Removal of Nitrogen by Nitrification– Denitrification We define a biological reaction as a reaction mediated by organisms. It encompasses both the organisms and the underlying chemical reactions. To fully apply the knowl- edge of biological reactions to the treatment of water and wastewater, the chemical nature of these reactions must be given center stage. In other words, to control the process of removing nitrogen by nitrification–denitrification, the intrinsic chemical reactions must be unraveled and fully understood. The organisms only serve as mediators (i.e., the producer of the enzymes needed for the reaction). Thus, on the most fundamental level, nitrogen removal is a chemical process (more accurately, a biochemical process), and the treatment for removal of nitrogen by nitrification– denitrification as used in this textbook is chemical in nature and the process is a chemical unit process. In fact, nitrification–denitrification removal of nitrogen can be effected by purely enzymatic means by providing the needed enzymes externally without ever using microorganisms. Similar to phosphorus, nitrogen is a very important element that has attracted much attention because of its ability to cause eutrophication in bodies of water. As stated in the chapter on phosphorus removal, the Chesapeake Bay in Maryland and Virginia is fed by tributaries from farmlands as far away as New York. Because of the use of nitrogen in fertilizers for these farms, the bay receives an extraordinarily large amount of nitrogen input that has triggered excessive growths of algae in the water body. Presently, large portions of the bay are eutrophied. This chapter discusses removal of nitrogen using the unit process of nitrification followed by denitrification. Half reactions are utilized in the discussion of the chemical reactions. Whether or not a particular reaction will occur can be determined by the free energy change of the reactants and products. Thus, half reactions are normally tabulated in terms of free energies. To understand the exact meaning of free energy as it relates to half reactions and thus to nitrogen removal, microbial thermodynamics is discussed. Carbon requirements, alkalinity dose requirements, and reaction kinetics as they apply to nitrogen removal are all discussed. A section on whether or not to remove nitrogen is also included. 15.1 NATURAL OCCURRENCE OF NITROGEN The element nitrogen is a nonmetal. It belongs to Group VA in the Periodic Table in the second period. Its electronic configuration is [He]2 s 2 2 p 3 . [He] means that the helium configuration is filled. The valence configuration represented by the 2, 15 TX249_Frame_C15.fm Page 659 Friday, June 14, 2002 4:43 PM © 2003 by A. P. Sincero and G. A. Sincero 660 the L shell, shows five electrons in the orbitals: 2 electrons in the s orbitals and 3 electrons in the p orbitals. This means that, like phosphorus, nitrogen can have a maximum oxidation state of + 5; its smallest oxidation state is 3 − . Examples are nitrous oxide (N 2 O, 1 + ); nitric oxide (NO, 2 + ); dinitrogen trioxide (N 2 O 3 , 3 + ); nitrogen dioxide (NO 2 , 4 + ); dinitrogen tetroxide (N 2 O 4 , 4 + ); and dinitrogen pentox- ide (N 2 O 5 , 5 + ). Our interest in nitrogen as it occurs in nature is in the form that makes it fertilizer to plants. These forms are the nitrites, nitrates , and ammonia . The nitrogen in ammonia exists in its smallest oxidation state of 3 − ; in nitrites, it exists as 3 +, and in nitrates, it exists as 5 + . These nitrogen species are utilized by algae as nutrients for growth. Also, because organic nitrogen hydrolyzes to ammonia, we will, in general, be concerned with this form of the nitrogen species. 15.2 TO REMOVE OR NOT TO REMOVE NITROGEN The formula of algae is (CH 2 O) 106 (NH 3 ) 16 H 3 PO 3 (Sincero and Sincero, 1996). Gleaning from this formula, to curtail its production in any water body such as the Chesapeake Bay, it is necessary to control only any one of the elements of nitrogen, phosphorus, oxygen, hydrogen, or carbon. It must be stressed that only one needs to be controlled, because absence of any element needed for the construction of the algal body prevents the construction of the body. This is analogous to a car. To disable this car, you only need to remove one wheel and you can never drive the car. Of course, oxygen, hydrogen, and carbon should never be controlled, because there are already plenty of them around. From the algae formula, the ratio of N to P is 16 / 1 = 16 mole for mole or 14(16) / 31 = 7.2 mass for mass. Table 15.1 shows various values of nitrogen and phosphorus concentrations in the water column and the corresponding N/P ratios in some coastal areas of Maryland (Sincero, 1987). For those ratios greater than 7.2, phosphorus will run out first before nitrogen does. In these situations, phosphorus should be controlled first and nitrogen should be left alone in the discharge, until further investigation reveals that the ratio has reversed. TABLE 15.1 Nitrogen and Phosphorus Ratios, Maryland Coastal Area Organic N, mg/L NH 3 –N, mg/L NO 2 –N, mg/L NO 3 –N, mg/L Total N, mg/L Total P, mg/L N/P Ratio 0.79 0.01 0.002 0.03 0.83 0.59 1.4 0.63 0.04 0.003 0.01 0.68 0.15 4.6 0.47 0.03 0.002 0.02 0.52 0.09 5.8 2.59 0.01 0.002 0.03 2.63 0.29 9.1 1.99 0.01 0.002 0.02 2.02 0.30 6.7 3.19 0.07 0.002 0.03 3.29 0.18 18.3 1.59 0.01 0.002 0.02 1.62 0.13 12.5 0.49 0.01 0.002 0.02 0.52 0.04 13.1 0.59 0.01 0.002 0.02 0.62 0.11 5.7 1.19 0.01 0.002 0.03 1.23 0.27 4.6 TX249_Frame_C15.fm Page 660 Friday, June 14, 2002 4:43 PM © 2003 by A. P. Sincero and G. A. Sincero 661 For those situations where the ratio is less than 7.2, nitrogen will run out first. In these cases, should nitrogen be controlled? Certain forms of algae, the blue-greens, can synthesize nitrogen from the air into ammonia, which they need for growth (Sincero, 1984). These particular species are very resistant and can survive anywhere where there is a water body. Thus, if this is the case in a particular body of water, it may be a waste of money to remove nitrogen, because the alga could simply get the nitrogen it needs from the air. Phosphorus should be removed, instead. These situations can become very political, however, especially with some environmentalists. Some authorities even claim that it is still advisable to remove both nitrogen and phosphorus (D’Elia, 1977). It is in this situation that modeling of the effect of the discharge of the nutrients nitrogen and phosphorus on the eutrophication potential of the water body should be investigated accurately and in great detail. 15.3 MICROBIAL THERMODYNAMICS The study of the relationships between heat and other forms of energy is called thermodynamics . All living things utilize heat, therefore, the science of thermody- namics may be used to evaluate life processes. An example of a life process is the growth of bacteria when wastewater is fed to them to treat the waste. Knowledge of microbial thermodynamics is therefore important to professionals involved in cleaning up wastewaters. Variables involved in the study of the relationship of heat and energy are called thermodynamic variables . Examples of these variables are temperature, pressure, free energy, enthalpy, entropy, and volume. In our short discussion of thermodynam- ics, we will address enthalpy, entropy, and free energy. As mentioned, whether or not a particular reaction, such as a biological reaction, is possible can be determined by the free energy change between products and reactants. Free energy, in turn, is a function of the enthalpy and entropy of the reactants and products. 15.3.1 E NTHALPY AND E NTROPY Let H represent the enthalpy, U the internal energy, P the pressure, and the volume of a particular system undergoing a process under study. The enthalpy H is defined as (15.1) Internal energy refers to all the energies that are present in the system such as kinetic energies of the molecules, ionization energies of the electrons, bond energies, lattice energies, etc. The system possesses all these energies by virtue of its being and are all integral (that is, internal) with the system. Let us derive the relationship between enthalpy and the heat exchange during a biological reaction, where biological reaction is a chemical reaction mediated by organisms. Biological reactions are carried out at constant pressure; hence, the heat exchange is a heat exchange at constant pressure. Designate this exchange as Q p . The first law of thermodynamics states that any heat added to a system minus any work W that the system is doing at the same time manifests itself in the form of an V – HUP V – += TX249_Frame_C15.fm Page 661 Friday, June 14, 2002 4:43 PM © 2003 by A. P. Sincero and G. A. Sincero 662 increase of the internal energy. In differential form, (15.2) The only work done in biological reactions is the work of pushing the surroundings (the atmosphere) in which the reaction is occurring. This is a pressure-volume work; hence, the term. Because the biological reaction is at constant pressure, differentiate the enthalpy equation at constant pressure. This produces (15.3) This may be combined with Equation (15.2) to eliminate dU producing (15.4) This equation concludes that change in enthalpy is a heat exchange at constant pressure between the system under study and its surroundings. Before we discuss entropy, define reversible process and reversible cycle. A reversible process is a process in which the original state or condition of a system can be recovered back if the process is done in the opposite direction from that in which it is currently being done. To perform a reversible process, the steps must be conducted very, very slowly, in an infinitesimal manner, and without friction. From the definition of a reversible process, the definition of a reversible cycle follows. A reversible cycle is a cycle in which the reversible process is applied in every step around the cycle. Heat added to a system causes its constituent particles to absorb the energy resulting in the system being more chaotic than it was before. If the heat is added reversibly, the ratio of the infinitesimal heat added to the temperature T during the infinitesimal time that the heat is added defines the change in entropy . If this addition is done around a reversible cycle, the state or condition of the system at the end of the cycle will revert back to its original state or condition at the beginning of the cycle. This must be so, since the whole process is being done reversibly in every step along the way around the cycle. Hence, the change in entropy around a reversible cycle is zero. Let S be the entropy and Q rev be the reversible heat added. In a given differential step, the heat added is dQ rev . The differential change in entropy in every differential step is therefore dS = dQ rev / I . Around the cycle, the change in entropy is the integral, thus (15.5) The symbol ͛ means that the integrand is to be integrated around the cycle and subscripts e and b refer to the end and the beginning of the cycle, respectively. If the process is not around a cycle, the previous subscripts simply mean the end and dU dQ p dW– dQ p Pd V –== Pd V dH dU Pd V – += dH dQ p = dS ∫ ° S e S b – ∆S dQ rev T ∫ ° 0=== = TX249_Frame_C15.fm Page 662 Friday, June 14, 2002 4:43 PM © 2003 by A. P. Sincero and G. A. Sincero 663 beginning of the process. In this case, the integral will not be zero and the equation is written as the integral (15.6) (15.7) Interpretations of enthalpy and entropy. The heat absorbed by the system causes more agitation of its constituent elements. This increased agitation and chaos is the entropy increase and is calculated by Eqs. (15.6) and (15.7). The entropy increase is an increase in disorder of the constituents of the system. The energy state of the system is increased, but because the energy supporting this state is nothing more than supporting chaos, this energy is a wasted energy. The equations therefore calculate the loss in energy of the system as a result of increased chaos or disorder. Consider a fuel such as coal, and burn it in a furnace. The burning of the coal occurs under constant atmospheric pressure. As the coal burns, heat is released; this heat is energy Q p , which may be used to produce electricity by using a boiler and a turbine generator. From Equation (15.4), Q p is equal to the enthalpy change ∆ H. We therefore conclude that before the coal was burned, it possessed an enthalpy H which, by virtue of Equation (15.4), is its energy content. By the entropy change during the process of burning, however, all this energy is not utilized as useful energy but is subtracted by the change in entropy. The electrical energy that is ultimately delivered to the consumer is less by an amount equal to the overall entropy change in the transformation of coal to electricity. In biological reactions, the fuel is the food. In biological nitrogen removal, nitrogen in its appropriate form is fed to microorganisms to be utilized as food. This food possesses enthalpy as does coal; and, similar to coal, its energy content cannot all be utilized as useful energy by the microorganism as a result of the inevitable entropy inefficiency that occurs in the process of consuming food. 15.3.2 F REE E NERGY Will a certain food provide energy when utilized by microorganisms? If the answer is yes, then the food will be eaten; and if it is in a wastewater, the wastewater will be cleaned up. The answer to this question can now be quantified by the combination of the concept of enthalpy and entropy. This combination is summed up in a term called free energy . Free energy G is defined as (15.8) Because H is an energy content and S is a wasted energy, G represents the useful energy (or, alternatively, the maximum energy) the fuel can provide after the wasteful Sd b e ∫ S e S b – ∆S Qd rev T b e ∫ === Q rev T = T constatnt= GHTS–= TX249_Frame_C15.fm Page 663 Friday, June 14, 2002 4:43 PM © 2003 by A. P. Sincero and G. A. Sincero energy (represented by S) has been subtracted from the energy content. Thus, the term free energy. Biological processes are carried out at a given constant temperature as well as constant pressure. Thus, differentiating the free-energy equation at constant temper- ature, (15.9) Note: In order for G to be a maximum (i.e., to be a free energy), Q must be the Q rev as depicted in the equation. 15.4 OXIDATION–REDUCTION REACTIONS OF NITROGEN FOODS Life processes involve electron transport. Specifically, the mitochondrion and the chloroplast are the sites of this electron movement in the eucaryotes. In the procary- otes, this function is embedded in the sites of the cytoplasmic membrane. As far as electron movement is concerned, life processes have similarity to a battery cell. In this cell, electrons move because of electrical pressure, the voltage difference. By the same token, electrons move in an organism because of the same electrical pressure, the voltage difference. In a battery cell, one electrode is oxidized while the other is reduced; that is, oxidation–reduction occurs in a battery cell. Exactly the same process occurs in an organism. In an oxidation–reduction reaction, a mole of electrons involved is called the faraday, which is equal to 96,494 coulombs. A mole of electrons is equal to one equivalent of any substance. Therefore, a faraday is equal to one equivalent. Let n be the number of faradays of charge or mole of a substance participating in a reaction, and let the general reaction be represented by the half-cell reaction of Zn as follows: (15.10) The couple Zn/Zn 2+ has an electric pressure between them. Now, take another couple such as Mg/Mg 2+ whose half-cell reaction would be similar to that of Equation (15.10). The couples Zn/Zn 2+ and Mg/Mg 2+ do not possess the same voltage potential. If the two couples are connected together, they form a cell. Their voltage potentials are not the same, so a voltage difference would be developed between their electrodes and electric current would flow. Let the voltage between the electrodes of the above cell be measured by a poten- tiometer. Designate this voltage difference by ∆E. In potentiometric measurements, no electrons are allowed to flow, but only the voltage tendency of the electrons to flow is measured. No electrons are allowed to flow, therefore, no energy is dissipated due to friction of electrons “rubbing along the wire.” Thus, any energy associated with this no-electron-to-flow process represents the maximum energy available. Because voltage is energy in joules per coulomb of charge, the energy associated can be dG dH TdS– dH Q rev –== Zn Zn 2+ 2e − +→ TX249_Frame_C15.fm Page 664 Friday, June 14, 2002 4:43 PM © 2003 by A. P. Sincero and G. A. Sincero calculated from the voltage difference. This associated energy corresponds to a no- friction loss process; it is therefore a maximum energy—the change in free energy— after the entropic loss has been deducted. Let n, the number of faradays involved in a reaction, be multiplied by F, the number of coulombs per faraday. The result, nF, is the number of coulombs involved in the reaction. If nF is multiplied by ∆E, the total associated energy obtained from the previous potentiometric experiment results. Because the voltage measurement was done with no energy loss, by definition, this associated energy represents the free energy change of the cell (i.e., the maximum energy change in the cell). In symbols, (15.11) The sign has been used. A convention used in chemistry is that if the sign is negative, the process is spontaneous and if the sign is positive, the process is not spontaneous. As mentioned, the battery cell process is analogous to the living cell process of the mitochondrion, the chloroplast, and the electron-transport system in the cyto- plasmic membrane of the procaryotes. Thus, Equation (15.11) can represent the basic thermodynamics of a microbial system. In the living cell, organic materials are utilized for both energy (oxidation) and synthesis (reduction). Microorganisms that utilize organic materials for energy are called heterotrophs. Those that utilize inorganics for energy are called autotrophs. Autotrophs utilize CO 2 and for the carbon needed for cell synthesis; the het- erotrophs utilize organic materials for their carbon source. Autotrophs that use inorganic chemicals for energy are called chemotrophs; those that use sunlight are phototrophs. The bacteria that consumes the nitrogen species in the biological removal of nitrogen are chemotrophic autotrophs. Algae are phototrophic autotrophs. Somehow, in life processes, the production of energy from release of electrons does not occur automatically but through a series of steps that produce a high energy- containing compound. This high energy-containing compound is ATP (adenosine triphosphate). Although ATP is not the only high energy-containing compound, it is by far the major one that fuels synthesis in the cell. ATP is the energy currency that the cell relies upon for energy supply. The energy function of ATP is explained as follows: ATP contains two high- energy bonds. To form these bonds, energy must be obtained from an energy source through electron transfer. The energy released is captured and stored in these bonds. On demand, hydrolysis of the bond releases the stored energy which the cell can then use for synthesis and cell maintenance. ATP is produced from ADP (adenosine diphosphate) by coupling the release of electrons to the reaction of organic phosphates and ADP producing ATP. ATP has two modes of production: substrate-level phosphorylation and oxidative phospho- rylation. In the former, the electrons released by the energy source are absorbed by an intermediate product within the system. The electron absorption is accompanied by an energy release and ATP is formed. The electron-transport system is simple. ∆GnF ∆E±= ± HCO 3 − TX249_Frame_C15.fm Page 665 Friday, June 14, 2002 4:43 PM © 2003 by A. P. Sincero and G. A. Sincero Fermentation is an example of a substrate-level phosphorylation process that uses intermediate absorbers such as formaldehyde. Substrate-level phosphorylation is inefficient and produces only a few molecules of ATP. In the oxidative phosphorylation mode, the electron moves from one electron carrier to another in a series of complex reduction and oxidation steps. The difference between substrate-level and oxidative phosphorylation is that in the former, the transport is simple, while in the latter, it is complex. For a hydrogen-containing energy source, the series starts with the initial removal of the hydrogen atom from the molecule of the source. The hydrogen carries with it the electron it shared with the original source molecule, moving this electron through a series of intermediate carriers such as NAD (nicotinamide adenine dinucleotide). The intermediate that receives the electron-carrying hydrogen becomes reduced. The reduction of NAD, for example, produces NADH 2 . The series continues on with further reduction and oxidation steps. The whole line of reduction and oxidation constitutes the electron- transport system. At strategic points of the transport system, ATP is produced from ADP and inorganic phosphates. The other version of oxidative phosphorylation used by autotrophs involves the release of electrons from an inorganic energy source. An example of this is the release of electrons from , oxidizing to , and the release of electrons from , oxidizing to . The transported electrons emerge from the system to reduce a final external electron acceptor. The type of the final acceptor depends upon the environment on which the electron transport is transpiring and may be one of the following: for aerobic environments, the acceptor is O 2 ; for anaerobic environments, the possible acceptors are , , and CO 2 . When the acceptor is , the system is said to be anoxic. The values of free energy changes are normally reported at standard conditions. In biochemistry, in addition to the requirement of unit activity for the concentrations of reactants and products, pressure of one atmosphere, and temperature of 25°C, the hydrogen ion concentration is arbitrarily set at pH 7.0. Following this convention, Equation (15.11) may be written as (15.12) The primes emphasize the fact that the standard condition now requires the {H + } to be 10 −7 moles per liter.The subscript o signifies conditions at standard state. In environmental engineering, it is customary to call the substance oxidized as the electron donor and the substance reduced as the electron acceptor. The electron donor is normally considered as food. In the context of nitrogen removal, the foods are the nitrites, nitrates, and ammonia. Equation (15.10) is an example of an electron donor reaction. Zn is the donor of the electrons portrayed on the right-hand side of the half-cell reaction. On the other hand, the reverse of the equation is an example of an electron acceptor reaction. Zn +2 would be the electron acceptor. McCarty (1975) derived values for free energy changes of half-reactions for various electron donors and acceptors utilized in a bacterial systems. The ones specific for the nitrogen species removal are shown in Table 15.2. NH 4 − NH 4 + NO 2 − NO 2 − NO 2 − NO 3 − NO 3 − SO 4 −2 NO 3 − ∆G′ o nF∆E o ′±= TX249_Frame_C15.fm Page 666 Friday, June 14, 2002 4:43 PM © 2003 by A. P. Sincero and G. A. Sincero 15.4.1 CRITERION FOR SPONTANEOUS PROCESS It is a law of nature that things always go in the direction of creating greater chaos— this is the second law of thermodynamics. Any system, except those at temperature equals absolute zero, is always disordered.* The energy required to maintain this disorder, we have found, is called entropy. As mentioned, any system possesses free energy at any instant, this energy being the net energy remaining after the entropy required to maintain the current disorder has been subtracted from the enthalpy (energy content). When the system goes from state 1 (current state) to state 2, its free energy at the latter state may or may not be the same as the former. If the free energy at state 2 TABLE 15.2 Half-Cell Reactions for Bacterial Systems in Nitrogen Removal kcal/electron–mol Reactions for cell synthesis: Ammonia as nitrogen source: — Nitrate as nitrogen source: — Reactions for electron acceptors: Oxygen as acceptior: −18.68 Nitrate as acceptor: −17.13 Nitrite as acceptor: — Reactions for electron donors: Domestic wastewater as donor (heterotrophic reaction): −7.6 Nitrite as donor: +9.43 Ammonia as donor: +7.85 * At absolute zero, all particles practically cease to move and are therefore structured and orderly. ∆∆ ∆∆ G o ′′ ′′ 1 5 CO 2 1 20 HCO 3 − 1 20 NH 4 + H + e − ++++ 1 20 C 5 H 7 NO 2 9 20 H 2 O+→ 1 28 NO 3 − 5 28 CO 2 29 28 H + e − +++ 1 28 C 5 H 7 NO 2 11 28 H 2 O+→ 1 4 O 2 H + e − ++ 1 2 H 2 O→ 1 5 NO 3 − 6 5 H + e − ++ 1 10 N 2 3 5 H 2 O+→ 1 3 NO 2 − 4 3 H + e − ++ 1 6 N 2 2 3 H 2 O+→ 1 50 C 10 H 19 NO 3 9 25 H 2 O+ 9 50 → CO 2 1 50 NH 4 + 1 50 HCO 3 − H + e − ++ ++ 1 2 NO 2 − 1 2 H 2 O+ 1 2 NO 3 − H + e − ++→ 1 6 NH 4 + 1 3 H 2 O+ 1 6 NO 2 − 4 3 H + e − ++→ TX249_Frame_C15.fm Page 667 Friday, June 14, 2002 4:43 PM © 2003 by A. P. Sincero and G. A. Sincero is the same as that in state 1, the system must be in equilibrium. If the free energy at state 2 is greater than that at state 1, then some outside free energy must have been added to the system. In Table 15.2, this is the case of nitrite as a donor and ammonia as a donor. External free energies of 9.43 kcal/electron-mol and 7.85 kcal/electron- mol, respectively, has been added to the system; these values are indicated by the plus signs. External sources of energy are being required, so these half-cell reactions cannot occur spontaneously; they are said to be endothermic (i.e., requiring external energies to effect the reaction). On the one hand, when the free energy at state 2 is less than that at state 1, some energy must have been released by the system to the surroundings, thus manifesting in the decrease of free energy. A decrease in free energy is indicated in the table with a negative sign. This energy has been released “voluntarily” by the system without some form of “coercion” from the surroundings. The release is spontaneous, and therefore the reaction is spontaneous. Note: Thus, this is the criterion for a spontaneous process: When the free energy change is negative, the process is spontaneous. Judging from Table 15.2, when the electrons that travel through the electron transport system are finally accepted by oxygen, a large amount of energy equal to 18.68 kcal/electron-mol is liberated. This liberated energy is then captured in the bonds of ATP. The same statement holds for the others whose free energy changes have negative signs. Thus, any material in wastewater, edible by organisms, will release energy, resulting in their destruction. The more energy that can be released, the easier it is to be treated using microorganisms. 15.5 MODES OF NITROGEN REMOVAL The physical removal of nitrogen using the unit operation of stripping was discus- sed in a previous chapter. The present chapter concerns only the removal of nitrogen by biochemical means, as mediated by microorganisms. The technique of the unit process is to release the nitrogen in the form of the gas N 2 to the atmosphere. This will first entail nitrifying the nitrogens using the species of bacteria Nitrosomonas and Nitrobacter. Nitrosomonas oxidizes the ammonium ion into nitrites, deriving from this oxidation the energy it needs. Nitrobacter then oxidizes the nitrites into nitrates, also deriving from this oxidation the energy that it needs. These oxidations into nitrites and nitrates is called nitrification. Nitrification is an aerobic process. After the nitrogen has been nitrified, the second unit process of denitrification is then applied. The denitrifying bacteria, which are actually heterotrophs, convert the nitrates into nitrogen gas, thus ridding the wastewater of nitrogen. Denitrification is an anaerobic process. 15.6 CHEMICAL REACTIONS IN NITROGEN REMOVAL In biochemical nitrogen removal, BNR, two steps are required: oxidation of nitrogen to nitrate and subsequent reduction of the nitrate to gaseous nitrogen, N 2 . The oxidation steps are mediated by Nitrosomonas and by Nitrobacter, as mentioned TX249_Frame_C15.fm Page 668 Friday, June 14, 2002 4:43 PM © 2003 by A. P. Sincero and G. A. Sincero [...]... mass of BOD5 or mass of organisms or cells per unit mass of COD For the case of Ydn, the units used in practice are in terms of mass of the organisms or cells per unit of mass of the nitrate nitrogen Unlike the conversion of Yc and Ydc from mass basis to mole basis which is harder, the conversion of Ydn from mass basis to mole basis is very straightforward; thus, we will address the conversion of the former... conditions are met 15.7.1 UNITS OF CELL YIELDS Three types of cell yields are used in the previous derivations: Yc, Ydn, and Ydc The units of Yc and Ydc are in terms of moles of organisms per unit mole of sewage The units of Ydn are in terms of moles of organism per unit mole of nitrate nitrogen The units normally used in practice for Yc and Ydc, however, are either in terms of mass of organisms or cells (approximated... rate of decay Substrate kinetics may be established by noting that as organisms grow, substrates are consumed Therefore, the rate of decrease of the concentration of the substrate is proportional to the rate of increase of the concentration of the organisms The first term on the right-hand side of Equation (15.71) is the rate of increase of microorganism that corresponds to the rate of decrease of the... reports cell yields (productions) for Nitrosomonas of 0.04 to 0.29 milligrams of the bacteria per milligram of NH4–N destroyed and cell yields for Nitrobacter of 0.02 to 0.084 milligrams of the bacteria per milligrams of NO2–N destroyed (Mandt and Bell, 1982) Yield or specific yield simply means the amount of organisms produced per unit amount of substrate consumed Also, for nitrification to be the dominant... underflow rate of flow from the secondary clarifier Xu = underflow mass concentration of organisms from the secondary clarifier The volume of the control volume is comprised of the volume of the reactor, volume of the secondary clarifier, and the volumes of the associated pipings The total rate of increase is d∫ V [X ]dV /dt ; the local rate of increase is ∂ ∫ V [X ]dV / ∂ t; ˆ and the convective rate of increase... – Ym The literature reports cell yields Ym for Nitrosomonas of 0.04–0.29 mg of the bacteria per mg of NH4–N destroyed and cell yields for Nitrobacter Yn of 0.02–0.084 mg of the bacteria per milligrams of NO2–N destroyed 1.5/113 Let Y m = 1.5 mg/mg = = 0.186; Y n = 0.05 mg/mg 1/14 0.05/113 = - = 0.0062 1/14 p″ = 30/14 = 2.14 mmol/L Therefore, 0.186 + 0.0062 ( 1 – 0.186 ) [ A nit... proceed with the reduction down to the formation of the ammonium ion This, however, will require the formation of bonds between the nitrogen atom and three atoms of hydrogen (to form NH3), plus the hydrogen bond between the ammonia + molecule and the hydrogen proton (to form the ammonium ion, NH 4 ) This needs extra energy Thus, the process stops at the liberation of the nitrogen gas This whole reduction... the size of the reactor can then be determined In general, there are three types of kinetics involved in the removal of nitrogen: nitrification kinetics, denitrification kinetics, and carbon kinetics 15.10.1 KINETICS OF GROWTH AND FOOD UTILIZATION The kinetics of the microbial process may be conveniently divided into kinetics of growth and kinetics of food (or substrate) utilization The kinetics of growth... gram-equivalent of NH4–N, based on Equation (15.15), assume m equivalents are incorporated into the cell of Nitrosomonas, C5H7NO2 Thus, the NH4–N equivalent remaining for the donor reaction of Equation (15.13) is 1 − m, equals (1 − m) 1 [((1/20)N)/1] (1/N) = - (1 − m) moles [The 1/20 came from Equation (15.15) used 20 1 to compute the equivalent mass of NH4–N.] Thus, - (1 − m) moles of NH4–N is 20 available for. .. -mass of COD mass of COD 113 mol COD ( 32 ) 1 mol cells = Y COD 113 25 mol sewage 2 Therefore, mol cells −4 Y c or Y dc = 7.08 ( 10 )Y COD mol sewage (15.56) 3 Example 15.1 A domestic wastewater with a flow of 20,000 m /d is to be denitrified The effluent from the nitrification tank contains 30 mg/L of NO3–N, 2.0 mg/L of dissolved oxygen, . (productions) for Nitrosomonas of 0.04 to 0.29 milligrams of the bacteria per milligram of NH 4 –N destroyed and cell yields for Nitrobacter of 0.02 to 0.084 milligrams of the bacteria per milligrams of. plenty of them around. From the algae formula, the ratio of N to P is 16 / 1 = 16 mole for mole or 14( 16) / 31 = 7.2 mass for mass. Table 15.1 shows various values of nitrogen. processes. An example of a life process is the growth of bacteria when wastewater is fed to them to treat the waste. Knowledge of microbial thermodynamics is therefore important to professionals involved