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Biological Nitrogen Removal

The Nitrogen Cycle and the Technical Removal Process

The relationship between the various nitrogen compounds and their transformation is presented in Fig. 10.2 as the nitrogen cycle. The transformation reactions include fixation, ammonifi cation, assimilation, nitrification and denitrification. The principle compounds in the nitrogen cycle are nitrogen gas, ammonium, organic nitrogen and nitrate (De Renzo 1978).

The atmosphere serves as a reservoir of N2 gas which is naturally transformed by electrical discharge (lightning) and by nitrogen-fixing organisms. Moreover, N2 gas is fixed by a technical manufacturing process known as the Haber-Bosch synthesis process since 1915. Industrial fixation was initially developed for the production of fertilizers and explosives:

C6H5CH31) + 3 HNO3 ^ C6H2CH3(NO2)32) + 3 H2O (10.3)

In the fixed state, nitrogen can continue through various reactions. Nitrogen gas is returned to the atmosphere by an explosive reaction of a mixture from NaNO3 and Ca(NO3)2 with NH4Cl to N2 gas (Foerst 1965). Nitrogen gas is also formed by the biological reduction through denitrification. The nitrogen cycle is applicable to surface water and the soil/groundwater environment, where many transforming reactions can occur. Nitrogen can be added by precipitation and dustfall, surface runoff, artificial fertilizers and the direct discharge of wastewater (Fig. 10.2).

Domestic wastewater contains organic nitrogen compounds and ammonium. These originate from protein metabolism in the human body. In fresh domestic wastewater, approximately 60% of the nitrogen is in the organic and 40% is in the inorganic form, such as NH+. The organic compounds include amino acids, proteins, ADP/ATP and urea as the basic organic sources of nitrogen and phosphorus.

Biological nitrification and denitrification together make up the most useful processes to remove nitrogen. During nitrification, ammonium is first oxidized to nitrite or nitrate by aerobic chemolitho-autotrophic bacteria. Nitrite and nitrate are then reduced to N2 gas in the denitrification process by chemoorgano-heterotrophic denitrifying bacteria under anoxic conditions. Nitrification and denitrification occur inside living bacteria in nature and in biological wastewater treatment processes.

In Sections 10.2.2 to 10.2.5 we discuss the microbiology, basic reactions, kinetics and performance of biological nitrogen removal processes by nitrification and denitrification.

1) Toluene, 2) TNT = 2,4,6-Trinitrotoluene

Organic Nitrogen Wastewater

Fig. 10.2 Principal compounds in the nitrogen cycle are nitrogen gas, ammonium, organic nitrogen and nitrate.

Fig. 10.2 Principal compounds in the nitrogen cycle are nitrogen gas, ammonium, organic nitrogen and nitrate.

Nitrification

10.2.2.1 Nitrifying Bacteria and Stoichiometry

The autotrophic bacteria oxidize inorganic nitrogen components to obtain energy for growth and maintenance, while they obtain carbon for cell building by the reduction of CO2. The principal genera in the activated sludge process, Nitrosomonas and Nitrobacter, are responsible for the oxidation of ammonium to nitrite (nitritifi-cation) and of nitrite to nitrate (nitratification), respectively.

Basic physiological and structural characteristics of Nitrosomonas and Nitrobacter are presented in Table 10.2.

The stoichiometry for catabolism of NH4 and NO2 oxidation are:

with AG0 = -240 ... -350 kJ mol-1 for Nitrosomonas and AG0 = -65 ... -90 kJ mol-1 for Nitrobacter (Halling-S0rensen and J0rgensen 1993; Wiesmann and Libra 1999).

Table 10.2 Basic comparison between nitrifying and denitrifying bacteria (Gerardi and Michael 2002; Halling-Sorensen and Jorgensen 1993).

Indication Nitrifiers Denitrifiers

Nitrosomonas Nitrobacter

Carbon source Cell shape Cell size O2 requirement pH range tG

Growth range of temperature

Inorganic (CO2) Coccus (spherical) 1.0 • 1.5 ^m Strictly aerobic 5.8-8.5 8-36 h 5-30°C

Inorganic (CO2)

Bacillus (rod-shaped)

Strictly aerobic

12-60 h

Organic carbon

Facultative aerobic 6.5-8.5 0.25-0.5 h

The overall oxidation of ammonium by both groups is obtained by adding Eqs. (10.4) and (10.5):

in which a large amount of oxygen is needed and the pH decreases in water with low buffer capacity if no pH control is performed.

Compared to the catabolism of nitrification, anabolism has more complex stoichiometric reactions. Due to the use of carbon dioxide as the carbon source, a much lower growth rate of nitrifying biomass results and it is difficult to study cellbuilding compared to aerobic heterotroph growth, especially if the cell-building of both nitrifying genera must be determined separately. Therefore, there are significant deviations among equations describing the metabolism of nitritification and nitratification (Sherrad 1977; Dombrowski 1991; US EPA 1993; Grady et al. 1999; Henze et al. 2002).

The stoichiometric reactions for the anabolism of NH+ and NO- oxidation are as follows, assuming that the empirical formulation of bacterial cells is C5H7O2N (Halling-S0rensen and J0rgensen 1993; Henze et al. 2002):

bacteria

10NO- + 5CO2 + NH+ + 2H2O ^ 10NO- + C5H7O2N + H+ (10.8)

bacteria

When compared to the catabolism of NH+, less energy is available for the growth of Nitrobacter in comparison to Nitrosomonas (see Eqs. 10.4 and 10.5). Both anabolic reactions usually take place at 5.5 <pH <8.3; therefore, Eq. (10.9) must be considered (see also Section 4.3):

The stoichiometric reactions of NH+ and NO- oxidation for catabolism and anabo-lism applied to 1 mol NH+ and NO- are given by Eq. (10.10) and Eq. (10.11) respectively (Wiesmann and Libra 1999):

0.0182C5H7O2N + 0.98NO- + 1.04H2O + 1.89H2CO3 ( . )

NO- + 0.02H2CO3 + 0.48O2 + 0.005NH+ + 0.005HTO- ^ ^^

Ammonium and nitrite are used as energy sources and CO2 as a carbon source for nitrifying bacteria. Ammonium is oxidized to nitrite over three steps and the oxidation of nitrite to nitrate is a single step (Eq. 10.12). The intermediate between hy-droxylamine and nitrite is not known (Henze et al. 2002).

It is assumed that, for every reaction step, almost the same amount of energy is produced. The energy produced by the oxidation from ammonium to nitrite is a factor of about 3.0-3.8 greater than that of the transformation from nitrite to nitrate (see Eqs. 10.4 and 10.5). Based on this fact, the biomass yield coefficient of YXa/NH4 or YXa/NO2 has to correspond to this relation (see Eqs. 10.18 and 10.19). Many authors have measured the growth of Nitrosomonas and Nitrobacter and described its stoichiometry, but the values are very different. The growth of new cells in the activated sludge process is referred to as an increase in the mixed liquor volatile suspended solids (MLVSS). Nitrifying bacteria obtain a relatively small amount of energy from the oxidation of ammonium and nitrite, resulting in long generation times and a small population MLVSS.

The specific growth rate of the nitrifying bacteria in activated sludge is much lower than that of aerobic organo-heterotrophs. Nitrifiers' poor ability to form flocs and the risk of being washed out of the system with their low growth rate can be overcome by the likelihood that they are adsorbed onto the surface of other floc particles. This characteristic is normally used for nitrification in biofilm reactors (see Chapter 7). Using membrane bioreactors (see Chapter 12) for nitrification is also very beneficial. The growth of nitrifying bacteria is affected by a number of environmental parameters such as dissolved oxygen concentration c', pH and the presence of inhibitors (see Section 10.2.2.3).

In order to determine the rate of NH4 oxidation in a CSTR, assuming steady state, the following expressions are used:

0 0

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