Chapter 10   Nitrogen                                                  return to Table of Contents

  [ I'm adding an audible pronunciation feature, when you see this icon, click on it (you may need QuickTime) : ]

                 The two nutrients of concern are nitrogen and phosphorous.  Both of these are fertilizers and either can be the limiting nutrient for plant growth in receiving streams.  Because these molecules affect so much, chapter 15 covers Ammonia and chapter 11 describes phosphorous.

                 The term 'straight fertilizer' is used for any material that supplies only one of the three principal macronutrients: nitrogen, phosphorus or potassium.  Mixed fertilizers are those that contain two or all three of the principal macronutrients.  Both may be liquids, but usually they are solids in granular form.  The manufacturer labels every container to show the grade of the fertilizer.  This is expressed by three numbers, which are the percentages of the three major nutrients.  Thus, a 4 -16 -18 fertilizer contains 4 percent nitrogen (N), 16 percent phosphoric oxide (P₂O₅), and 18 percent potassium oxide (K₂O).¹  Nitrogen will generally stimulate the growth of foliage; phosphate assists root growth, energy storage and flower formation while potassium contributes to cell wall strength. Roses seem to prefer 18-24-16 (NPK) while other plants thrive on different amounts.

Adequate supplies of these nutrients (NPK) lead to rapid growth and reproduction of algae, the eutrophication stage for the water system.  The explosion in growth rate for the algae is called a “bloom”.  Algae are plants and during times of sunlight add oxygen to the water as a byproduct of photosynthesis, during the dark hours they absorb oxygen.  When the bloom reaches maximum growth, the algae begin to die.  Decomposition of the algae by bacteria also contributes to the oxygen depletion in the water.

                 Nitrogen in wastewater is most commonly present in the form of bound organic nitrogen.  Biological treatment is required to convert the organic nitrogen; first to ammonia, next to nitrite, then nitrate.  Ammonia is produced under anaerobic conditions while the nitrate is the product of aerobic digestion.  If nitrate is produced, the nitrogen reduction has come to a dead-end.  Currently, it is not economically feasible to further reduce nitrate levels in water.  Ammonia can be, however; several methods are available:  air-stripping, ion-exchange and breakpoint chlorination.

               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, nitrate, and nitrite are the most important nitrogen forms in wastewater treatment.  Kjeldahl nitrogen is organic plus ammonia nitrogen.

                 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. These bugs are usually associated with root system of plants such as legumes, but they may be present in a lagoon environment.  If so, input of nitrogen into a lagoon provides bacteria with an essential nutrient which eventually is converted into a biomass, which after decay contributes ammonia, nitrite and nitrate.  While aerating a lagoon with air (~79% Nitrogen), the nitrogen combines very little (like in air, although the nitrogen dissolves in the splashing along with the oxygen) with anything unless helped by bugs.   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 only other effect of air's nitrogen that is important in biological treatment is what happens in aeration with air in deep tanks: oxygen is the most consumed molecule while nitrogen, dissolved above saturation at normal pressure, upon release in the effluent the nitrogen bubbles out and sludge is difficult to settle since in effect we have dissolved nitrogen flotation.  In such cases degasification (denitrogenation) is needed to help settling. This aspect is often not considered by deep tank designers.

                 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.  Other than one exceptional case where a green algae has been shown to fix atmospheric nitrogen, this process is carried out strictly by bacteria.  The productivity of many ecosystems is limited by the supply of fixed forms of nitrogen.  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.

                 It is estimated that microorganisms convert approximately 200 million metric tons of nitrogen to fixed forms of nitrogen per year compared to about 30 million metric tons produced by industrial production of nitrogen fertilizers.  The fixation of atmospheric nitrogen depends on the nitrogenase enzyme system.  Nitrogenase is very sensitive to oxygen and nitrogen fixation and, therefore, nitrogen fixation often is restricted to habitats with appropriately low levels of free oxygen; the nitrogenase enzyme is protected in some systems by leghaemoglobin, which supplies oxygen to the organisms for respiration without denaturing the nitrogenase.  ATP, adenosine triphosphate, is required to drive the reactions catalyzed by the nitrogenase enzyme system.  The fixation of atmospheric nitrogen requires a high energy input (approximately 30 ATP/N₂ fixed) and in terrestrial ecosystems is largely dependent on the availability of relatively high concentrations of organic matter for use in generating ATP.

                 Nitrogen fixation in soil

                  In terrestrial habitats, the microbial fixation of atmospheric nitrogen is carried out by free-living bacteria and by bacteria living in symbiotic association with plants.  Symbiotic nitrogen fixation by rhizobium is most important in agricultural fields, where this bacterium lives in association with various crop plants.  In forests, other symbiotic nitrogen-fixing bacteria, including actinomycetes, live in association with various trees and make significant contributions to the nitrogen needed to support the growth of forests.  When growing in association with plants or animals, symbiotic nitrogen-fixing bacteria, such as rhizobium species, generally exhibit rates of nitrogen fixation that are two to three orders of magnitude higher than are accomplished by free-living nitrogen-fixing soil bacteria.  Rhizobium species associated with alfalfa, for example, can account for an input of 250 kg of nitrogen fixed per hectare per year, as compared to 2.5 kg of nitrogen fixed per hectare per year for free-living nitrogen-fixing Azotobacter species.

                 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.” ⁹

                 Nitrogen exists in many forms in the environment .  The movement and transformation of these nitrogen compounds through the biosphere is characterized by the nitrogen cycle.  The atmosphere serves as a reservoir of nitrogen in the form of nitrogen gas.  Although virtually inexhaustible ( the atmosphere is 79 percent nitrogen), the nitrogen must be combined with hydrogen or oxygen before it can be assimilated by higher plants; the plants, in turn are consumed by animals.  Fixation of nitrogen means the incorporation of inert, gaseous nitrogen into chemical compounds such that it can be used by plants and animals.  Ammonification is the change from organic nitrogen to the ammonium form.  Synthesis, or assimilation, is a biochemical mechanism that uses ammonium or nitrate compounds to form plant protein.  Nitrification is the biological oxidation of ammonium.  This is done in two steps, first to the nitrite form [NO₂], then to the nitrate form [NO₃]. 

Nitrification is a biological process accomplished primarily by two types of microorganisms: Nitrosomonas and Nitrobacter, autotrophs. The proper conditions must exist for Nitrosmomonas to be able to separate the nitrogen from the hydrogen in the ammonium molecule and replace the hydrogen with oxygen molecules. Sufficient oxygen and the appropriate temperature and microbiological food must be present to accomplish this process. Nitrobacter also rely on oxygen to complete the stabilization of the nitrite molecule into the more stable nitrate substance.

Denitrification is the biological reduction of nitrate to nitrogen gas.  It can proceed through several steps in the biochemical pathway, with the ultimate production of nitrogen gas.  A fairly broad range of heterotrophic bacteria are involved in this process.  The nitrogen cycle is complex and follows many paths.

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, nitrate, and nitrite are the most important nitrogen forms in wastewater treatment.  The term Kjeldahl (KELL-doll) 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 which is important to test.  If oxidized forms of nitrogen affect the removal of ammonia, then those 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).

Distillation and specific ion meters are two methods of measuring the amount of ammonia in a sample. 

 

                    

Top of the Document
On to Chapter 11
My question sets for self exam		errors?