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"Assuming secondary treatment with 90% C-BOD removal and assimilation of 5 mg/L NH4-N. The remaining TKN is either oxidized to nitrate (nitrified) or remains unoxidized (unnitrified). Total Kjeldahl nitrogen (TKN) consists of organic-N (usually quickly ammonified) + ammonium-N. Based on 4.33 mg O2 demand/mg NHz(+-N oxidized.

"Assuming secondary treatment with 90% C-BOD removal and assimilation of 5 mg/L NH4-N. The remaining TKN is either oxidized to nitrate (nitrified) or remains unoxidized (unnitrified). Total Kjeldahl nitrogen (TKN) consists of organic-N (usually quickly ammonified) + ammonium-N. Based on 4.33 mg O2 demand/mg NHz(+-N oxidized.

Originally, biological sewage treatment plants produced nitrified effluents, since it was recognized that the presence of nitrate indicated thorough stabilization of the wastewater. However, once the BOD test became available, plants realized that they could reduce the energy costs associated with supplying oxygen (a major portion of the overall cost of treatment) by providing only carbonaceous removal. Thus, most U.S., plants for many years were designed and operated so as not to nitrify.

However, this meant that ammonium was being discharged to the receiving water, where it might also undergo nitrification and exert an N-BOD. Two classic papers in the 1960s, one examining the Thames estuary in London and the other the Grand River in Michigan, showed that this did in fact occur in some waters, seriously depleting DO. However, in some other receiving waters there was little evidence of nitrification. It now appears that nitrification can be expected in smaller rivers and streams, where the nitrifiers grow mainly attached to streambed surfaces (including vegetation), and in estuaries, where planktonic nitrifiers have time to grow. In larger rivers, where streambed surfaces are relatively small compared to the water volume, nitrifiers appear to settle out and to grow too slowly to oxidize most of the ammonia.

Ammonium may also enter streams from agricultural runoff, stormwater from suburban and urban areas (including erosion from fertilized lawns), and from natural organic inputs, such as falling leaves and bird droppings. Of course, this nitrogen also can represent an oxygen demand.

In denitrification, nitrate is used as an alternate electron acceptor to oxygen. Thus in systems in which carbon oxidation and nitrification take place, denitrification can result in "recovery" of some of the oxygen utilized for nitrification. For example, in equation (13.10), 1 mol of nitrate is "used" to oxidize 1.25 mol of carbohydrate. This nitrate required 2 mol of oxygen when it was produced from ammonia [equation (13.15)]. If the same amount of carbohydrate were oxidized by oxygen, 1.25 mol of O2 would be required. Thus, using the nitrate saves using 1.25 mol of O2, essentially "recovering" 62.5% (100 x 1.25/2) of the oxygen that was used to form the nitrate. Some advanced wastewater treatment plants that nitrify take advantage of this to reduce aeration costs while removing nitrogen as N2.

Toxicity of Nitrogen Compounds If neither the treatment plant nor the receiving water nitrifies, the ammonia from discharges and runoff will be present in the stream. This can be a serious problem in that nonionized ammonia can be extremely toxic to aquatic organisms. It is recommended that nonionized ammonia concentrations in freshwater remain below 0.02 mg/L (20 ppb) to protect the most sensitive water uses (such as trout production), or below 0.04 mg/L in most other cases. (These values are an exception to the general reporting convention for nitrogen and are based on NH3 rather than NH3-N.) The amount of nonionized ammonia is strongly dependent on pH (Section 13.2), but at the slightly alkaline pH values of some streams, these levels can be exceeded. Thus, whether the stream nitrifies or not, there may be good arguments to require nitrification within the treatment plant, and it is becoming more common again.

Example 13.6 In Example 13.4, a water sample had 3.0 mg/L total ammonia- + ammonium-N. If that sample was taken from a stream below a wastewater treatment plant, does the stream meet an nonionized NH3 standard of 0.04 mg/L?

Answer The nonionized NH3 concentration calculated for that example was 0.047 mg/L NH3-N. Thus, the standard is not being met.

Ammonia is also toxic to microorganisms, including nitrifiers, and at high levels can interfere with treatment. As long as the nitrifiers are active, feed concentrations to a reactor can be high, but a sudden increase in concentration or pH can lead to failure. The increased concentration of nonionized ammonia slows down nitrifier activity, decreasing ammonia oxidation rates and leading to a rapid buildup in concentration and further increased toxicity. Typically, well-designed and well-operated reactors can handle up to 500 mg/L (or even higher) influent total ammonia concentrations, depending on pH, because the concentration within the reactor will remain well below 100 mg/L.

Nitrite is also highly toxic to aquatic organisms. U.S. recommendations at one time were to maintain concentrations of nitrite-N below 0.06 mg/L, and some European countries have suggested a 0.02 mg/L limit. Fortunately, nitrite rarely accumulates in the environment, since it is an intermediate for both nitrification and denitrification. Apparently for this reason, there is no longer a U.S. nitrite criterion.

Both nitrite and nitrate may be toxic to humans, and drinking water standards have been set at 1 mg/L for nitrite-N and 10 mg/L for nitrate-N. Upon ingestion, nitrate may be reduced to nitrite in the digestive tract. Once absorbed into the bloodstream, nitrite binds (preferentially over oxygen) with hemoglobin, leading to a mild to severe asphyxia known as methemoglobinemia. This is of particular concern for bottle-fed infants, in whom it is called ''blue baby disease'', as it can be fatal. Nitrite can also react with certain amines released during degradation of amino acids, forming nitrosamines, some of which are potent carcinogens.

Nutrient Enrichment Usually, either nitrogen or phosphorus is the nutrient that limits the amount of photosynthetic growth that can occur in a water body. Typically, while phosphorus is limiting in lakes and reservoirs, nitrogen is the limiting nutrient in streams and coastal waters. Since a variety of organisms can utilize each form, addition of nitrogen as either ammonium or nitrate where N is limiting can lead to excessive growth of plants, algae, and/or cyanobacteria, speeding the process of eutrophication (Section 15.2.6). This is a major reason that some wastewater treatment plants are required to practice some form of advanced treatment to remove nitrogen, not merely to nitrify it.

Alkalinity Consumption As can be seen from equation (13.13), each mole of ammonium oxidized to nitrite produces 2 mol of acidity. This uses up 2 mol of the alkalinity (acid-neutralizing, or pH-buffering, capacity) of the water in which it occurs. If the water does not initially have enough alkalinity, this will result in a drop in pH. This can, in turn, inhibit nitrification as the pH drops as low as 5.5, as well as resulting in other harmful effects in wastewater treatment or perhaps soil. (Ammonia concentrations are usually diluted enough that this level of acidification of a stream will not occur.) To prevent this, nitrifying treatment plants may have to add supplemental alkalinity to raise the pH to more desirable or permitted levels (usually, > 6.5). Acidity produced by nitrification can also be one of the reasons for adding lime (usually, it is really crushed limestone, CaCO3) to agricultural fields or fertilized lawns and gardens.

Example 13.7 In a wastewater treatment plant, 20 mg/L of ammonium-nitrogen is nitrified to nitrate. How much alkalinity is consumed?

Answer From equation (13.13), 2 mol of alkalinity is needed for every mole of nitrogen oxidized. By convention, alkalinity is expressed as CaCO3 equivalents, with

2 mol of acid neutralized per mole (100 g). Thus 1 mol of CaCO3 is needed for every mole of ammonium:

Alkalinity consumed

&l 4 V 14 g NH4+-N y V1 mol NH4+-N7 \1 mol CaCO3 J

= 20(7.14 mg CaCO3/mg N) = 143 mg/L CaCO3 alkalinity

Denitrification produces 1 mol of alkalinity for each mole of nitrite or nitrate reduced to nitrogen gas [e.g., equations (13.10) and (13.11)]. Thus half of the alkalinity lost through nitrification can be recovered by denitrification while removing the nitrogen (as a gas) from the system. This will often eliminate the need for alkalinity addition in wastewater treatment, but would usually be undesirable in agriculture, where the nitrogen loss is typically a concern.

Disinfection Nitrite and chlorine rapidly react to form nitrate and chloride, which has no disinfecting power. For example, in drinking water, where hypochlorous acid is the predominant active chlorine species;

Thus, every milligram of NO2~-N uses up 71/14 = 5.1 mg of active chlorine (which is always expressed on the basis of Cl2, with molecular weight of 71).

Ammonia also reacts with chlorine, but the products depend on the relative concentrations of the two. At low Cl/N molar ratios (< 1), mainly monochloramine (NH2Cl), forms:

As the Cl/N ratio increases, dichloramine (NHCl2) forms:

Mono- and dichloramine are referred to as combined chlorine. Although they still represent active chlorine, they have much less disinfecting power than HOCl (~1 to 2% as effective). This is one reason that disinfection of wastewater (which usually contains ammonia) is less effective than disinfection of drinking water (which typically does not).

Finally, at still higher ratios, the chlorine oxidizes the ammonia to produce nitrogen gas or nitrate, or occasionally, trichloramine (NCl3), depending on pH and the specific ratio.

This is referred to as breakpoint chlorination (Figure 13.23), and these products have no disinfecting power. Thus, depending on the Cl/N ratio, adding more chlorine can actually result in a decrease in the remaining active chlorine, or chlorine residual. Once all of the ammonia is converted to nitrogen gas, nitrate, or trichloramine, any additional chlorine addition results in an increase in the chlorine residual.

Breakpoint Chlorination
Chlorine Dose Figure 13.23 Breakpoint chlorination.

Other In the soil, ammonium is often held at cation-exchange sites of clay minerals. Nitrification mobilizes this nitrogen, making it easier for plants to absorb if the ammonium in their rhizosphere has been depleted, but also easier for it to leach into the ground-water. On the other hand, assimilation of ammonium or nitrate by bacteria can make this nitrogen immobilized and unavailable to plants. Thus, if waste materials with a high carbon/nitrogen ratio, (such as some crop residues and some solid wastes) are applied to soil, the microbial activity on the carbonaceous materials can lead to a depletion of available soil N for plants. This is referred to as nitrogen robbing.

Some potable waters naturally contain ammonia, and others may add it intentionally. This results in the formation of chloramines, which although weaker disinfectants, are less likely to react with organic compounds in the water to produce trihalomethanes and other potential carcinogens. However, nitrifiers have been found growing as biofilms attached to the walls of the water distribution pipes in some of these systems, where their relative resistance to chlorination aids in their survival. Concerns arising from these observations include the production of nitrite (which inactivates chlorine), organic material (promoting growth of heterotrophs), and acidity (potentially contributing to corrosion, which might further increase concentrations of lead in the water).

During anaerobic digestion of sewage sludges, concentrations of ammonium (from ammonification), magnesium, and phosphate will sometimes become high enough to lead to the precipitation of magnesium ammonium phosphate, or struvite (MgNH4PO4). Heavy deposition of this mineral can clog pipes, coat heat exchangers, and otherwise interfere with digester operation.

Because of the limited metabolic diversity of nitrifiers, some compounds act as selective inhibitors, limiting nitrification while having little or no effect on other organisms. Allyl thiourea was one compound used early for this purpose in both agriculture (to prevent loss of nitrogen through nitrification followed by leaching or denitrification) and the BOD test (to prevent interference in the measurement of the C-BOD, Section 13.1.3), but it biodegrades fairly rapidly. Other compounds have also been used, most containing sulfur and/or nitrogen. The compound now recommended for the C-BOD test is 2-chloro-6-(trichloromethyl) pyridine, also called N-serve, nitrapyrin, and TCMP. For research purposes, nitrite oxidation has been inhibited using potassium perchlorate, KClO3, but this compound may also inhibit nitrate reduction. It is also possible that a compound entering a wastewater treatment plant may interfere with nitrification without having a noticeable effect on other treatment processes.

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