Environmental Significance Of Nitrogen Species

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While nitrogen is seen to be an essential component of all living things, excessive concentrations of certain nitrogen species in some compartments of the environment can lead to significant environmental problems. This is true of some nitrogen species in the atmosphere as well as in terrestrial and aquatic environments.

Atmospheric Concerns with Nitrogen Species

The three major environmental problems associated with nitrogen species in the atmosphere are photochemical smog, global warming, and stratospheric ozone depletion. Photochemical smog results when partially oxidized organic matter, NO*, and sunlight come together under certain meteorological conditions, resulting in a series of complex chemical and photochemical reactions that lead to the production of high ozone concentrations and organic chemicals that together produce eye irritation, reduced air visibility, crop damage, and severe adverse health impacts in humans. The automobile has been a primary producer of two of the ingredients of photochemical smog, partially oxidized organic matter and NOj;. The most serious problems occur in dense urban areas where there are many automobiles.

Wide use of fossil fuels over the past century has resulted in an increase in atmospheric carbon dioxide, which acts as a blanket to prevent heat from radiating from the earth, a phenomenon, come to be known as the greenhouse effect, that is increasing the earth's temperature. However, the gaseous oxides of nitrogen also exhibit a greenhouse effect. While carbon dioxide is believed responsible for about 55 percent of the increased changes to the radiative temperature between 1980 and 1990, increased NO, production from fuel and biomass combustion and particularly from denitrification (N20) as a result of increased commercial fertilizer usage is estimated to be responsible for 6 percent of the increase.® While the amount of NO* in the atmosphere would appear to be small compared with C02, one molecule of N20 has a heat-trapping ability equivalent to 200 molecules of CO2.

While NOjj is partially responsible for increased ozone production as part of photochemical smog production in urban areas near the earth's surface, it is somewhat surprising that it also plays a role in the destruction of ozone in the stratosphere. Stratospheric ozone plays a key role in protecting life on earth from the harmful effects of excessive ultraviolet radiation. The widespread use of chlorofluo-rocarbons (CFCs) is known to have resulted in significant destruction of the protective stratospheric ozone, but NO* is playing a role as well. N20 and N02 are both converted to NO in the atmosphere, and NO reaching the stratospheres reacts with ozone to result in its depletion.

'R. Rosswall, "Greenhouse Gases and Global Change: International Collaboration," Env. Set. Tech., 25(4): 567, 1991.

Aquatic Concerns with Nitrogen Species

An Indicator of Sanitary Quality It has long been known that polluted waters will purify themselves, provided that they are allowed to age for sufficient periods of time. The hazard to health or the possibility of contracting disease by drinking such waters decreases markedly with time and temperature increase, as shown in Fig. 25.2.

Prior to the development of bacteriological tests for determining the sanitary quality of water (about 1893), those concerned with the public health were largely dependent upon chemical tests to provide circumstantial evidence of the presence of contamination. The chloride test was one of these (see Sec. 21.2), but it gave no evidence of how recently the contamination had occurred. Chemists working with wastes and freshly polluted waters learned that most of the nitrogen is originally present in the form of organic (protein) nitrogen and ammonia. As time progresses, the organic nitrogen is gradually converted to ammonia nitrogen, and later on, if aerobic conditions are present, oxidation of ammonia to nitrite and nitrate occurs. The progression of events was found to occur somewhat as shown in Fig. 25.3, and more refined interpretations of the sanitary quality of water were based upon this knowledge. For example, waters that contained mostly organic and ammonia nitrogen were considered to have been recently polluted and therefore of great potential danger. Waters in which most of the nitrogen was in the form of nitrate were considered to have been polluted a long time previously and therefore offered little threat to the public health. Waters with appreciable amounts of nitrite were of highly questionable character. The bacteriological test for colifonn organisms provides circumstantial evidence of much greater reliability concerning the hygienic safety of water, and it has eliminated the need for extended nitrogen analysis in most water supplies.

In 1940 it was found that drinking waters with high nitrate content often caused methemoglobinemia in infants. From extended investigations in Iowa, Minnesota,

Figure 25.2

Surface waters; health hazard in relation to age of pollution.

Figure 25.2

Surface waters; health hazard in relation to age of pollution.

chapter 25 Nitrogen

0 so

Time, days

Figure 25.3

Changes occurring in forms of nitrogen present in polluted water under aerobic conditions.

0 so

Time, days

Figure 25.3

Changes occurring in forms of nitrogen present in polluted water under aerobic conditions.

and Ohio, where the problem has been most acute, it has been concluded that the nitrate content should be limited.2 For this reason, the U.S. EPA has set a maximum contaminant level requiring that the nitrate-nitrogen concentration not exceed 10 mg/L and the nitrite-nitrogen concentration not exceed 1 mg/L in public water supplies.

Methemoglobinemia is actually a result of interaction of nitrite with hemoglobin, the nitrite being formed from nitrate reduction in the digestive system. For this reason, a maximum contaminant level for drinking water has now been set for nitrite as well as nitrate. Nitrite can also interact with amines chemically (especially when chlorinating for disinfection3) or enzymatically4 to form nitrosamines, which are strong carcinogens. The formation of N-nitrosodimethylamine (NDMA) by these processes has been found to result during wastewater treatment and has become an issue recently in wastewater reuse projects and contaminated groundwater supplies. Ammonia reacts with chlorine to form chloramines, which are slower-acting disinfectants than free chlorine as discussed in Sec. 20.2. Ammonia is sometimes added

JK. F. Maxcy, Report on Relation of Nitrate Nitrogen Concentration in Well Waters to the Occurrence of Methemoglobinemia in Infants. Natl. Acad. Sci.-Research Council Sanit. Eng. and Environment Bull., 264, 1950.

3J. Choi and R. L. Valentine, "Formation of JV-nitrosodimethylamine (NDMA) from reaction of monochloramine: a new disinfection by-product," Water Research, 36: 817-824 (2002). "M, Alexander, "Biodégradation and Bioremediation," Academic Press, San Diego, 1994.

to drinking water supplies when a disinfection residual in water mains is desired as chloramines do not decompose as rapidly as chlorine.

Nutritional and Related Problems All biological processes employed for wastewater treatment are dependent upon reproduction of the organisms employed, as discussed in Sec. 6.6. In planning waste treatment facilities it becomes important to know whether the waste contains sufficient nitrogen for the organisms. If not, any deficiency must be supplied from outside sources. Determinations of ammonia and organic nitrogen are normally made to obtain such data.

Nitrogen is one of the fertilizing elements essential to the growth of algae. Such growth is often stimulated to an undesirable extent in bodies of water that receive either treated or untreated effluents, because of the nitrogen and other fertilizing matter contributed by them. Nitrogen analyses are an important means of gaining information on this problem.

Oxidation in Rivers and Estuaries The autotrophic conversion of ammonia to nitrite and nitrate requires oxygen, indicated in Eqs. (25.9) and (25.10), and so the discharge of ammonia nitrogen and its subsequent oxidation can seriously reduce the dissolved-oxygen levels in rivers and estuaries, especially where long residence times required for the growth of the slow-growing nitrifying bacteria are available. Also, these organisms are produced in large numbers by highly efficient aerobic biological waste treatment systems, and their discharge with the treated effluent can cause rapid nitrification to occur in waterways. Disinfection of effluents (e.g., with chlorine or ultraviolet light) minimized this problem. Nitrogen analyses are important in assessing the possible significance of the problem, and in the operation of treatment processes designed to reduce ammonia discharge.

Control of Biological Treatment Processes Determinations of nitrogen are often made to control the degree of purification produced in biological treatment. With the use of the BOD test, it has been learned that effective stabilization of organic matter can be accomplished without carrying the oxidation into the nitrification stage. This results in reducing time of treatment and air requirements where ammonia removal is not otherwise mandated.

Advantage is taken of denitrification for removing nitrogen from wastes where this is required to prevent undesirable growths of algae and other aquatic plants in receiving waters. Ammonia and organic nitrogen are first biologically converted to nitrite and nitrate by aerobic treatment. The waste is then placed under anoxic conditions, where denitrification converts the nitrite and nitrate to nitrogen gas, which escapes to the atmosphere. For denitrification to occur, organic matter, which is oxidized for energy white the nitrogen is being reduced, must be present. Methanol was a favorite form of organic matter, but, due to its increased cost, other materials including untreated wastewater itself are commonly used. When nitrification is required to protect oxygen resources in streams, advantage can be taken of the oxidizing potential of nitrate present in the plant effluent to reduce oxygen requirements for treatment. Here the nitrate-containing effluent is recycled back to be mixed with settled wastewater under anoxic conditions to effect some organic oxidation through denitrifica-

tion. The reduced quantity of organics remaining are then subject to the usual aerobic step, but now less oxygen is required for treatment. An additional advantage here is that nitrate is simultaneously removed in the denitrification step, which now occurs as a first step, rather than as the last step in the treatment train. Hie formation of N2 by denitrification is sometimes a problem in the activated sludge process of wastewater treatment. Prolonged detention of activated sludge in final settling tanks allows formation of sufficient nitrogen gas to buoy the sludge, if nitrates are present in adequate amounts. This is often referred to as the "rising" sludge problem. As this discussion suggests, analyses for nitrogen forms in wastewater treatment can be of great importance in process control and in achieving overall treatment objectives.

In some states, ammonia-nitrogen limitations have been imposed because of suspected toxic effects upon fish life. It is well-known that un-ionized ammonia is toxic but that the ammonium ion is not. Since the relationship between the two is pH-dependent,

a discussion is in order. Figure 25.4 shows the relationship between free ammonia and ammonium ion that exists for several concentrations of ammonia nitrogen over the pH range of interest in most natural waters. Free ammonia in concentrations above about 0.2 mg/L can cause fatalities in several species of fish. . Applying the usual safety factor, a National Research Council Committee has recommended that no more than 0.02 mg/L free ammonia be permitted in receiving waters.5 From this

Mandalas Ciles

Figure 25.4

The effect of pH and ammonia nitrogen concentration (NHj + NH|) on the concentration of free ammonia in water, pH

Figure 25.4

The effect of pH and ammonia nitrogen concentration (NHj + NH|) on the concentration of free ammonia in water,

'Committee on Water Quality Criteria, "Water Quality Criteria, 1972," Superintendent of Documents, Washington, DC, 1972.

and the data presented in Fig. 25.4, it appears that ammonia toxicity will not be a problem in receiving waters with pH below 8 and ammonia-nitrogen concentrations less than about 1 mg/L.

Control of ammonia for the given reasons is generally accomplished by nitrification. In some cases, limitations are placed upon the amount of total nitrogen that can be present in an effluent. This condition can be met by removal of ammonia in some instances, but more often it requires nitrification and denitrification. Because of these new requirements, methods of measuring all forms of nitrogen have become important.

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