The Effect of Nitrogen Discharge

The effects of nitrogen discharge will be mentioned briefly in this section to be able to relate the methods of nitrogen removal with the expected effects of their application.

The four major effects are:

1. Fertilization (eutrophication) of aquatic ecosystems

2. Oxygen depletion in aquatic ecosystems.

3. Toxicity to aquatic life.

4. Contamination of ground water by nitrate and its effect on the public health.

The word eutrophic generally means "nutrient rich." Naumann introduced in 1919 the concepts of oligotrophy and eutrophy. He distinguished between oligotrophic lakes containing little planktonic algae and eutrophic lakes containing much phytoplankton.

The eutrophication of lakes in Europe and North America has grown rapidly during the last few decades due to the increased urbanization and the increased discharge of nutrients per capita.

The production of fertilizers has grown exponentially in this century as demonstrated in Fig. 1.5, and the concentration of nutrients in many lakes reflects the same exponential growth, (Ambtihl, 1969).

The word eutrophication is used increasingly in the sense of the artificial addition of nutrients, mainly nitrogen and phosphorus, to water. Eutrophication is generally considered to be undesirable, although it is not always so.

The green color of eutrophic lakes makes swimming and boating less safe due to increased turbidity. Furthermore, from an aesthetic point of view the chlorophyll concentration should not exceed 100 mg m"3. However, the most critical effect from an ecological viewpoint is the reduced oxygen content of the hypolimnion, caused by the decomposition of dead algae. Eutrophic lakes might show high oxygen concentrations at the surface during the summer, but low oxygen concentrations in the hypolimnion, which may cause fishkill.

On the other hand an increased nutrient concentration may be profitable for shallow ponds used for commercial fishing, as the algae directly or indirectly form food for the fish population.

Figure 1.5.The production of fertilizers (t yr"1), as demonstrated for N and P205, has grown exponentially (the y-axis is logarithmic).

About 16-20 elements are necessary for the growth of freshwater plants, as shown in Table 1.3, where the relative quantities of essential elements in plant tissue are shown.

The present concern about eutrophication relates to the rapidly increasing amounts of phosphorus and nitrogen, which are normally present at relatively low concentrations. Of these two elements phosphorus is often considered the major cause of eutrophication, as it was formerly the growth-limiting factor for algae in the majority of lakes but, as demonstrated in Fig. 1.5, its usage has greatly increased during the last decades. Nitrogen is a limiting factor in a number of East African lakes as a result of the nitrogen depletion of soils by intensive erosion in the past. Nitrogen may, however, become limiting to growth in lakes and in coastal zones as a result of the tremendous increase in the phosphorus concentration caused by discharge of waste water, which contains relatively more phosphorus than nitrogen. While algae use 4-10 times more nitrogen than phosphorus, waste water generally contains only 3 times as much nitrogen as phosphorus.

Table 1.3.

Average fresh-water plant composition on wet basis

Element Plant content (percentage)

Oxygen

80.5

Hydrogen

9.7

Carbon

6.5

Silicon

1.3

Nitrogen

0.7

Calcium

0.4

Potassium

0.3

Phosphorus

0.08

Magnesium

0.07

Sulfur

0.06

Chlorine

0.06

Sodium

0.04

Iron

0.02

Boron

0.001

Manganese

0.0007

Zinc

0.0003

Copper

0.0001

Molybdenum 0.00005

Cobalt

0.000002

Nitrogen accumulates in lakes to a lesser extent than phosphorus and a considerable amount of nitrogen is lost by denitrification (nitrate to gaseous N2).

The growth of phytoplankton is the key process in eutrophication and it is therefore of great importance to understand the interacting processes regulating its growth.

Primary production has been measured in great detail in many large lakes. This process represents the synthesis of organic matter, and can be summarized as follows:

This equation is necessarily a simplification of the complex metabolic pathway of photosynthesis, which is dependent on sunlight, temperature and the concentration of nutrients. The composition of phytoplankton is not constant (note that Table 1.5 only gives an average concentration), but reflects to a certain extent the chemical composition of the water. If, for example, the phosphorus concentration is high, the phytoplankton will take up relatively more phosphorus -termed the luxury uptake.

The sequence of events leading to eutrophication often occurs as follows. Oligotrophic waters often have a N:P ratio of more than or equal to 10, which means that phosphorus is less abundant relative to the needs of phytoplankton than nitrogen. If sewage is discharged into the lake the ratio will decrease since, the N:P ratio for municipal waste water is about 3:1. Consequently, nitrogen will be less abundant than phosphorus relative to the needs of phytoplankton. Municipal waste water contains typically 30 mg I"1 N and 10 mg I"1 P. In this situation, however, the best remedy for the excessive algal growth is not necessarily to remove nitrogen from the sewage, because the mass balance might show that nitrogen-fixing algae would produce an uncontrollable input of nitrogen into the lake.

It is necessary to set up a mass balance for the nutrients. This will often reveal that the input of nitrogen from nitrogen-fixing blue green algae, dry and wet deposition and tributaries is already contributing too much to the mass balance for any effect to be produced by nitrogen removal from the sewage. On the other hand the mass balance may reveal that most of the phosphorus input (often more than 95%) comes from the sewage, and so demonstrates that it is better management to remove phosphorus from the sewage rather than nitrogen. It is, therefore not a matter of which nutrient is limiting, but which nutrient can most easily be made to limit the algal growth.

These considerations have implied that the eutrophication process can be controlled by a reduction in the nutrient budget. For this purpose a number of eutrophication models have been developed, which take a number of processes into account. For details, see Jorgensen (1976), Jorgensen et al., (1978), Jorgensen et al., (1986) and Jorgensen (1988).

Generally however, it is possible to conclude that reduction of the eutrophication in aquatic ecosystems requires a solution which is tailored to the particular case. Some will require reduction in the phosphorus inputs, some in the nitrogen inputs and some will require reductions in inputs of both nutrients. Nitrogen reductions seem to be most important for the eutrophication control in lakes and marine environment during the summer time, while spring run-off often transfers large amounts of nitrogen to the aquatic environment, making it difficult to control nitrogen as the limiting factor.

Maintenance of a high oxygen concentration in aquatic ecosystems is crucial for survival of the higher life forms in aquatic ecosystems. At least 5 mg /1 is needed for many fish species. At 20-21 °C this corresponds to 5/9 = 56% saturation. The oxygen concentration is influenced by several factors, of which the most important are the decomposition of organic matter, and the nitrification of ammonia (ammonium) according to the following process:

Ammonia is formed by decomposition of organic matter. Proteins and other nitrogenous organic matter are decomposed to simpler organic molecules such as amino acids, which again are decomposed to ammonia. Urea and uric acid, the waste products from animals, are also broken down to ammonia. Nitrifying microorganisms can use ammonia as an energy source, as the oxidation of ammonia is an energy-producing process. This decomposition chain is illustrated in Fig. 1.6, where it can be seen that the free energy (chemical energy) is decreased throughout the chain.

The nitrification process can be described by the following first order kinetic expression:

dN dt

where

Nt = concentration of ammonium at time = t No = concentration of ammonium at time = 0 Kn = rate constant, nitrification a>

c LU

Proteins

Amino acids ) \

*

f

/ A

Urea,uric acid )

Nitrite

Nitrate

Figure 1.6. Decomposition chain: from protein to nitrate.

Nt and No may here be expressed by the oxygen consumption corresponding to the ammonium concentration. Values for KN and No are given for some characteristic cases in Table 1.4. Kh is dependent on the temperature as illustrated in the following expression:

where T = the temperature (°C), KT = a constant in the interval 1.06-1.08.

Characteristic values, Kn, and No (20 °C)

Kn (1 /24h)

No

Municipal waste water

0.15-0.25

80-130

Mechanical-treated muni

cipal waste water

0.10-0.25

70-120

Biological-treated muni

cipal waste water

0.05-0.20

60-120

Potable water

0.05

0-1

River water

0.05-0.10

0-2

The relation between ammonium concentration and oxygen consumption according to (1.2) may be calculated as (2 * 32)/14 = 4.6 mg 02 per mg NH4+ - N, but due to bacterial assimilation of ammonia this ratio is reduced to 4.3 mg 02 per mg NH4+- N in practice.

It is easy to see from the values of ammonium nitrogen or total nitrogen in municipal waste water that the oxygen consumption for nitrification is significant. If a total nitrogen concentration of 28 mg N /1 is presumed, the oxygen consumption for nitrification becomes 128 mg /1 , which may be compared with the BOD5 of municipal waste water on about 200 - 250 mg / I . The growth of nitrifying microorganisms is, however, relatively slow, which implies that the nitrification is not completed in aquatic ecosystems with short retention times. Ecological models (Jorgensen, 1988 and Jorgensen and Johnsen, 1989) can be used to characterize the role of the oxygen depletion caused by nitrification and therefore the consequences for the aquatic life of nitrifying ammonium in waste water before discharge. The conclusion will, however, generally be that nitrification of municipal waste water is required for all discharge into inland water ecosystems. Many industrialized countries have therefore introduced an effluent standard for ammonium and organic nitrogen concentrations.

While nutrients are necessary for plant growth, they may produce a deterioration in life conditions for other forms of life. Ammonia is extremely toxic to fish, while ammonium, the ionized form is harmless. As the relation between ammonium and ammonia is dependent on pH: (see also Section 7.1)

where pK = -log K and K = equilibrium constant for process (1.7).

The pH value as well as the total concentration of ammonium and ammonia is thus important. This is demonstrated in Table 1.5. This implies that the situation is very critical in many hypereutrophic lakes during the summer, when photosynthesis is most pronounced, as the pH increases when the acidic component C02 is removed or reduced by this process. The annual variations of pH in a hypereutrophic lake are shown in Fig. 1.7. pK is about 9.24 - 9.30 in distilled water at 18 - 25° C, but increases with increasing salinity. It implies that the concentrations shown in Table 1.5 are higher in sea water.

It is a clear conclusion from these considerations that ammonium discharge into aquatic ecosystems, in particular inland waters, is not desirable and that municipal waste water therefore must be nitrified before discharge. The pubic health hazard is associated with nitrate in groundwater, which occur due to leaching of nitrate; see Fig. 1.3. Nitrate in drinking water is associated with methemoglobinemia, which affects infants less than three months, because of their lack of an enzyme capable of oxidizing nitrite.

Table 1.5

Concentrations of ammonium nitrogen (ammonium + ammonia), in mg per.l, which contains an unionized ammonia concentration of 0.025 mg

NH3 / I at various pH and temperatures

Table 1.5

Concentrations of ammonium nitrogen (ammonium + ammonia), in mg per.l, which contains an unionized ammonia concentration of 0.025 mg

NH3 / I at various pH and temperatures

°C

pH = 7.0

pH = 7.5

pH = 8.0

pH = 8.5

pH = 9.0

pH = 9.5

5

19.6

6.3

2

0.65

0.22

0.088

10

12.4

4.3

1.37

0.45

0.16

0.068

15

9.4

5.9

0.93

0.31

0.12

0.054

20

6.3

2

0.65

0.22

0.088

0.045

25

4.4

1.43

0.47

0.17

0.069

0.039

30

3.1

1

0.33

0.12

0.056

0.035

Figure 1.7. The seasonal variation in pH in a hypereutrophic lake (Lake Glumso, Denmark).

Month

Figure 1.7. The seasonal variation in pH in a hypereutrophic lake (Lake Glumso, Denmark).

When water with a high concentration of nitrate is used in preparing infant formulas, nitrate is reduced to nitrite in the stomach after ingestion. The nitrites react with hemoglobin in the blood to form methemoglobin, which is incapable of carrying oxygen in comparison to hemoglobin. The result is suffocation accompanied by bluish tinge to the skin, which explains the use of the term "blue babies" in conjunction with methemoglobinemia.

From 1945-1975 about 2000 cases of methemoglobinemia were reported in the U.S. and Europe with a mortality rate of 7-8%. Because of the difficulties in diagnosing the disease and because no reporting is required, the actual incidence may be many times higher (Kaufman, 1974).

WHO and most countries have set up standards for nitrate in drinking water. Typical standards are: U.S. 10 mg nitrate- N /1 and in most European countries 30 -100 mg nitrate/I.

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