Water and Wastewater Engineering
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21-28 WATER AND WASTEWATER ENGINEERING
21-2. Repeat Problem 21-1 with an assumption that a coagulant has been added and
2 10
5
cm
3
/ cm
3
.
21-3. WEF (1998) recommends that the clarifier linear flow-through velocity be limited to
0.020 m/s to prevent scour. Using a for sticky particles and a Darcy-Weisback
friction factor of 0.03, estimate the specific gravity of a 200 m diameter particle
that will be scoured at the recommended veloc
ity.
21-4. If the flow-through velocity in Problem 21-3 is raised to 0.025 m/s, what size particle
will be scoured?
21-5. Using the data for Cynusoidal City (Problem 20-18 in Chapter 20), estimate the peak
four-hour flow rate and calculate the peak-to average ratio.
21-6. Using the data from Metuchen (Problem 20-19 in Chapter 20), estimate the peak
four-hour flow rate and c
alculate the peak-to average ratio.
21-7. Assuming there is no equalization and that the primary tank for Cynusoidal City
(Problem 20-18 in Chapter 20) has a design detention time at average flow of
2.0 hours, a volume of 2,880 m
3
, and a diameter of 29 m, determine the actual deten-
tion time and overflow rate for the peak four-hour flow rate.
21-8. Assuming there is no equalization and that the primary tank for Metuchen
(Problem 20-19 in Chapter 20) has a design detention time at average flow of 2.0 hours,
a volume of 704 m
3
, and a diameter of 15 m, determine the actual detention time and
overflow rate for the peak four-hour flow rate.
21-9. A monitoring well has been placed near a primary settling tank. For the sketch shown
in Figure P-21-9 determine the elevation of the water in the monitoring well that
should not be exceeded if the tank is to be dewatered.
21-10. For the sketch shown in Figure P-21-10, estim
ate the thickness of the concrete that
must be placed to prevent the tank from “floating” if the groundwater table rises to
a depth 1.0 m below grade.
21-11. Design a splitting box for two identical circular clarifiers. The total flow rate is
8,450 m
3
/ d. To complete the design, provide the diameter of the inlet pipe, the
dimensions of the splitting box, the weir length for each clarifier, and a sketch that
shows the plan, profile, and cross section with dimensions. Assume a peaking factor
of 3.0 and a head of 10 cm on a sharp-crested weir.
Concrete @
2,400 kg/m
3
0.45 m
Monitoring well
GWT
Elev. 183.30 m
5.0 m
FIGURE P-21-9
Tank flotation elevation.
Concrete @
2,400 kg/m
3
GWT
Elev. 3.66 m
1.0 m
4.7 m
FIGURE P-21-10
Tank thickness to prevent flotation.
PRIMARY TREATMENT 21-29
21-12. Design a splitting box for three circular clarifiers. The total design flow rate to the
three clarifiers is 43,000 m
3
/ d. Two identical clarifiers have diameters of 30 m. The
third clarifier has a diameter 15 m. The depth of each clarifier is 4.5 m. To complete
the design provide the diameter of the inlet pipe, the dimensions of the splitting box,
the weir length for each clarifier, and a sketch that shows the plan, profile, and cross
section with d
imensions. Assume a a head of 20 cm on a sharp-crested weir. Use the
design flow rate to size the inlet pipe.
21-13. Design a circular clarifier for a flow rate of 8,450 m
3
/ d. Assume the following:
Center feed
Overflow rate 30 m
3
/ d · m
2
S i de water depth 4.3 m
Feedwell detention time 20 min
EDI detention time 10 s
S l udge hopper volume 0.5 m
3
To complete the design provide the following:
D i a meter of tank
Diameter and depth of feedwell
Diameter and depth of EDI
D i mensions of sludge hopper
Check of velocity across sludge zone
Calculation of the weir loading rate
A sketch of the plan and profile with dimensions and a detail of the weir
cross-section configu
ration.
21-14. Design a circular clarifier for a flow rate of 34,560 m
3
/ d. Assume the following:
Center feed
Overflow rate 30 m
3
/ d · m
2
S i de water depth 4.3 m
Feedwell detention time 20 min
EDI detention time 10 s
S l udge hopper volume 1.5 m
3
To complete the design provide the following:
D i a meter of tank
Diameter and depth of feedwell
Diameter and depth of EDI
D i mensions of sludge hopper
Check of velocity across sludge zone
Calculation of the weir loading rate
A sketch of the plan and profile with dimensions and a detail of the weir
cross-section configu
ration.
21-30 WATER AND WASTEWATER ENGINEERING
21-8 REFERENCES
Albertson, O. E. and P. Alfonso (1995) “Clarifier Performance Upgrade,” Water Environment and
Technology, March, pp. 56–59.
ATV (1988) “Sludge Removal Systems for Secondary Sedimentation Tanks of Aeration Plants,”
Abwassertechnische Vereinigung, Korrespondenz Abwasser, vol. 35, no. 13, pp. 182–193.
Benefield, L. D., J. F. Judkins, Jr., and A. David Parr (1984) Treatment Plant Hydraulics for
Environmental Engineers, Prentice-Hall, Englewood Cliffs, New Jersey, pp. 108–118.
Camp, T. R. (1936) “A Study of the Rational Design of Settling Tanks,” Sewage Works Journal, vol. 8,
no. 9, pp. 742–758.
Camp, T. R. (1942) “Grit Chamber Design,” Sewage Works Journal, vol. 14, pp 368–381.
Daukss, P. and M. Lunn (2007) “Best Practices for Secondary Clarifier Performance,” presented
at Michigan Water Environment Association Annual Meeting, Boyne Highlands,
Michigan.
Davis, M. L. and D. A. Cornwell (2008) Introduction to Environmental Engineering, McGraw-Hill,
New York, p. 447.
GLUMRB (2004) Recommended Standards for Wastewater Facilities, Great Lakes–Upper Mississippi
River Board of State and Provincial Public Health and Environmental Managers, Health Education
Services, Albany, New York, pp. 70-1–70-7.
Kawamura, S. (1981) “Hydraulic Scale Model Simulation of the Sedimentation Process,” Journal of
American Water Works Association, vol. 73, no. 7, pp. 372–379.
Kawamura, S. and J. Lang (1986) “Re-evaluation of Launders in Re
ctangular Sedimentation Basins,”
Journal of Water Pollution Control Federation, vol. 58, no. 12, pp. 1,124–1,128.
Krebs, P., D. Vischer, and W. Gujer (1992) “Improvement of Secondary Clarifier Efficiency by by Porous
Walls,” Water Science Technology, vol. 26, nos. 5/6, pp. 1,147–1,156.
Metcalf & Eddy (1991) Wastewater Engineering: Collection and Pumping of Wastewater, McGraw-Hill,
New York, p. 109.
Metcalf & Eddy (2003) Wastewater Engineering: Treatment and Reuse, 4th e
d., McGraw-Hill, Boston,
Massachusetts, pp. 396–417.
O’Melia, C. R. (1972) “Coagulation and Flocculation,” in W. J. Weber, Jr. (ed.), Physiochemical
Processes for Water Quality Control, Wiley-Interscience, New York, pp. 94–95.
Parker, D. S., W. J. Kaufmann, and D. Jenkins (1972) “Floc Breakup in Turbulent Flocculation
Processes,” Journal of Sanitary Engineering Division, American Society of Civil Engineers, vol. 98,
pp.79–99.
Parker, D. S., M. Esquer, M. Hetherington et al. (2000) Assessment and Optimization of a Chemically
Enhanced Primary Treatment System,” Proceedings of the 73rd Annual Water Environment
Federation Technical Exposition and Conference, Anahiem, California, October 14–18, Water
Environment Federation, Alexandria, Vergenia.
Pettit, M. V. (2006) “Rectangular Clarifiers,” in Clarifier Design, 2nd ed., Water Environment Federation
Manual of Practice no. FD-8, pp. 499–581.
Reardon, R. D. (2006) “High-Rate and Wet Weather Clarifier Design Concepts and Considerations,”
Clarifier Design, 2nd ed., Water Environment Federation Manual of Practice no.
FD-8, p. 71–77.
Stukenberg, J. R., L. C. Rodman, J. E. Touslee (1983) “Activated Sludge Clarifier Improvements,”
Journal of Water Pollution Control Federation, vol. 55, no. 4, pp. 341–348.
Tekippe, R. J. (2006) “Circular Clarifiers,” in Clarifier Design, 2nd
ed., Water Environment Federation
Manual of Practice no. FD-8, pp. 397–488.
U.S. EPA (1974) Process Design Manual for Upgrading Existing Wastewater Treatment Plants, United
State Environmental Protection Agency, Washington, D.C., pp. 6–7.
PRIMARY TREATMENT 21-31
U.S. EPA (1975) Process Design Manual for Suspended Solids Removal, U.S. Environmental Protection
Agency Report No. EPA-625/1-75-003a, Washington, D.C.
Wahlberg, E. J., T. M. Keinath, and D. S. Parker (1994) “Influence of Activated Sludge
Flocculation Time on Secondary Clarific ation” Water Environment Research, vol. 66, no. 6,
pp. 779–786.
Wahlberg, E. J. (2006) “Primary Clarifier Design Concepts and Considerations,” in Clarifier Design,
2nd ed., Water Environment Federation Manual of Prac
tice no. FD-8, pp. 9–42.
WEF (1998) Design of Municipal Wastewater Treatment Plants, 4th ed., Water Environment Federation
Manual of Practice 8, Alexandria, Virginia, pp. 10-1–10-70.
WEF (2006) in Clarifier Design, 2nd ed., Water Environment Federation Manual of Practice
no. FD-8.
Y o ung, J. C., J. L. Cleasby, and E. R. Baumann (1978) “Flow and Load Variations in Treatment Plant
Design,” Journal of the Environmental Engineering Divis
ion, American Society of Civil Engineers,
vol. 104, EE2, pp. 289–303.
22-1
22 -8 OPERATION AND MAINTENANCE
22 -9 CHAPTER REVIEW
22 -10 PROBLEMS
22 -11 DISCUSSION QUESTIONS
22 -12 REFERENCES
WASTEWATER MICROBIOLOGY
22-1 INTRODUCTION
22 -2 ROLE OF MICROORGANISMS
22-3 CLASSIFICATION OF MICROORGANISMS
22 -4 MICROBIAL BIOCHEMISTRY
22 -5 POPULATION DYNAMICS
22 -6 DECOMPOSITION OF WASTE
22 -7 MICROBIOLOGY OF SECONDARY
TREATMENT UNIT PROCESSES
22
CHAPTER
22-2 WATER AND WASTEWATER ENGINEERING
22 -1 INTRODUCTION
This chapter provides an introduction to wastewater microbiology. Emphasis is placed on micro-
bial biochemistry because it forms the basis for selecting the appropriate unit process for treating
the wastewater as well as establishing the fundamental relationships for design calculations.
22-2 ROLE OF MICROORGANISMS
The stabilization of wastewater is accomplished biologically using a variety of microorganisms.
The microorganisms convert colloidal and dissolved carbonaceous organic matter into various
gases and into protoplasm. * Because protoplasm has a specific gravity slightly greater than that
of water, it c
an be removed from the treated liquid by gravity settling.
It is important to note that unless the protoplasm produced from the organic matter is
removed from the solution, complete treatment will not be accomplished because the proto-
plasm, which itself is organic, will be measured as BOD in the effluent. If the protoplasm is not
removed, the only treatment that will be achieved is that associated with the bacterial conversion
of a portion of the organic matter originally present to various gaseous end prod ucts.
In addition to stabilization of carbonaceous organic matter, some organisms can remove
nutrients that are responsible for eutrophication. A focus of the following discussion is the
explanation of the environmental factors that promote the growth and biochemistry of these
organisms.
22 -3 CLASSIFICATION OF MICROORGANISMS
Classification by Energy and Carbon Source
The relationship between the source of carbon and the source of energy for the microorganism
is important. Carbon is the basic building block for cell synthesis. A source of energy must be
obtained from outside the cell to enable synthesis to proceed. The goal in wastewater treatment
is to convert both the carbon and
the energy in the wastewater into the cells of microorganisms,
which can be removed from the water by settling or filtration. Therefore, the processes are
designed and operated to encourage the growth of organisms that use organic material for both
their carbon and energy source.
If microorganisms use organic material as a supply of carbon, they are c
alled heterotrophic.
Autotrophs require only CO
2
to supply their carbon needs. Organisms that rely only on light for
energy are called phototrophs. Chemotrophs e xtract energy from organic or inorganic oxidation/
reduction reactions. Organotrophs use organic materials, while lithotrophs o xidize inorganic
compounds (Rittmann and McCarty, 2001).
Classification by Oxygen Relationship
Bacteria also are classified by their ability or inability to utilize oxygen in oxidation-reduction
reactions. Obligate aerobes are microorganisms that must have oxygen. When wastewater contains
oxygen and can support obligate aerobes, it is called aerobic.
*In the lexicon of wastewater treatment, this is called biosolids, or, more colloquially, sludge.
WASTEWATER MICROBIOLOGY 22-3
Obligate anaerobes are microorganisms that cannot survive in the pres ence of oxygen.
Wastewater that is devoid of oxygen is called anaerobic. Facultative anaerobes can use oxygen
in oxidation/reduction reactions and, under certain conditions, they can also grow in the absence
of oxygen.
Under anoxi
c c onditions, a group of facultative anaerobes called denitrifiers utilize nitrites
(NO )
2
and nitrates (NO )
3
instead of oxygen. Nitrate nitrogen is converted to nitrogen gas in the
absence of oxygen. This process is called anoxic denitrification.
Classification by Temperature
Each species of bacteria reproduces best within a limited range of temperatures. Four temperature
ranges are used to classify bacteria. Those that grow best at temperatures below 20 C are called
psychrophiles. Mesophiles grow best at temperatures between 25 C and 40 C. Between 45 C and
60 C, the thermoph
iles grow bes t. From about 60 C to near boiling, hyperthermophiles grow
best. The growth range of facultative thermophiles e xtends from the thermophilic range into the
mesophilic range. Growth is not limited to these ranges. Bacteria will grow at slower rates over a
larger range of temperatures and will survive at a very large range of temperatures. For example,
Escherichia coli, classified as a
mesophile, will grow at temperatures between 20 C and 50 C
and will reproduce, albeit very slowly, at temperatures down to 0 C. If frozen rapidly, they and
many other microorganisms can be stored for years with no significant death rate. Once the opti-
mum temperature range is exceeded, growth rate drops off rapid
ly due to the denaturation of key
proteins.
Some Microbes of Interest in Wastewater Treatment
Bacteria. The highest population of microorganisms in a wastewater treatment plant will
belong to the bacteria. They are single celled organisms that use soluble food. Conditions in the
treatment plant are adjusted so that chemoheterotrophs predominate. No particular species is
selected as “the best.”
Fungi. Fungi are multicellular, nonphotosy
nthetic, heterotrophic organisms. Fungi are obligate
aerobes that reproduce by a variety of methods including fission, budding, and spore formation.
Their cells require only half as much nitrogen as bacteria so that in a nitrogen deficient wastewater,
they predominate over the bacteria (McKinney, 1962).
Algae. This group of microorganisms are photoautotroph
s and may be either unicellular
or multicellular. Because of the chlorophyll contained in most species, they produce oxygen
through photosynthesis. In the presence of sunlight, the photosynthetic production of oxygen is
greater than the amount used in respiration. At night they use up oxy
gen in respiration. If the
daylight hours exceed the night hours by a reasonable amount, there is a net production of
oxygen. Algae are of benefit in stabilization lagoons for wastewater treatment when they supply
oxygen in excess of respiration. Other than production of oxygen, they do not contribute to the
stabilization of waste because they use carbon dioxid e or bicarbonates
as a sourc e of carbon
rather than organic carbon. They are a liability when they leave in the lagoon effluent because
they contribute to the total suspended particulate concentration and may cause discharge limits
to be exceeded.
22-4 WATER AND WASTEWATER ENGINEERING
Protozoa. Protozoa are single-celled organisms that can reproduce by binary fission ( dividing
in two). Most are aerobic chemoheterotrophs, and they often consume bacteria. They are desirable
in wastewater effluent because they act as polishers in consuming the bacteria.
Rotifers and Crustaceans. Both rotifers and crustaceans are ani
mals—aerobic, multicellular
chemoheterotrophs. The rotifer derives its name from the apparent rotating motion of two sets of
cilia on its head. The cilia provide mobility and a mechanism for catching food. Rotifers consume
bacteria and small particles of organic matter.
Crustaceans, a group that include
s shrimp, lobsters, and barnacles, are characterized by their
shell structure. They are a source of food for fish and are not found in wastewater treatment
systems to any extent except in under-loaded lagoons. Their presence is indicative of a high level
of dissolved oxygen and a ver
y low level of organic matter.
22 -4 MICROBIAL BIOCHEMISTRY
Energy Capture
Living organisms capture energy released from oxidation-reduc tion reactions. Enzymes are the
organic catalysts produced by microorganisms and used by them to speed the rate of energy-
yielding and cell-building reactions. The major source of energy is o
xidation-reduction reactions
that involve transfer of electrons from one atom to another or from one molecu le to another.
Electron carriers move the electrons from one compound to another. The initial electron donor
is called the primary electron donor. The final electron acceptor is called the terminal electron
acceptor. In aerobic systems, the spent electron combines with m
olecular oxygen to form water.
The electron carriers may be divided into two classes: those that are diffusible throughout the
cell’s cytoplasm and those that are attached to enzymes in the cytoplasmic membrane. The diffusible
carriers include the coenzymes nicotinamide-adeni
ne dinucleotide (NAD
) and nicotinamide-
adenine dinucleotide phosphate (NADP
). NAD
i s involved in energy-generating ( catabolic )
reactions. NADP
+
i s involved in biosynthetic ( anabolic ) reactions. Electron carriers attached to
the cytoplasmic membrane include NADH dehydrogenases, flavoproteins, cytochromes, and
quinonnes (Rittmann and McCarty, 2001).
The reactions of NAD
+
and NADP
+
are
NAD H e NADH H
22
(22-1)
NADP H e NADPH H
22
(22-2)
NAD
(or NADP
) extracts two protons and two electrons from a molecule that is being
oxidized. In turn they are converted to the reduced forms NADH and NADPH, respectively. The
reaction free energy for each of thes e reactions is 62 kJ. This means that energy must be
taken from the organic molecule in order for NADH (or NADPH) to be formed. When NADH
(or NADPH) give
s up the electrons to another carrier, it is reduced back to NAD
(or NADP
). It
also gives up the chemical energy.
If oxygen is the terminal electron acceptor, the energy that is released can be determined
from the overall free energy of the NADH and O
2
half reactions (Rittmann and McCarty, 2001):
NADH H NAD H e
22
(22-3)
WASTEWATER MICROBIOLOGY 22-5
1
2
22
22
OH e HO
(22-4)
Net NADH O H NAD H O:
1
2
22
(22-5)
The reaction free energy for 22-3 and 22-4 is 62 kJ and 1 57 kJ, respectively. In an aerobic
system, the energy that is transferred from the organic chemical to NADH is ultimately released
to oxygen. The overall energy yield for use by the organism is 219 kJ/mole of NADH.
Nitrate, sulfate, and carbon dioxide are other i
mportant terminal electron acceptors in waste-
water treatment. The overall oxidation-reduction reactions when these electron acceptors accept
the electrons from NADH are (Rittmann and McCarty, 2001):
NADH NO H NAD N H O
2
5
7
5
1
5
6
5
3 22
(22-6)
NADH SO H NAD H S HS H O
4
2
1
4
11
8
1
8
1
8
22
(22-7)
NADH CO H NAD CH H O
1
4
1
4
1
2
242
(22-8)
The reac tion free energy for these reactions is 206 kJ, 20 kJ, and 1 5 kJ, respectively.
When nitrate is the terminal electron acceptor, the system is called anoxic. When sulfate or car-
bon dioxide is the terminal electron acceptor, the system is called anaerobic. This energy analy-
si
s indicates that the energy available with nitrate as the electron acceptor is similar to that with
oxygen, but sulfate and carbon dioxide yield much less energy per NADH.
The practical implication of these calculations of available energy is that there is a
hierarchy of oxidation-reduction reactions. Because aerobic oxi
dation provides more energy
for microorganism growth, it will proceed in preferenc e to anoxic (nitrate) oxidation. Like-
wise, anoxic oxidation will proceed in preference to anaerobic (sulfate and carbon dioxide)
oxidation.
The energy is captured by the organism by transferring the energy from intermediate electron
carriers
to energy carriers. The primary example of an energy carrier is adenosine triphosphate
(ATP). When energy is released from an electron carrier, it is used to add a phosphate group to
adenosine diphosphate (ADP):
ADP H PO ATP H O
3 42
(22-9)
In this reaction ADP acquires 32 kJ while NADH has given up more than six times this
amount of energy when oxygen is the terminal electron acceptor. Theoretically, under aerobic
conditions, six moles of ATP could be form ed from each mole of NADH. Because the actual
reaction
s do not capture 100 percent of the standard free energy, only three m oles of ATP are
formed.
22-6 WATER AND WASTEWATER ENGINEERING
Metabolism
The sum total of all the chemical processes of the cell is called metabolism. It may be separated
into catabolism, which is the process of obtaining energy, and anabolism, which is the process of
synthesis of cellular components. Both the catabolic processes and the anabolic processes
are very
complex. A simplified overview of the catabolic processes is presented in the following paragraphs.
The general stages of catabolism of fats, carbohydrates, and proteins is diagramed in Figure 22-1 .
Hydrolysis o ccurs in Stage I. It may be described as the splitting of a polymer by adding water to a
covalent bond
( Figure 22-2 ). The reaction is catalyzed by a hydrolyase enzyme.
The formation of acetyl-CoA * from fatty acids and glucose is illustrated in Figures 22-3
and 22-4 .
A n e xpanded view of the citric acid cycle (also known as the Krebs cycle after its author, or
the tricarboxylic acid cycle ) i
s shown in Figure 22-5 . Two new compounds are introduced in the
diagram: flavin adenine dinucleotide (FAD), a less energetic carrier than NAD
, and guanosine
triphosphate (GTP), an analog of ATP.
Fats Carbohydrates Proteins
Fatty acids
and glycerol
Glucose and
other sugars
acetyl-CoA
CoA
CO
2
e
Citric
acid
cycle
CoA
Stage I
hydrolysis
Stage II
acetyl-CoA
formation
Stage III
acetyl-CoA
oxidation
e
Amino
acids
FIGURE 22-1
The three general stages of catabolism of fats, carbohydrates, and proteins under
aerobic conditions. Reversing the processes gives anabolism. ( Source: Rittmann and
McCarty, 2001.)
*CoA is coenzyme A. An enzyme may be considered an organic catalytic agent. It consis ts of a protein portion ( apoenzyme )
an
d, in some cases, a prosthetic group ( coenzyme ) that is part of the enzyme as well.