Chapter 9 Nitrification

Transcription

1 Chapter 9 Nitrification

2 9.0 Nitrification - The microbiological oxidation of NH 4+ -N to NO 2- -N and NO 3- -N. NH 4+ -N removal is a mandated process for some wastewaters because i) NH 4 + consumes oxygen up to 4.57 g O 2 / g NH 4 +-N. ii) NH 4 + is toxic to aquatic macroorganisms In addition, wastewater treatment that involves denitrification of NO 3- -N frequently requires nitrification to convert the input NH 4+ -N to NO 3- -N.

3 9. 0 Nitrification to remove NH 4+ -N from drinking-water supplies also is practiced i) to make the water biologically stable ii) to eliminate the free-chlorine demand that produces chloramines when free chlorine is desired. NH 3 + HOCl NH 2 Cl (monochloroamine) + H 2 O

4 The nitrifying bacteria are autotrophs, chemolithotrophs, and obligate aerobes. Energy source: light ---- phototroph Chemical ---- chemotroph chemolithotroph chemoorganotroph Carbon source: CO autotroph organic carbon heterotroph Photoautotroph: cyanobacteria, some purple and green bacteria autotrophs, Being autotrophs, the nitrifiers must fix and reduce inorganic carbon. Photoheterotroph: some purple and green bacteria Chemoheterotroph = chemoorganotroph: most bacteria, some archaea Chemoautotroph = chemolithotroph: a few bacteria and many archaea Methylotroph: 1 carbon compound as carbon source

5 chemolithotrophs, Their chemolithotrophic nature makes f s o and Y still smaller, because their nitrogen e-donors release less energy per electron equivalent than do organic e-donors, H 2, or reduced sulfur.

6 chemolithotrophs, Of course, the low Y value translate into a small maximum specific growth rate ( ) and a large, Θ min x. Therefore, nitrifiers are slow growers. qy [3.7] min S 0 K Yqˆ 0 S b bk 3.26

7 obligate aerobes - They use O 2 1) for respiration (energy), and 2) as a direct reactant for the initial monooxygenation of NH 4 + to NH 2 OH (hydroxylamine) - The latter use of oxygen may be the reason why nitrifiers are relatively intolerant of low dissolved-oxygen concentrations; nitrifier catabolism is slowed by oxygen limitation at concentration that have no effect on many heterotrophs.

8 - Nitrification is a two-step process. - In the first step, NH 4 + is oxidized to NO 2 - according to the following energy-yielding rxn. (The most famous genus; Nitrosomonas) 1 6 NH 1 O 4 1 NO 6 1 H 3 1 H O [9.1] G o ' kj per e eq - the second stage of the nitrification reaction is the oxidation of NO 2 - to NO 3 - :(The most famous genus; Nitrobacter, Nitrospira) 1 2 NO 2 G o 1 1 O2 NO3 4 2 ' kj per e eq [9.2] - Since nitrifiers exist in environment in which organic compounds are present, It might be curious that nitrifiers have not evolved to use organic molecules as their carbon source. ~ probably related to their evolutionary link to photosynthetic microorganisms.

9 For ammonium oxidizers Heterotrophs For nitrite oxidizers 20 (mgbod L /mgvssa-d) f so : very low for each group, compared to the typical f o s value of for aerobic heterotrophs Nitrifiers conserve very few electrons in biomass

10 For ammonium oxidizers Heterotrophs For nitrite oxidizers 20 (mgbod L /mgvssa-d) The low f s o values translate directly to low Y values (Caution: Unit is different form each other!!). In units of g VSS a / g OD, the difference of Y becomes greater. Y = 0.1 for ammonium oxidizer, Y =0.45 for heterotrophs Y =0.33x (14/6) / (32x1/4) = 0.1 (g VSS/g OD) [eq. 9.1] 1 NH 6 1 O2 4 1 NO H 3 1 H2 6 4 O [9.1]

11 For ammonium oxidizers Heterotrophs 9 d -1 For nitrite oxidizers Maximum specific growth rate of both organisms are low ( < 1 d 1 at 20 o C ): qy Therefore, the limiting value of [ min ] lim 1 Yqˆ b [3.7] [ min ] lim [3.27] is large ( > 1 d). (See next slide) The relatively high values of Ko quantify that nitrifiers are not tolerant of low DO concentrations

12 For ammonium oxidizers Heterotrophs Θ x min =0.11 d For nitrite oxidizers Smin = 0.17 mgbod L /L(complex substrates) [Θ min x ] lim must be large ( > 1 d ) 1 [ min ] lim [3.27] Y ˆ q b Smin < 1 mg/l Effluent NH 4 +, NO 2 - can be very low levels min Thus, as long as the SRT is maintained well above and sufficient dissolved oxygen is present, the nitrification can be highly efficient

13 Primary consumers of organic waste: heterotrophic bacteria (Generic parameters at 20 0 C). Limiting substrate Y (true yield for synthesis) qˆ ˆ Yˆ q K (substrate utilization) BOD L (maximum specific growth rate) 9 d -1 (conc. Giving one-half the maximum rate) b (endogenous decay coefficient) 0.15 d f d (biodegradable biomass fraction) 0.8 min x 0.45 mg VSS a /mg BOD L 20 mg BOD L /mg VSS a - d simple substrate : 1 mg BOD L /L complex substrate : > 10 mg BOD L /L (value at which washout begins) 0.11 d safety factor d S min (the minimum substrate conc. capable of supporting steady-state biomass) safety factor 36 4 d simple substrate : mg BOD L /L complex substrate : > 0.17 mg BOD L /L

14 9.1 Temperature effect - The temperature effecs are very important -Nitrification is sometimes considered impossible for low-water temperatures. -However, in fact, stable nitrification can be maintained at 5 o C or lower, as long as the SRT remains high enough For example, for 5 o C, a safety factor of only 5 requires that Θ x min be 3.6 x 5 = 18 d (quite large). Thus SRT should be larger than 18 days. -One problem with low-temperature nitrification is that û becomes quite small, making recovery of nitrification after a wash out a very slow process. Thus, avoiding nitrifier washout due to excess sludge wasting, low DO, or inhibition must be an absolute priority, particularly for low temperatures.

15 The directly comparable parameters: f so,,, S min ˆ min -Very similar for two types of nitrifiers. -This circumstance is completely logical, since both are aerobic chemolithoauthotrophs oxidizing N (next slide) -They almost always coexist in the same habits, experiencing the same SRT and oxygen concentrations. -Reflects their biochemical and ecological similarities

16 chemolithotrophs, Their chemolithotrophic nature makes f s o and Y still smaller, because their nitrogen e-donors release less energy per electron equivalent than do organic e-donors, H 2, or reduced sulfur.

17 Overall, balanced equation for the complete oxidation of NH + 4 to NO 3- N by nitrifiers having x 15 d, f s NH O CO C 5 H 7 O N NO H 2 O 1.973H [9.3] refer to next slide - The low net formation of nitrifier biomass : Y net = 113 x 0,0261/ 14 = 0.21 g VSS a /g N - Nitrification creates a major oxygen demand. OD = x 32 / 14 = 4.14 g O 2 / g NH 4+ -N consumed - Produces almost two (1.973) strong-acid equivalents (1.973) per mole of NH 4 + removed Thus the alkalinity consumption is x 50/14 =7.05 g as CaCO 3 /g NH 4+ -N - The first step, ammonium oxidation, is responsible for the acid production

18 2.5 Overall Reactions for Biological Growth 2) Obtain the overall reaction (R) including energy and synthesis using portions of electrons, fe (= 0.6) and fs (= 0.4) fe*re: 0.02 C 6 H 5 COO NO H + -> 0.12 CO N HCO H 2 O fs*rs: C 6 H 5 COO NH HCO 3 - -> 0.02 C 5 H 7 O 2 N H 2 O R : C 6 H 5 COO NO NH H + -> 0.02 C 5 H 7 O 2 N CO N HCO H 2 O [2.34] The overall reaction for net synthesis of bacteria that are using benzoate as an e- donor, nitrate as an e- acceptor, and ammonium as nitrogen source.

19 Nitrifiers produce SMPs which can be consumed by heterotrophic bacteria - - Most of the nitrifier-produced SMP are BAP (biomass-associated products) - SMPs are part of the decay process of the nitrifiers and reduce the net synthesis of the nitrifiers - A way in which nitrifiers create e-donors for heterotrophs and increase the heterotrophic biomass * UAP ( substrate-utilization-associated products ) growth associated they are generated from substrate utilization and biomass growth they are not intermediates of catabolic pathways * BAP ( biomass-associated products ) non-growth associated related to decay and lysis of cell release of soluble cellular constituents through lysis and solubilization of particulate cellular components

20 Chemical inhibition - Highly sensitive to chemical inhibition. - The very slow growth rate of nitrifiers magnifies the negative impacts of inhibition and, in part, makes it appear that nitrifiers are more sensitive than are faster growing bacteria. - Furthermore, some apparent inhibitors are e-donors whose oxidation depletes the DO and may cause oxygen limitation. - Most relevant inhibitors are: unionized NH 3 (at higher ph), undissociated HNO 2 (usually at low ph), anionic surfactants, heavy metals, chlorinated organic chemicals, and low ph

21 9.2 Common Process Considerations A successful nitrification process suspended growth or biofilmmust account for the reality that - heterotrophic bacteria always are present - and competing with the nitrifiers for dissolved oxygen and space. two disadvantages - The nitrifiers relatively high K o value puts them at a disadvantage in the competition for oxygen. - Their slow growth rate is a disadvantage when competing for any space that requires a high growth rate overcome by ensuring that - the nitrifiers have a long SRT, typically greater than 15 d, - larger value may be needed in the presence of toxic materials, a low D.O. concentration, or low temperature.

22 9.2 Common Process Considerations Even with the relatively long SRT of extended aeration, nitrifying process often have relatively small safety factors. - for economic reasons - AS [Θ x min ] lim for nitrification is 1-3 d, the reactor volume is too great when the safety factor is greater than about 10, because the Θ x / Θ ratio cannot be increased indefinitely to compensate. - Of course, operating with a small safety factor increases the risk of washout due to solids loss or inhibition and increases the needs for operator attention. - Unfortunately, the risk is high, and instability in nitrification is a common problem in treatment operations

23 9.3 Activated Sludge Nitrification : One-sludge Versus Two-Sludge

24 9.3 Activated Sludge Nitrification : One-sludge Versus Two-Sludge One-sludge nitrification - One-sludge nitrification is the process configuration in which heterophic and nitrifying bacteria coexist in a single mixed liquor that simultaneously oxidizes organic BOD and ammonium - Use the term one sludge to emphasize the ecological relationship between the nitrifiers and hetrotrophs

25 9.3 Activated Sludge Nitrification : One-sludge Versus Two-Sludge Two-sludge nitrification - Two-sludge nitrification is an attempt to reduce the competition between heterotrophs and nitrifiers by oxidizing most of the organic BOD in a first stage - The first sludge is essentially free of nitrifiers, while the second sludge has a major fraction of nitrifiers One-sludge nitrification can be carried out in sequencing batch reactors (SBR) - SBR involves sequential periods of filling, aerobic reaction, settling, and effluent draw-off in one tank - In most ways, SBR nitrification resembles any other one-sludge system.

26 9.7 The ANAMMOX Process - Recently (1999), a novel bacterium in the planctomycetes group has been discovered for its ability to anaerobically oxidize NH 4+ -N to N 2, not NO It is called the ANAMMOX microorganism, Anaerobic Ammonium Oxidation. - The discovery of ANAMMOX is one of the most startling ones in environmental biotechnology. Ammonium: electron donor Nitrite: electron acceptor NH 4 NO2 N2 2H2O Autotrophs: the reduction of inorganic carbon via oxidation of nitrite to nitrate, nitrite as the nitrogen source 5CO NO 2 14NO2 3H2O H C5H7O2 N 2 3

27 9.7 The ANAMMOX Process The yield and specific growth rate are low. Y = 0.14 g VSS a /g NH 4+ -N, = 0.065/d This gives an overall stoichiometry of approximately: NH NO CO2 0.02H N C5H 7O2 N 0.024NO H2O ANAMMOX s favoring condition - exceptional biomass retention: a very long SRT - stable operation - the presence of nitrite - lack of oxygen - lack of e - donor: cause the reduction of nitrite via denitrification In the presence of e donor, nitrite is reduced to N 2 NO 3 NO 2 NO N2O N2