CHAPTER 10. Denitrification

Transcription

1 CHAPTER 10 Denitrification

2 Introduction Denitrification NO dissimilatory reduction of 3 or NO to N gas ( NO 3 or NO is the electron acceptor used in energy generation ) Denitrification is widespread among heterotrophic and autotrophic bacteria. (many of which can shift between oxygen respiration and nitrogen respiration) applied when the complete removal of N is required to prevent eutrophication. In order to have denitrification, the nitrogen must be of its oxidized forms, NO 3 or NO, so denitrification is frequently is coupled to nitrification, which is needed to create the oxidized nitrogen. Tertiary denitrification : the water does not contain the necessary electron donor and thus an exogenous electron donor must be provided. Onesludge denitrification : the water contains an electron donor that can drive denitrification.

3 10.1 Physiology of Denitrifying Bacteria Denitrification process It proceeds in a stepwise manner: NO NO NO 3 NO NO N O N e H = NO H O 3 e NO e N H H = NO = N H O O H O e H = N H O O Nitrate Re ductase Nitrite Re ductase Nitric Oxide Re ductase Nitrous Oxide Re ductase Very low conc. of the electron donor or too high DO can lead to accumulation of denitrification intermediates. (NO,NO and N O) i) a low conc. of the electron donor limits the supply of electrons to drive the reductive halfreactions. ii) a high DO tends to repress the nitrite and nitrous oxide reductases before the nitrate reductase is suppressed.

4 10.1 Physiology of Denitrifying Bacteria ph values outside the optimal range of 7 to 8 can lead to accumulation of intermediates. In low acidity waters, ph control can become an issue, because denitrification produces strong base CH 3COOH NO3 H O N H CO3 OH H NO3 N OH H O [10.1] [10.] 8/5 equivalents of strong base produced when 8/5 mol of NO3 N is reduced. an alkalinity increase of (50)/(14) = 3.57g as CaCO 3 /g consumed. NO 3 NN For the acetate case, the effect is altered slightly, because mol of a weaker acid (H CO 3, pka 63)replace 6.3) 1 mol of a weak acid (CH 3 COOH, pka = 4.3)

5 10.1 Stoichiometric and kinetic parameters for denitrifiers Stoichiometric and kinetic parameters for denitrifiers in the early application of denitrification, intensive study was conducted for systems with relatively high NO3N (electron acceptor) or little BOD (electron donor) levels such as agricultural runoff (high NO3N) and advanced treatment of secondary effluent (little BOD). Thus, research addressed exogenous electron donnors and carbon sources. Because methanol was relatively inexpensive, a very large database on methanol has been developed. But the large database on methanol cannot be applied directly for situations in which another organic molecule is the donor. In table 10.1, OD ; the mass of O required for complete oxidation of the donor. (8g OD = one e eq. OD=BOD L for organic donor )

6 10.1 Stoichiometric and kinetic parameters for denitrifiers

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9 10.1 Stoichiometric and kinetic parameters for denitrifiers In summary, the heterotrophic denitrifiers have kinetic characteristics similar to aerobic heterotrophs. Because they are facultative aerobes, the shifts from O respiration to NO 3 or NO respiration causes only a small 0 decrease in and Y, which gives only modest increases in f s [ θ min χ ] lim = 1 Yqˆ b [3.7] [ θ min x ] lim Thus, denitrification processes should perform similarly to aerobic processes used for BOD L removal. On the other hand, the kinetic characteristics of heterotrophic denitrifiers and autotrophic denitrifiers are very different. Autotrophic denitrifiers have 0 lower f s, are much slower growers, and thus require substantially longer SRT. see table 10.1 While high DO is required for maximum nitrification, high DO slows or stop denitrification. Therefore process design and operation must reconcile these conflicting physiological characteristics.

10 10.1 Stoichiometric and kinetic parameters for denitrifiers The denitrifying bacteria often use NO 3 or NO as the N source for cell synthesis. The added electron cost of reducing the N source to the 3 oxidation state 0 in cell reduces f s and the true yield. Using NO 3 as N source requires 8 extra electron equivalents per mole of biomass (C 5 H 7 O N) compared with NH 3 as N source. NO 3 N 5e 3e C 5 H 7 O N requires 0 electron equivalents (Nsource; NH 4 ) to reduce the C the oxidation state zero but it requires 8 electron equivalents to reduce the C,N when the NO 3 is Nsource (see table.4), so OD (oxygen demand) of biomass: 8e eq mol cells ( comparewith 1 mol cells 113g cells 0e eq mol cells N 3 8 g O = 1.98 g OD/ g cells ( cell > NO3) e eq 1 mol cells 113g cells 8 g O = 1.4 g OD/ g cells ( cell > NH 3) e eq

11 .5 Overall Reactions for Biological Growth

12 10. Tertiary Denitrification Denitrification processes: Tertiary denitrification : exogenous electron donor is needed and added. One sludge denitrification : donor already present in the wastewater Tertiary denitrification Water or wastewater containing NO 3 or NO, but little or no electron donor. i) Agricultural runoff contaminated with nitrogen fertilizers ii) Drinking water supplies in agricultural regions (high N contaminated raw water ) iii) Effluent from aerobic biological process (i.e., secondary treatment)

13 10. Tertiary Denitrification Electron donor : the rate limiting step in almost all circumstances i) Organic electron donor Most commonly supplied They promote the accumulation of heterotrophic denitrifiers. In terms of physiology and kinetics, very similar to the aerobic heterotrophs used for BOD oxidation Similar design criteria Historically methanol was chosen for its economic benefits, not because it is a better exogenous electron donor than any other choice CH 3 OH NO H C 5 H 7 O N N H O 0.119CO Waste streams from the food processing and beverage industry are good choice because of high BOD conc. & very high C/N ratio. low N is desirable because the goal of denitrification is total removal of N.

14 10. Tertiary Denitrification ii) Inorganic electron donor H is an excellent electron donor for autotrophic denitrification low cost per electron equivalent compared to organic compounds less biomass production than with heterotrophs no reduced nitrogen (NH 4 ) added lack of safe and efficient H transfer system (disadvantage) It recently stimulated to develop a membranedissolution device. Reduced sulfur most common of reduced S is elemental sulfur, S(s). Normally embedded in a solid matrix that includes a solids base, such as CaCO 3, because the oxidation of S(s) generates strong acid. 6 3 S(s) NO3 H O SO 4 N H [10.3]

15 I0. Membrane bioreactor for Nitrate removal (Degremont) 외부환경요인 ( 원수수질등 ) 에따른처리효율의변동 외부탄소원및대사산물에의한처리수의재오염가능 높은회수율 (Water Recovery) 성취과제

16 10. Autotrophic Membrane Bioreactor for nitrate removal Membranedissolution process (Bruce Rittmann, Arizona State Univ.) H Raw water NO 3, NO Post Treatment NO 3, NO N Autotrophic Bioreactor Gas Permeable Membrane for Biofilm Growth Disadvantage of Membranedissolution process i) 독립영양미생물 (autotrophs) 의느린성장속도및낮은활성에의한제거효율저하 ii) 미생물및대사산물에의한처리수의재오염 ( 후처리필요 ) iii) 막내부의수소부분압상승에의한폭발위험성

17 10. Ion Exchange Membrane Bioreactor, IEMB (J.G. Crespo, Nova de Lisbona Univ., Portugal) Ion Exchange Membrane Bioreactor (IEMB) Donnan Dialysis i Biological Denitrification

18 10. Ion Exchange Membrane Bioreactor, IEMB K Cl NO 3 (J.G. Crespo, Nova de Lisbona Univ., Portugal) K Cl NO 3 Principle of Donnan Dialysis Nonporous and dense ionexchange membrane Driving force : Concentration difference : Electrical Potential Difference : Counter currention transport Used to remove the toxic heavy metal and to Used to remove the toxic heavy metal and to soften the drinking water

19 생물 10. Ion Exchange Membrane Bioreactor, IEMB (J.G. Crespo, Nova de Lisbona Univ., Portugal) Principle of IEMB 처리수 생물막조 HCO 3 K Cl Donnan Dia alysis Membrane 처리수조 막 K HCO 3 Cl CO NO 3 탄소원 미량영양분 Counterion (Cl ) N CO NO NO 3 원수