitrogen) that can be assimilated into biomass is restricted, almost exclusively, to a limited number of bacterial species.  Most microorganisms and all plants and animals are unable to use atmospheric nitrogen directly and depend on the availability of fixed forms of nitrogen for incorporation into their cellular biomass.  Ammonia is the first detectable product of nitrogen fixation.  It is assimilated into amino acids and subsequently synthesized into proteins and nucleic acids.  Proteins, amino acids, and inorganic ammonium ions are used as a source of nitrogen by many organisms that are unable to assimilate atmospheric nitrogen directly.

                 Ammonia may be oxidized to nitrite then to nitrate in varying degrees depending on temperature, time and available oxygen.  Nitrate is seldom found in raw wastewater or primary effluent, but the secondary effluent will have some because of the biological treatment process.              

                 To illustrate the concept of nutrient cycles. a simplified version of the nitrogen cycle will be used as an example.  A wastewater treatment plant discharges nitrogen in the form of nitrate in the plant effluent to the receiving waters.  Algae take up the nitrate and produce more algae.  The algae are eaten by fish which convert the nitrogen to amino acids, urea and organic residues.  If the fish die and sink to the bottom, these nitrogen compounds can be converted to ammonium.  In the presence of dissolved oxygen and special bacteria, the ammonium is converted to nitrite then to nitrate, and finally the algae can take up the nitrate and start the cycle all over again.

                 “Nitrogenous material can enter the aquatic environment from either natural or human-caused sources.  The proper delineation of these sources an often be clouded, since the apparent quantities from natural sources can include nitrogen generated from human activity.  For example, while nitrogen fixation by lightning may be expected in rainfall, the combustion of fossil fuels or the application of liquid ammonia agricultural fertilizers, which subsequently releases to the air through volatilization can increase rainfall concentration of Nitrogen substantially.  The pervasiveness of human impact on the environment limits the certainty with which naturally occurring Nitrogen and pollution source Nitrogen can be differentiated quantitatively.  Measurement of naturally occurring baseline Nitrogen levels are best made in more remote, underdeveloped and pristine areas; yet interpretation of these data should still be considered uncertain to some degree.”¹

The compounds of nitrogen are of interest to the wastewater treatment plant operator because of the importance of nitrogen in the life processes of all plants and animals.  The chemistry of nitrogen is complex because of the several forms that nitrogen can assume.  Ammonia, organic nitrogen, nitrate, and nitrite are the most important nitrogen forms in wastewater treatment.  The term Kjeldahl nitrogen refers to organic plus ammonia nitrogen.  In wastewater, the definitions of TKN and TN are: TKN = organic nitrogen + ammonia.   TN (Total Nitrogen) = TKN + nitrate + nitrite = organic nitrogen + ammonia + nitrate + nitrite.  Therefore, the difference is the oxidized forms of nitrogen, nitrate and nitrite.  Depending on the treatment process, organic loading, and other considerations, one needs to decide what is important.  If oxidized forms of nitrogen affect the removal of ammonia, then the concentrations are important.  In secondary treatment process, nitrate and nitrite are usually tested, at least in summer months.  TKN is important because organic loading (from organic nitrogen) represents oxygen demand.

Total Kjeldahl nitrogen (TKN) is a parameter that is frequently used as an indicator of industrial pollution and sewage (Boler 1992). This parameter includes nitrogen from ammonia, amino acids, polypeptides, and proteins (Boler 1992).  The concerns of excessive TKN levels are largely the same as for excessive total nitrogen levels, though the pressures are generally more from point sources such as sewage treatment plants and industrial point sources.

Total nitrogen is the combined measurement of various forms of nitrogen in water including nitrate, nitrite, ammonia, and organic nitrogen. Such nitrogenous compounds along with other nutrients, serve as an important nutrient base for primary productivity.  When the concentration of these nutrients consistently exceeds natural levels, however, a nutrient imbalance is produced.  This imbalance can lead to undesirable changes in the biological community and can drive an aquatic system into an accelerated rate of eutrophication.  Typically, the eutrophication process leads to a change in the structure of the algal community, including severe algal blooms for extended periods of time.   In turn, the decomposition of these large algal blooms usually leads to a depletion in dissolved oxygen concentrations (Friedemann and Hand 1989).