Water and Wastewater Engineering
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WASTEWATER MICROBIOLOGY 22-17
The microbiology, stoichiometry, growth kinetics, and environmental control factors of these
unit processes are described in the following paragraphs. These are the fundamental basis for
selection and design of secondary treatment alternatives.
Aerobic Oxidation
The unit processes used to remove BOD may be either suspended growth where microorganisms
are suspended in the wastewater or attached growth where the microorganisms grow on a solid
surface.
Microbiology. Aerobic heterotrophic bacteria predominate. Protozoa also play a role by con-
suming free bacteria and colloidal particles.
Stoichiometry. The stoichiometry
of the oxidation may be described by the following generic
equations:
O x idation and synthesis
COHNS O nutrients CO NH C H
bacteria
223 5 7
⎯→⎯⎯⎯ NNO other end products
organic
2
new cells
matter
(22-20)
where COHNS is a generic representation of organic matter and C
5
H
7
NO
2
is a generic repre-
sentation of new cells.
Endogenous respiration
CHNO O CO HO NH energ
bacteria
5 72 2 2 2 3
552⎯→⎯⎯⎯ yy
cells
(22-21)
When the organic matter is used up, the cells begin to consume their own cell tissue to obtain
energy for cell maintenance. This process is called endogenous respiration.
Growth Kinetics. The form of the rate expressions for net biomass growth and substrate utili-
zation are given by Equations 22-15 and 22-18 . These are use
d to develop design parameters in
Chapter 23.
Environmental Factors. Although successful carbonac eous removal can be achieved over a
range of 6.0 to 9.0, the optimum pH is near neutral. A common minimum limit for DO concentra-
tion is 2.0 mg/L (Metcalf & Eddy, 2003).
Nitrification
Nitrification is the term used to describe the two-step process in which ammonia
(NH
)
4
is oxidized
to nitrite
(NO )
2
that is, in turn, oxidized to nitrate
(NO )
3
Nitrification may be accomplished by
either suspended growth or attached growth unit processes.
22-18 WATER AND WASTEWATER ENGINEERING
Microbiology. Aerobic autotrophic bacteria must predominate to accomplish nitrification. Two
genera are commonly recognized. Ammonia is oxidized to nitrite by Nitrosomonas. Nitrite is
oxidized to nitrate by Nitrobacter. In the last two decades a number of other autotrophic genera
have been identified that will also perform these functions (Met
calf & Eddy, 2003).
Stoichiometry. The oxidation steps that yield energy are:
2 3 242
42
NH O NO H
Nitroso-bacteria
2
⎯→⎯⎯⎯⎯⎯ HHO
2
(22-22)
22
2
NO O NO
2
Nitro-bacteria
3
⎯→⎯⎯⎯⎯⎯
(22-23)
The total oxidation reaction may be written as
NH O NO H H O
4 3
22
22
→
(22-24)
From the total oxidation reaction, the oxygen required for total oxidation of ammonia is 4.57 g of
O
2
/g of N. Of this amount, 3.43 g of O
2
/g is used for nitrite production and 1.14 g of O
2
i s used
for oxidation of nitrite.
Neglecting cell tissue, the amount of alkalinity required to buffer the total oxidation reaction
can be estimated from the following reaction:
NH HCO O NO CO H O
4 33
22 23
222
→
(22-25)
Thus, for each gram of ammonia nitrogen (as N) that is converted, 7.14 g of alkalinity as CaCO
3
is required.
The generic biomass synthesis reaction is
4 5
225 72 2
CO HCO NH H O C H NO O
3 4
→
(22-26)
where C
5
H
7
NO
2
is the generic representation of bacterial cells.
Growth Kinetics. The growth rate of Ni trosomonas controls the overall c onversion reaction.
For nitrification systems operated at tem peratures below 28 C, ammonia oxidation kinetics are
rate limiting. Thus, designs are based on saturation kinetics for ammonia oxidation:
n
nm
n
dn
N
KN
k
⎛
⎝
⎜
⎞
⎠
⎟
(22-27)
where
n
specific growth rate for nitrifying bacteria, g new cells/g cells · d
nm
maximum specific growth rate, g new cells/g cells · d
N nitrogen concentration, g/m
3
K
n
half velocity constant, substrate concentration at one-half the maximum specific
substrate utilization rate, g/m
3
k
dn
endogenous decay coefficient for nitrifying organisms, g VSS/g VSS · d
WASTEWATER MICROBIOLOGY 22-19
The
nm
values for nitrifying organisms are much lower than
m
for heterotrophic organ-
isms. The values reported for
nm
vary from 0.25 to 0.77 g VSS/g VSS · d (Randall et al., 1992).
Because these values cover a wide range, whenever possible bench-scale or in-plant testing are
highly recommended to evaluate site-specific nitrification values.
Nitrification rates are affected by the DO concentration in suspended growth processes. The
rates increase up to DO conc
entrations of 3 to 4 mg/L. To account for the effects of DO, Equation
22-27 is modified as follows:
n
nm
no
dn
N
KN
DO
KDO
k
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
(22-28)
where K
o
half saturation constant for DO, g/m
3
DO dissolved oxygen concentration, g/m
3
At low to moderate organic loadings, the kinetic model coefficients are generally adequate.
At high organic loadings, these models will over predict the nitrification rates (Metcalf & Eddy,
2003).
Because the growth rate for nitrifying organisms is less than that for heterotrophic organ-
isms, the fraction of organisms in the reactor is considerably less
than the fraction of heterotro-
phic organisms in a single stage carbon oxidation-nitrification process. The fraction of nitrifying
organisms ( f
N
) can be estimated with the following equation (Crites and Tchobanoglous, 1998):
f
N
016
06 016
3
5
.
..
()
()(
NH removed
BOD removed NH
33
removed )
(22-29)
Environmental Factors. In addition to maintenance of sufficient DO as noted above, the pH must
be controlled within a narrow range. Nitrification will occur at pH values in the range of 6.8 to 8.0.
Typically the range of pH is held to 7.0 to 7.2 (Metcalf & Eddy, 2003). Where the alkalinity is low, it is
added in the form of lime, soda ash, sodium bicarbonate, or magnesium hydroxide d
epending on cost.
Toxic effects from a wide variety of organic compounds, metals , and un-ionized ammonia
have been observed.
Denitrification
To reduce the potential for eutrophication, nitrate may be reduced to nitric oxide, nitrous oxide,
and nitrogen gas. Two modes of biological nitrate removal can occur: assimilating nitrate
reduction and dissimilating nitrate reduction. Assimilating nitrate reduction involves the reduction
of nitrate to ammonia for cell s ynthesis. It occu
rs when NH
4
-N is not available. It is indepen-
dent of DO concentration. Dissimilating nitrate reduction is coupled to the respiratory electron
transport chain. Nitrate or nitrite is used as an electron acceptor for the oxidation of organic or
inorganic electron donors.
The most common process used in municipal wastewater treatment plants is known as the
Mod
ified Ludzak-Ettinger (MLE) process. It consists of an anoxic tank followed by an aeration
tank. Nitrification occurs in the aeration tank. Nitrate produced in the aeration tank is recycled back
to the anoxic tank. Organic substrate in the influent wastewater provides the electron donor for
o xidation-reduction reactions using nitrate. The process is also known as preanoxic deni
trification.
22-20 WATER AND WASTEWATER ENGINEERING
An alternative process called postanoxic denitrification removes the BOD first in an aeration
tank. Denitrification occurs in a second tank that is anoxic. The elec tron donor source is from
endogenous decay. This process has a much slower rate of reaction than the MLE process. Often
an exogenous carbon sour
ce such as methanol or acetate is added to provide sufficient rbCOD
for nitrate reduction and to increase the rate of denitrification. Postanoxic processes may include
both suspended growth and attached growth systems.
Microbiology. Both heterotrophic and autotrophic organisms are capable of denitrification.
A large nu
mber of genera have been identified. Most of the bacteria are fac ultative anaerobes.
Pseudomonas species are the most common.
Stoichiometry. The nitrate reduction reactions involve the following steps:
NO NO NO N O N
3 2
→→ →→
22
(22-30)
The reaction stoichiometry for the three common electron donors is as follows:
Influent wastewater
CHON NO N CO HO NH O
H
Ge
310 19 3 2223
10 5 10 3 10
→
nneric
wastewater
composition
(22-31)
Methanol
5 6 3 5 76
3 222
CH OH NO N CO H O OH
3
→
(22-32)
Acetate
5 841068
3 222
CH COOH NO N CO H O OH
3
→
(22-33)
In each of these reactions one equivalent of alkalinity is produced for each equivalent of NO
3
- N
reduced. This equates to 3.57 g of alkalinity as CaCO
3
per gram of nitrogen reduced. This means
that about one-half of the alkalinity consumed in nitrification (7.14 g as CaCO
3
) can be recovered
in denitrification.
The oxygen equivalent of nitrate and nitrite is useful in design when the total oxygen required
for nitrification-denitrification is to be calculated. For nitrate it is 2.86 g of O
2
/g of NO
3
- N. For
nitrite it is 1.71 g of O
2
/g of NO
2
- N.
The amount of rbCOD required to provide a sufficient amount of electron donor for nitrate
removal may be estimated as (Metcalf & Eddy, 2003):
g of rbCOD/g NO -N
3
286
1142
.
. Y
n
(22-34)
where Y
n
net biomass yield, g VSS/g rbCOD.
WASTEWATER MICROBIOLOGY 22-21
Growth Kinetics. The substrate utilization rate expression given by Equation 22-19 is modi-
fied by a term to show a lower utilization rate in the anoxic zone:
r
Y
SX
KS
su
m
s
(22-35)
where fraction of denitrifying bacteria in the biomass, g VSS/ g VSS
The value of h a s been found to vary from 0.20 to 0.80 for preanoxic denitrification reactions
(Stensel and Horne, 2001). For postanoxic suspended growth and attached growth processes, the
term is not required because the biomass is pre
dominately denitrifying bacteria.
DO can inhibit nitrate reduction by repressing the nitrate reduction enzyme. A DO concentra-
tion as low as 0.13 mg/L has been observed to cause denitrification to stop. The effect of nitrate
and DO concentration is accounted for by two correction factors for Equation 22-35 (Barker and
Dold, 1997):
r
Y
SX
KSK
K
su
m
ss
O
1
3
33
NO
NO
NO
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
,
O
K
DO
⎛
⎝
⎜
⎞
⎠
⎟
(22-36)
where K
O
# DO inhibition coefficient for nitrate reduction, mg/L
K
s, NO3
half velocity coefficient for nitrate limited reaction, mg/L
The value of K
O
# is system specific. Values in the range of 0.1 to 0.2 mg/L have been proposed
for K
O
# and 0.1 mg/L for K
s, NO3
(Barker and Dold, 1997).
Environmental Factors. Alkalinity is produced in the denitrification process and the pH is
generally elevated. No significant effect on the denitrification rate has been reported for pH val-
ues between 7.0 and 8.0.
Phosphorus Removal
In biological phosphorus removal (BPR or Bio-P) or enhanced biological phosphorous removal
(EBPR) as it is sometimes called, the phosphorus in the wastewater is incorporated into cell mass
in excess of levels needed for cell synthesis and maintenance. This is accomplished by moving
the biom
ass from an anaerobic to an aerobic environment. The phosphorus contained in the bio-
mass is removed from the process as sludge.
Microbiology. The original work on enhanced Bio-P identified Acinetobacter a s the respon-
sible genus. Subsequent work has identified Bio-P bacteria in other genera such as Arthrobacter,
Aeromonas, Nocardia, and Pseudomonas. The Bio-P organisms in these genera are referred to as
phosphorus accumulating organisms (PAOs).
Based on the work of Comeau et al. (1986), Wentzel et al. (1986) developed a mechanistic
model used to explain BPR. This model proposes that (Stephens and Stensel, 1998):
Complex chemical oxygen demand (COD) is fermented to acetate by facultative bacteria under anaerobic
conditions. The bacteria assimilate acetate in the anaerobic zone and convert it to polyhydroxybutyrate
(PHB) using reducing equivalents provided from the tricarboxylic acid (TCA)
cycle. Stored polyphosphate
is degraded to provide adenosine triphosphate (ATP) necessary for PHB formation, and the polyphos-
phate degradation is accomplished by the release of orthophosphorus and magnesium, potassium, and
22-22 WATER AND WASTEWATER ENGINEERING
calcium. Under aerobic conditions, the PHB is oxidized to synthesize new cells and to produce reducing
equivalents needed for ATP formation. Phosphate and inorganic cations are taken up, reforming poly-
phosphate granules. The amount of phosphate taken up under aerobic conditions excee
ds the phosphorus
released during anaerobic conditions, resulting in excess phosphorus removal.
Other proposed models incorporate glycogen into the phosphate removal mechanism. Based on labo-
ratory measurements of cellular carbohydrates, Mino et al. (1987) speculated that under anaerobic c
ondi-
tions, glycogen serves as the electron donor for PHB formation. Then, under aerobic conditions, glycogen
is synthesized from PHB oxidation to replenish the glycogen reserves needed under anaerobic conditions.
In two papers, Smolders et al. (1994a and 1994b) presented a stoichiometric model that in
cludes the role of
glycogen for acetate uptake and PHB storage in the anaerobic phase and a stoichiometric model for PHB
oxidation and glycogen and polyphosphate formation during the aerobic phase. . . . Both models produce
PHB from acetyl-CoA formed from acetate, but the Wentzel model relies on the TCA cycle for NADH,
whereas glycogen is
produced under aerobic conditions in the Mino/Smolders model.
These models are illustrated in Figure 22-11 .
Stoichiometry. Common heterotrophic bacteria in activated sludge have a phosphorus com-
position of 0.01 to 0.02 g P/g biomass. PAOs are capable of storing phosphorus in the form of
phosphates. In the PAOs, the phosphorus content may be as high as 0.2 to 0.3 g P/g biomass.
A cetate uptake is critical in d
etermining the amount of PAOs and, thus, the amount
of phosphorus that can be removed by this pathway. If s ignificant amounts of DO or nitrate
enter the anaerobic zone, the acetate will be depleted before it is taken up by the PAOs. Bio-P
removal is not used in systems that are des igned for nitrification without providing a means of
denitrification.
The am
ount of phosphorus removal can be estimated from the amount of rbCOD in the waste-
water influent. The following assumptions are used to evalu ate the stoichiometry of biological
phosphorus removal: (1) 1.06 g of acetate/g of rbCOD will be produced as the COD is fermented
Anaerobic conditions:
Common pathway
Glycogen pathway
TCA pathway
CH
3
COOH
Acetyl-CoA
TCA
cycle
NADH
NADH
NADNAD
AT P
ADP
M
H
PO4
e
Glycogen
H
e
(PO
4
)n (PO
4
)n
1
PHB
Glycogen produced under aerobic growth
PHB Acetyl-CoA Pyruvate Glycogen
FIGURE 22-11
Comparison of mechanisms for anaerobic acetate u ptake, phosphorus
release, and PHB formation using either the TCA cycle in the Wentzel model
or glycogen in the Mino/Smolders models for NADH. ( Source: Stephens
and Stensel, 1998.)
WASTEWATER MICROBIOLOGY 22-23
to volatile fatty acids (VFAs), (2) a cell yield of 0.3 g VSS/g of acetate, and (3) a cell phosphorus
content of 0.3 g of P/ g VSS. With thes e assumptions, it is estimated that 10 g of rbCOD is
required to remove 1 g of phosphorus (Metcalf & Eddy, 2003).
It should be noted that all VFAs are not equivalent in phosphorus uptake. Table 22-2 illus-
trates the importan
ce of acetic acid . From an operational perspective a simple measurement of
total VFAs may not be sufficient to identify performance deficiencies.
Growth Kinetics. Bio-P growth kinetics fall in the same order of magnitude as that of other
heterotrophic bacteria. A maximum specific growth rate at 20 C is given as 0.95
g/g · d by Barker
and Dold (1997).
Environmental Factors. Steady-state rbCOD or acetate availability is required for good
phosphorus removal. Periods of starvation or low rbCOD concentrations result in lowering of
i ntracellular storage reserves of glycogen, PHB, and polyphosphates. This leads to decreased
phosphorus removal efficiency. This is a potential scenario at
start-up of the plant when
wastewater flow rates and loads are low. It is also a potential scenario during the diurnal cycle of
BOD load because of nighttime decreases in anthropogenic activity.
S ystems with excessive anaerobic contact times and without significant VFA production will
experience phosphorus release with no u
ptake of acetate. Excessive anaerobic contact results in
the release of orthophosphate without the addition of acetate as the bacteria use stored polyphos-
phate for an energy source. During subs equent aeration, the oxidation of PHB provides energy
for bacteria to assimilate not only the released phosphorus but additional phosphorus from the
influent was
tewater to build polyphosphate reserves. Not all of the phosphorus that is released in
the absence of acetate consumption can be taken up because there is not enough PHB storage to
provide the energy for excess uptake during the aerobic period.
Likewise, excessive aeration time will result in less phosphorus uptake. This is the result of
the competitive role of glycogen in the for
mation and utilization of PHB. From Figure 22-11 it
may be noted that glycogen is degraded under anaerobic conditions to provide energy for PHB
formation. Under aeration a portion of the PHB is converted to glycogen. If less glycogen is
available during anaerobic contact, then less PHB formation is expected. Less PHB results in less
phosphorus
uptake during aeration. If glycogen reserves are depleted in the aerobic period because
of excessive aeration, a greater portion of the PHB formed is used to replenish glycogen reserves.
This results in less PHB available for phosphorus uptake under aerobic conditions.
TABLE 22-2
Volatile fatty acids and phosphorus uptake
Volatile fatty acid P uptake/VFA COD consumed
Acetic 0.37
Butyric 0.12
Isobutyric 0.14
Isovaleric 0.24
Propionic 0.10
Valeric 0.15
A dapted from WEF, 2006.
22-24 WATER AND WASTEWATER ENGINEERING
S ystem performance is not affected by DO as long as the aerobic zone DO is above 1.0 mg/L.
The pH must be above 6.5 for appreciable phosphorus removal. The recommended molar ratios
of Mg, K, and Ca to phosphorus are 0.71, 0.50, and 0.25, respectively (Wentzel et al., 1989).
Selector
A selector i s a bioreactor design that favors the growth of floc-forming bacteria instead of fila-
mentous bacteria so that the biomass has better settling and thickening properties. The selector
designs are based on either kinetic or metabolic mechanisms.
Kinetic-Based Selector. While filamentous bacteria are more effic ient for substrate u tiliza-
tion at low substrate concentration
s, the floc-forming bacteria have a higher growth rate at high
soluble substrate concentrations. One or more reactors with short detention times (minutes) is
used for the kinetic selector. The ratio of the mass of COD or BOD to the mass of microorganisms
is high. It is on the order of 3 to 5 g of BOD/g of suspended solids per day. To achieve the kinetic
concept, the DO must be on the order of 6 to 8 mg/L. This allows the floc-forming bacteria to
have a high growth rate.
Metabolic-Based Selector. The filamentous bacteria cannot use nitrate or nitrite for an electron
acceptor. Because of this metabolic advantage, floc-forming bacteria that can use nitrate or nitrite,
and can store polyphosphates, are favored over filamentous organisms that do not settle well.
Thus, biological nutrient processes
inherently have good sludge settling characteristics because
the environmental conditions favor floc-forming bacteria over filamentous bacteria.
22 -8 OPERATION AND MAINTENANCE
The most common operational problems encountered in the operation of activated sludge treat-
ment systems are bulking sludge, rising sludge, and Nocardia foam. These problems result from
changes in the microbial ecology of the reactor. They can be solved by maintaining an environ-
ment that inhibits unfavorable mi
crobial populations.
Bulking Sludge
A bulking sludge i s one that has poor settling characteristics and does not compact well. This
frequently leads to discharge of floc particles and consequ ent permit violations for suspended
solids . There are two principal types of sludge bulking. The first is caused by the growth of
filamentous organisms, and the second is c aused by water trapped in the bacterial floc, thus
reducing the density of the agglomerate and resulting in poor settling.
Filamentous bacteria have been blamed for much of the bulking problem in activated sludge.
Although filamentous organisms are effective in removing organic
matter, they have poor floc-
forming and-settling characteristics. Bulking may also be caused by a number of other factors,
including long, slow-moving collection-system transport; low available ammonia nitrogen when
the organic load is high; low pH, which could favor acid-favoring fungi; and the lack of macro-
nutrient
s, which stimulates predomination of the filamentous actinomycetes over the normal floc-
forming bacteria. The lack of nitrogen also favors slime-producing bac teria, which have a low
specific gravity, even though they are not filamentous. The multicellular fungi cannot compete
with the bacteria normally but can compete un
der specific environmental conditions, such as low
WASTEWATER MICROBIOLOGY 22-25
pH, low nitrogen, low oxygen, and high carbohydrates. As the pH decreases below 6.0, the fungi
are less affected than the bacteria and tend to predominate. As the nitrogen concentrations drop
below a BOD
5
: N ratio of 20:1, the fungi, which have a lower protein level than the bacteria, are
able to produce normal protoplasm, while the bacteria produce nitrogen-deficient protoplasm.
Limited DO concentration has been noted more frequ ently than any other cause. The DO must
be at least 2 mg/L.
Control of bulking sludge by design includes providing adequate aeration capacity, a wide
range of return an
d waste sludge pumping rates, and appropriate clarifier geometry. The use of an
activated sludge selector upstream of the conventional process is a relatively new design alternative
to provide an environment that favors floc-forming microorganisms over filamentous organisms.
Operational controls include adjustment of pH, BOD
5
to nitrogen ratio, and oxygen concentration.
Rising Sludge
A sludge that floats to the surface after apparently good settling is called a rising sludge. Rising
sludge results from denitrification, that is, reduction of nitrates and nitrites to nitrogen gas in the
sludge blanket (layer). Much of this gas remains trapped in the sludge blanket, causing globs of
sludge to rise to the surfa
ce and float over the weirs into the receiving stream.
Rising-slu dge problems can be overcome by increasing the rate of return sludge flow, by
increasing the speed of the sludge-collecting mechanism, by decreasing the mean cell residence
time, and, if possible, by decreas
ing the flow from the aeration tank to the offending tank.
Foaming
E xcessive foam that floats on aeration tanks has been attributed to two bacteria genera: Nocardia
and Microthrix parvicella. These are filamentous organisms. Nocardia growth is common where
surface scum is trapped in either aeration basins or secondary clarifiers.
The use of a selector process reduces the abilit
y of the Nocardia to c ompete for food, and,
thus, reduces foaming. The foaming may also be minimized by avoiding trapping the foam,
returning skimmings to the aeration basins, and by the use of surface sprays. The presence of
Nocardia has also been associated with fats and edible oils in was
tewater. The use of prevention
programs that prohibit disc harge of oil and grease from restaurants and meat packing facilities
can help control potential Nocardia problems (Metcalf & Eddy, 2003).
Visit the text website at www.mhprofessional.com/wwe for supplementary materials
and a gallery of photos.
22 -9 CHAPTER REVIEW
When you have completed studying this chapter, you should be able to do the following without
the aid of your textbook or notes:
1 . Explain the difference between the following organism classifications:
a. Autotrophs and heterotrophs
b. Obligate aerobes, obligate anaerobes, and facultative anaerobes
c. Aerobes and denitrifiers
22-26 WATER AND WASTEWATER ENGINEERING
2. Explain the role of electron carriers, the primary electron donor, and the terminal
electron acceptor.
3. For each type of decomposition (aerobic, anoxic, and anaerobic), list an electron
a cceptor, important end products, and relative advantages and disadvantages as a waste
treatment process.
4. Lis
t the growth requirements of bacteria and explain why a bacterium needs them.
5. Sketch and label the bacterial growth curve for a pure culture. Define or explain each
phase labeled on the curve.
6. Explain the difference between oxidation/synthesis and endogenous respiration.
7. Explain why alkalinity decreases in nitrifi
cation and increases in denitrification.
8. Describe the mechanistic model for BPR.
9. Describe the phenomenon known as bulking sludge and explain its primary cause.
10. Give two possible design remedies and two operational controls to minimize bulking
sludge.
11. Describe the phenomenon known as r
ising sludge and explain its primary cause.
12. Give two operational controls to correct a rising sludge problem.
W ith the aid of this text, you should be able to do the following:
13. Determine energy yield for oxidation-reduction reactions given half reactions and G
0
for the half reaction.
14. Estimate the oxygen demand of endogenous oxidation of cells (g of O
2
/g of cells)
assuming cell oxidation can be described by Equation 22-21 .
15. Show by calculation that the oxygen required for total oxidation of ammonia is 4.57 g
of O
2
/g of N.
16. Show by calculation that 7.14 g of alkalinity as CaCO
3
is required for each gram of
ammonia nitrogen (as N) that is oxidized.
17. Evaluate the effects of DO concentration on nitrification.
18. Show by calculation that 3.57 g of alkalinity as CaCO
3
is produced for each equivalent
of NO
3
-N) that is reduced.
19. Evaluate the effects of DO concentration on denitrification.
22 -10 PROBLEMS
Table 22-3 on page 22-27 provides half-reactions that are required to solve problems in this section.
22-1. Develop Equation 22-6 from the half-reactions and determine the energy yield.
22 -2. Develop Equation 22-7 from the half-reactions and determine the energy yield.