chapter 9 nitrification - snuwemt.snu.ac.kr/lecture 2013-2/env/ch 9 nitrification 2013-2.pdf ·...

Chapter 9 Nitrification

Nitrification - The microbiological oxidation of NH4

+-N to NO2--N and NO3

--N.

NH4+-N removal is a mandated process for some

wastewaters because

i) NH4+ consumes oxygen up to 4.57 g O2 / g NH4+-N.

ii) NH4+ is toxic to aquatic macroorganisms

In addition, wastewater treatment that involves denitrification of NO3

- -N frequently requires nitrification to

convert the input NH4+-N to NO3

--N.

9.0

Nitrification to remove NH4+-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.

NH3 + HOCl NH2Cl (monochloroamine) + H2O

9. 0

The nitrifying bacteria are autotrophs,

chemolithotrophs,

and obligate aerobes.

autotrophs, Being autotrophs, the nitrifiers must fix and

reduce inorganic carbon.

Energy source: light ---- phototroph Chemical ---- chemotroph

chemolithotroph chemoorganotroph

Photoautotroph: cyanobacteria, some purple and green bacteria

Carbon source: CO2 ---- autotroph organic carbon ----- heterotroph

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

9.1 Biochemistry and Physiology of Nitrifying Bacteria

chemolithotrophs, Their chemolithotrophic

nature makes fso and Y still smaller, because their nitrogen e-donors release less energy per electron equivalent than do organic e-donors, H2, or reduced sulfur.


chemolithotrophs, Of course, the low Y value translate into a small

maximum specific growth rate ( ) and a large , Θx min . Therefore, nitrifiers are slow growers.


( ) [ ]26.3ˆ0

0min

bKbqYSSK−−

+=χθ

]7.3[∧∧

Yq=µ

∧µ

obligate aerobes - They use O2 1) for respiration (energy), and 2) as a direct

reactant for the initial monooxygenation of NH4+ to NH2OH

(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.


- In the first step, NH4+ is oxidized to NO2

- according to the following energy-yielding rxn. (The most famous genus; Nitrosomonas)

]1.9[61

31

61

41

61

2224 OHHNOONH ++=+ +−+

eqeperkJGo −−=∆ 79.45'

eqeperkjG

NOONO

o −

−−

−=∆

=+

07.37

]2.9[21

41

21

'

322

- the second stage of the nitrification reaction is the oxidation of NO2-

to NO3- :(The most famous genus; Nitrobacter, Nitrospira)


- Nitrification is a two-step process.

- 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.

fso: very low for each group, compared to the typical fo

s value of 0.6-0.7 for

aerobic heterotrophs

For ammonium oxidizers

For nitrite oxidizers

Nitrifiers conserve very few electrons in biomass

The low fso values translate directly to low Y values (Caution: Unit is different form

each other !!).

Heterotrophs

0.6-0.7

20

(mgBODL/mgVSSa-d)


In units of g VSSa/ g OD, Y = 0.1 for ammonium oxidizer, Y =0.45 for heterotrophs



Maximum specific growth rate of both organisms are low : less than 1 d –1 at 20 oC. The limiting value of is large.

Heterotrophs

9 d-1


]27.3[ˆ1][ lim

min

bqY −=χθ

limmin ][ χθ

The relatively high values of Ko quantify that nitrifiers are not tolerant of low DO concentrations

Smin < 1 mg/L Effluent NH4+, NO2- can be very low levels



[Θxmin] lim must be large, greater than 1 d

Thus, as long as the SRT is maintained well above and sufficient dissolved oxygen is present, the nitrification can be highly efficient

Heterotrophs

0.11 d

0.17

mgBODL/L(complex substrates)


]27.3[ˆ1][ lim

min

bqY −=χθ

minχθ

Primary consumers of organic waste: heterotrophic bacteria (Generic parameters at 20 0C).

Limiting substrate BODL

Y (true yield for synthesis) 0.45 mg VSSa/mg BODL

(substrate utilization) 20 mg BODL /mg VSSa- d

(maximum specific growth rate) 9 d-1

K (conc. Giving one-half the maximum rate)

simple substrate : 1 mg BODL /L complex substrate : > 10 mg BODL /L

b (endogenous decay coefficient) 0.15 d

fd (biodegradable biomass fraction) 0.8

(value at which washout begins) 0.11 d safety factor 100 11 d safety factor 36 4 d

Smin (the minimum substrate conc. capable of supporting steady-state biomass)

simple substrate : 0.017 mg BODL /L complex substrate : > 0.17 mg BODL /L

minxθ

qY ˆˆ =µ

q̂

- 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 oC or lower, as long as the SRT remains high enough For example, for 5 oC, a safety factor of only 5 requires that Θx 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.

9.1 Temperature effect

The directly comparable parameters: fso, , , Smin µ̂

-Very similar for two types of nitrifiers.

-This circumstance is completely logical, since both are aerobic chemolithoauthotrophs oxidizing N.

-They almost always coexist in the same habits, experiencing the same SRT and oxygen concentrations.

-Reflects their biochemical and ecological similarities


minχθ

Overall, balanced equation for the complete oxidation of NH4+ to

NO3-N by nitrifiers having 067.0,15 ==Θ sfdx

]3.9[973.1921.0

973.00261.01304.0815.1

2

3275224

+

−+

++

+=++

HOH

NONOHCCOONH

- The low net formation of nitrifier biomass : Ynet = 0.21 g VSSa/g N

refer to next slide

- Nitrification creates a major oxygen demand.

it is 1.815 x 32 / 14 = 4.14 g O2 / g NH4+-N consumed

- Produces almost two (1.973) strong-acid equivalents (1.973) per mole of NH4+

removed

Thus the alkalinity consumption is 1.973 x 50/14 =7.05 g as CaCO3/g NH4+-N

- The first step, ammonium oxidation is responsible for the acid production


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 C6H5COO- + 0.12 NO3- + 0.12 H+ -> 0.12 CO2 + 0.06 N2 + 0.002 HCO3

-

+ 0.1 H2O

fs*Rs: 0.0133 C6H5COO- + 0.02 NH4+ + 0.0067 HCO3

- -> 0.02 C5H7O2N + 0.0067 H2O

R : 0.0333 C6H5COO- + 0.12 NO3- + 0.02 NH4

+ + 0.12 H+

-> 0.02 C5H7O2N + 0.12 CO2 + 0.06 N2 + 0.0133 HCO3- + 0.1067 H2O [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.

2.5 Overall Reactions for Biological Growth

- - 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

Nitrifiers produce SMPs which can be consumed by heterotrophic bacteria


- 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 NH3 (at higher pH), undissociated HNO2 (usually at low pH), anionic surfactants, heavy metals, chlorinated organic chemicals, and low pH

Chemical inhibition


Even with the relatively long SRT of extended aeration, nitrifying process often have relatively small safety factors.

- for economic reasons

- [Θx min

]lim is 1-3 d, the reactor volume is too great when the safety factor is greater than about 10,

- Because the Θx / Θ ratio cannot be increased indefinitely to compensate.

- Unfortunately, the risk is high, and instability in nitrification is a common problem in treatment operations

- 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.

9.2 Common Process Considerations

A successful nitrification process – suspended growth or biofilm- must account for the reality that

- heterotrophic bacteria always are present

- and competing with the nitrifiers for dissolved oxygen and space.

- The nitrifiers’ relatively high Ko 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

two disadvantages

- 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.

9.2 Common Process Considerations

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 ammonium and organic BOD

- Use the term one sludge to emphasize the ecological relationship between the nitrifiers and hetrotrophs


• 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)

- involve 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.

9.7 The ANAMMOX Process

- Recently (1999), a novel bacterium in the planctomycetes group has been

discovered for its ability to anaerobically oxidize NH4+-N to N2, not NO2

-. - 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

Autotrophs: the reduction of inorganic carbon via oxidation of nitrite to nitrate, nitrite as the nitrogen source

OHNNONH 2224 2+=+ −+

−+− +→+++ 3275222 23145 NONOHCHOHNOCO

The yield and specific growth rate are low. 0.14 g VSSa/g NH4

+-N, 0.065/d

This gives an overall stoichiometry of approximately:

ANAMMOX’s favoring condition

- exceptional biomass retention: a very long SRT

- stable operation

- the presence of nitrite

- lack of oxygen

- lack of donor: cause the reduction of nitrite via denitrification

9.7 The ANAMMOX Process

OHNONOHCNHCONONH 232752224 95.1024.0017.002.0085.026.1 +++→+++ −+−+

chapter 9 nitrification - snuwemt.snu.ac.kr/lecture 2013-2/env/ch 9 nitrification 2013-2.pdf ·...

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