Water and Wastewater Engineering
Подождите немного. Документ загружается.
WASTEWATER MICROBIOLOGY 22-27
22 -3. Develop Equation 22-8 from the half-reactions and determine the energy yield.
22 -4. Determine by calculation the mass of oxygen required for total oxidation of ammonia
in g of O
2
/g of N.
22-5. Determine by calculation the alkalinity in g as CaCO
3
/ g of ammonia nitrogen, as
N, that is required to buffer the total oxidation reaction.
22-6. If the influent ammonium concentration is 21.8 mg/L, estimate the amount of alka-
linity (in mg/L) that must be added to buffer the oxidation reaction assuming that a
residual alkalinity of 80 mg/L as CaCO
3
is required to keep the pH at approximately
7. Assume the influent alkalinity is 250 mg/L as CaCO
3
.
22 -7. If in Problem 22-6 , the amount of alkalinity in the raw wastewater is 305 mg/L as
CaCO
3
, what will the pH be after nitrification? Assume that the CO
2
concentration in
the wastewater resulting from BOD oxidation and ammonia nitrification is 25 mg/L.
Use the following equation for the estimate:
pH log
HCO
HCO
3
pK
1
2 3
[]
[]
⎧
⎨
⎩
⎫
⎬
⎭
where pK
1
6.38.
22 -8. Using a value of K
0
0.5, plot a curve of
n
versus DO using Equation 22-28 . Metcalf
& Eddy (2003) suggests that nitrification rates increase up to DO levels of 3 to 4 mg/L.
Based on your plot, do you agree with this assessment?
22-9. Determine by calculation the alkalinity in g as CaCO
3
/g of nitrate nitrogen that may
be recovered from the reduction reaction.
TABLE 22-3
Half-reactions and their Gibb’s standard free energy at pH7.0
Half-reaction
G
0
,
kJ per electron
equivalent
1
8
2
1
8
4
1
4
2
CO H e CH H O
25.53
NAD
+
+ 2H
+
+ 2e
NADH + H
+
62
NADP
+
+ 2H
+
+ 2e
NADPH + H
+
62
1
2
1
2
1
2
2
NO H e NO H O
3 2
41.65
1
5
6
5
1
10
2
3
5
2
NO H e N H O
3
72.20
1
8
2
19
16
1
16
2
1
16
1
2
2
SO H e H S HS H O
4
20.85
1
6
2
5
4
1
12
2
1
12
1
2
2
0SO H e H S HS H
3
011.03
Sources: Metcalf & Eddy, 2003; Rittmann and McCarty, 2001.
22-28 WATER AND WASTEWATER ENGINEERING
22-10. Using a value of K #
O
0.10 mg/L, estimate the reduction in substrate utilization ( r
su
)
in nitrate reduction if the DO concentration is 0.10 mg/L.
22-11. Using a value of K #
O
0.10 mg/L, estimate the reduction in substrate utilization ( r
su
)
in nitrate reduction if the DO concentration is 0.20 mg/L.
22 -11 DISCUSSION QUESTIONS
22 -1. Consider the following experiment:
A glucose solution is placed in a series of BOD bottles with a seed of wastewater
microorganisms. The solution contains nitrate ions, sulfate ions, and oxygen. The
BOD bottles are sealed. Over several days to several weeks time, sequential bot-
tles are opened. Each is analyzed
for
NO
3
, O
2
, and SO
4
2
List the order in which
these compounds are expected to “disappear” and explain why.
22 -2. You are touring the research labs of the environmental engineers at your university.
Two biological reactors are in a controlled temperature room that has a temperature
of 35 C. Reactor A has a strong odor. Reactor B ha
s virtually no odor. What electron
acceptors are being used in each reactor?
22 -3. In its start-up phase a BPR process is not removing phosphorus as anticipated by
design calculations. What laboratory analyses would you ask to be performed to help
assess the problem? Why?
22 -4. Large globs of m
atlike sludge are observed in the supernatant of the secondary clari-
fier of an activated sludge plant.
This is a
sludge.
22 -12 REFERENCES
Barker, P. S. and P. L. Dold (1997) “General Model for Biological Nutrient Removal in Activated Sludge
Systems: Model Presentation,” Water Environment Research, vol. 69, no. 5, pp. 969–984.
Comeau, Y., K. J. Hall, R. E. W. Hancock, and W. K. Oldham (1986) “Biochemical Model for Enhanced
Biological Phosphorus Removal,” Water Research, vol. 20, no. 12, pp. 1,511–1,522.
Crites, R. and G. Tchobanoglous (1998) Small Decentralized Wastewater Management Systems, WCB
McGraw-Hill, Boston, Massachusetts, pp. 438.
Curds, C. R. (1973) “A Theoretical Study of Factors Influencing the Microbial Population Dynamics of
the Activated Sludge Process,” Water Research, Vol. 7, pp. 1269–1284.
Davis, M. L. and D. A. Cornwell (2008) Introduction to Environmental Engineering, McGraw-Hill, New
York. p. 456.
Lawrence, A. W. and P. L. McCarty (1970) “A Unified Basis for Biological Treatment Design and
Operation,” Journal of Sanitary Engineering Division, American Society of Civil Engineers,
vol. 96, no. SA3, pp. 757–778.
M cKinney, R. E. (1962) Microbiology for Sanitary Engineers, McGraw-Hill, NewYork, p. 40.
Mino, T., A. Viswanath, T. Yoshiaki, and T. Matsuo (1987) “Effect of Phosphorus Accumulation on
Acetate Metabolism in the Biological Phosphorus Removal Process,” Proceedings of Biological
Phosphorus Removal Wastewater International Association, Water Polluti
on Research Control
Specialty Conference, Rome, Italy.
WASTEWATER MICROBIOLOGY 22-29
Monod, J. (1949) “The Growth of Bacterial Cultures,” Annual Review of Microbiology, vol. 3, pp. 371–394.
Metcalf & Eddy (2003) Wastewater Engineering: Treatment and Reuse, 4th ed., McGraw-Hill, Boston,
Massachusetts, pp. 563–886.
Pelczar, M. J. and R. D. Reid (1958) Microbiology, McGraw-Hill, NewYork, p. 424.
Randall, C. W., J. L. Barnard, and H. D. Stensel (1992) “Design and Retrofit of Wastewater Treatment
Plants for Biological Nutrient Removal,” Water Quality Management Library, vol. 5, Technomic
Publishing Co., Lancaster, PA.
Rittmann, B. E. and P. L. McCarty (2001) Environmental Biotechnology: Principles and Applications,
McGraw-Hill, Boston, Massachusetts, pp. 13–36, 293.
S molders, G. J. F., J. van derMeij, M. C. M. van Loosdrecht, and J. J. Heijnen (1994a) “Model of the
Anaerobic Metabolism of the Biological Phosphorus Rem
oval Process: Stoichiometry and pH
Influences,” Biotechnology and Bioengineering, vol. 43, p. 461.
S molders, G. J. F., J. van derMeij, M. C. M. van Loosdrecht, and J. J. Heijnen (1994b) “Stoichiometric
Model of Aerobic Metabolism of the Biological Phosphorus Removal Process,” Biotechnology and
Bioengineering, vol. 44, p. 837.
Stensel, H. D. and G. Horne (2001) “Evaluation of Denitrification Kinetic
s at Wastewater Treatment
Facilities,” Research Symposium Proceedings, 73rd Annual Water Environment Federation
Conference, Anaheim, California, October 14–18.
Stephens, H. L. and H. D. Stensel (1998) “Effect of Operating Conditions on Biological Phosphorus
Removal,” Water Environment Research, vol. 70, no. 3, pp. 362–369.
WEF (2006) Biological Nutrient Removal (BNR) Operation in Wastewater Treatment Plants, Water
Environment Federation, Water Environment Federation Manual of Practice 30, Alexandria,
Virgina, p. 112.
Wentzel, M. C., L. H. Lotter, R. E. Loewenthal, an
d G. v. R. Marais (1986) “Metabolic Behavior of
Acinetobacter spp. in Enhanced Biological Phosphorous Removal—A Biochemical Model,” Water,
South Africa, vol. 12, p. 209.
Wentzel, M. C., R. E. Loewenthal, G. A. Ekama, and G. v. R. Marais (1989) “Enhanced Polyphosphate
Organism Cultures in Activated Sludge Systems—Part 1: Enhanced Culture Development,” Water,
South Africa, vol. 14, p. 81–92.
23-1
23-7 SUSPENDED GROWTH DESIGN PRACTICE
23-8 MEMBRANE BIOREACTOR DESIGN
PRACTICE
23-9 CHAPTER REVIEW
23-10 PROBLEMS
23-11 DISCUSSION QUESTIONS
23-12 REFERENCES
SECONDARY TREATMENT BY SUSPENDED
GROWTH BIOLOGICAL PROCESSES
23-1 INTRODUCTION
23-2 PROCESSES FOR BOD REMOVAL
AND NITRIFICATION
23-3 PROCESSES FOR DENITRIFICATION
23-4 PROCESSES FOR PHOSPHORUS REMOVAL
23-5 BIOLOGICAL TREATMENT WITH
MEMBRANE SEPARATION
23-6 SUSPENDED GROWTH DESIGN
PRINCIPLES
23
CHAPTER
23-2 WATER AND WASTEWATER ENGINEERING
23-1 INTRODUCTION
The major purpose of conventional s econdary treatment is to oxidize the readily biodegradable
BOD that escapes primary treatment and to provide further removal of suspended solids. Because
of the increasing recognition of the deleterious effects of nutrients, secondary treatment often
inclu
des treatment of nitrogen and phosphorus. Processes for biological treatment of BOD and
these nutrients are included in this chapter.
This chapter focuses on processes utilizing suspensions of microorganism s ( suspended
growth ) to treat wastewater. Those processes that utilize films of microorganisms attac
hed to
inert media ( attached growth ) to treat wastewater are discussed in Chapter 24.
This chapter is organized into three parts: processes for treatment, design principles, and
design practice.
23-2 PROCESSES FOR BOD REMOVAL AND NITRIFICATION
Oxidation Ponds
Treatment ponds have been used to treat wastewater for many years, particularly as wastewater
treatment systems for small communities. Many terms have been used to describe the different
types of systems employed in wastewater treatment. Oxidation pond has been wid ely use
d as a
collective term for all types of pond s. Originally, an oxidation pond was a pond that received
partially treated wastewater, whereas a pond that received raw wastewater was known as a
sewage lagoon.
In this discussion, oxidation pond is used as an all-inclusive term that refers to a pond or
lagoon used to treat organic waste by biologi
cal and physical processes. These processes would
commonly be referred to as self-purification if they took place in a stream. Waste stabilization
pond and stabilization lagoon are synonymous terms. Oxidation ponds are further classified as
follows
(Caldwell et al., 1973):
1 . Aerobic ponds: Shallow ponds, less than 1 m in depth, where dissolved oxygen is main-
tained throughout the entire depth, mainly by the action of photosynthesis.
2. Facultative ponds: Ponds 1 to 2.5 m deep, which have an anaerobic lower zone, a facul-
tative middle zone, and an aerobic upper zone maintained by photosynthesi
s and surface
reaeration.
3. Anaerobic ponds: Deep ponds that receive high organic loadings such that anaerobic
conditions prevail throughout the entire pond depth.
4. Maturation or tertiary ponds: Ponds used for polishing effluent from other biological
processes. Dissolved oxygen is furnished through photosynthesis and
surface reaeration.
This type of pond is also known as a polishing pond.
5. Aerated lagoons: Ponds oxygenated through the action of surface or diffused air aeration.
Approximately 40 percent of the municipal wastewater treatment plants in the United States are
oxidation ponds. Of the five types, facultative ponds and aerated lagoons predom
inate with facul-
tative ponds outnumbering aerated lagoons by almost three to one (WEF, 1998). While the ponds
are adequate for BOD oxidation, they do not provide biological nutrient removal.
SECONDARY TREATMENT BY SUSPENDED GROWTH BIOLOGICAL PROCESSES 23-3
Aerobic Ponds. The aerobic pond is a shallow pond in which light penetrates to the bottom,
thereby maintaining active algal photosynthesis throughout the entire system. During the daylight
hours, large amounts of oxygen are supplied by the photosynthesis process; during the hours of
darkness, wind mixing of the shallow water mass
generally provides a high degree of surface
reaeration. Stabilization of the organic material entering an aerobic pond is accomplished mainly
through the action of aerobic bacteria.
Facultative Ponds. These ponds are by far the most common type selected as wastewater treat-
ment systems for small communities. About 90 percent of these pond
s are located in communities
of 5,000 people or fewer. Facultative ponds are popular because their long retention times facilitate
the management of large fluctuations in wastewater flow and strength with no significant effect
on effluent quality. The operating and maintenance costs are less than those of other biological
systems that provide equivalent treatment. If land is
not expensive, the capital cost is less.
The pond oxidation processes range from aerobic in the upper levels to anaerobic in the
lower levels.
Anaerobic Ponds. The organic loading and the availability of dissolved oxygen determine
whether the biological activity in a treatment pond will occur under aerobic or anaerobic condi-
tions. A pond may be m
aintained in an anaerobic condition by applying a BOD
5
load that exceeds
oxygen production from photosynthesis. Photosynthesis can be reduced by decreasing the surface
area and increasing the depth. Anaerobic ponds become turbid from the presence of reduced
metal sulfides. This restrict
s light penetration to the point that algal growth becomes negligible.
Anaerobic ponds are used primarily as a pretreatment process and are particularly suited
for the treatment of high-temperature, high-strength wastewater. However, they have been used
successfully to treat municipal wastewater as well.
Maturation or Tertiary Ponds. These ponds receive wastewater effluent from secondary
treat
ment systems. They are not secondary treatment processes.
Aerated Lagoons. These ponds are oxygenated through the action of surface or diffused air
aeration. They are a type of activated sludge process. They differ primarily in that earth structures
are used and there is no recycling of biologic
al solids.
Activated Sludge
The ac tivated sludge process derives its name from the biological mass formed when air is
continuously injected into the wastewater. In this process, microorganisms are mixed thoroughly
with the organic compounds contained in wastewater under conditions
that stimulate their growth
through use of the organic compounds as substrate. As the microorganisms grow and are mixed
by the agitation of the air, the individual organisms flocculate to form an active mass of microbes
(biologic floc) called activated sludge.
In the conventional aerobic oxidation process, wa
stewater flows continuously into
an aeration tank ( Figure 23-1 ) where air is injected to mix the activated sludge with the
wastewater and to supply the oxygen needed for the organisms to oxidize the organic
compounds . The mixture of activated sludge and wastewater in the aeration tank is called
mi
xed liquor. The concentration of ac tive biomass is called mixed liquor volatile suspended
solids (MLVSS). The concentration of active biomass plus inert solids is termed mixed liquor
23-4 WATER AND WASTEWATER ENGINEERING
suspended solids (MLSS). In a conventional aerobic oxidation process, the mixed liquor flows
from the aeration tank to a second ary clarifier where the activated sludge is settled out. Most
of the settled sludge is returned to the aeration tank (it is called return sludge ) to maintain the
high population of m icrobes that permits rapid breakdown of the organi
c compounds. Because
more activated sludge is produced than is desirable in the process, some of the return sludge
is diverted or wasted to the sludge handling system for treatment and dis posal. The mean cell
residence time (MCRT or
c
), also called solids retention time (SRT) or sludge age, i s d efined
as the average amou nt of time that microorganisms are kept in the system.
In conventional activated sludge systems, the wastewater is typically aerated for six to eight
hours in long, rectangular aeration basins. Sufficient air is provid
ed to keep the sludge in suspension.
The air is injected near the bottom of the aeration tank through a system of diffusers. The volume
of sludge returned to the aeration basin is typically 20 to 30 percent of the wastewater flow.
The activated sludge process is controlled by wasting a portion of the microorganisms each
day
in order to maintain the proper amount of microorganisms to efficiently oxidize the biode-
gradable COD (bCOD). * Wasting means that a portion of the microorganisms is discarded from
the process. The discarded microorganisms are called waste activated sludge (WAS). A balance
is then achieved between growth of new organis
ms and their removal by wasting. If too much
FIGURE 23-1
A ctivated sludge aeration tank under air.
( Source: E. Lansing WWTP, photo courtesy of Harley Seeley of the Instructional Media Center, Michigan State
University.)
*See Table 18-4 in Chapter 18 for a complete list of abbreviations and their definitions.
SECONDARY TREATMENT BY SUSPENDED GROWTH BIOLOGICAL PROCESSES 23-5
sludge is wasted, the concentration of microorganisms in the mixed liquor will become too low
for effective treatment. If too little sludge is wasted, a large concentration of microorganisms will
accumulate and, ultimately, overflow the secondary tank and flow into the receiving stream.
There are many process
arrangements of the basic activated sludge process. Table 23-1 lists
some of the configurations used for oxidation of biodegradable COD and nitrification.
Because nitrifying bacteria grow much more slowly than heterotrophic bacteria, systems
designed for nitrification generally have much longer hyd
raulic and solids retention times. These
are discussed in the following paragraphs.
Plug Flow. This is the conventional arrangement where flow enters one end of a long narrow
tank and exits at the other ( Figure 23-2a ). The solid line in Figure 23-3 illustrates how substrate
and oxygen demand vary along the length of the tank. Oxygen demand is highe
st in the first
20 percent of the tank because of substrate oxidation. Oxygen demand along the remainder of
the tank is due to endogenous respiration.
If the concentration of substrate is high, it may lead to complete depletion of dissolved oxygen.
Oxygen depletion may be detrimental to some of the microbial population. It m
ay also result in
fermentation or partial oxidation that results in organic acid production and a drop in pH.
Step Feed. Although it is frequently called step aeration, step feeding is a more accurate descriptor
( Figure 23-2b ). In this modification of the plug flow configuration, the influent flows into the
aeration tank at several locations along its length. The benefit of this
arrangement is that it reduces
oxygen demand at the head end of the tank. This is shown by the small-dashed line in Figure 23-3 .
Complete Mix. This process is a completely mixed stirred tank reactor (CSTR). It is illustrated
in Figure 23-2c . Because the influent is
“immediately” diluted with the contents of the tank,
the substrate concentration and dissolved oxygen (DO) are uniform over the reactor volume. This
is illustrated by the large-dashed line in Figure 23-3 . While this arrangement overcomes the high
initial loading and DO problems in a plug flow system, its removal effi
ciency is not as high.
Oxidation Ditch. An oval or race-track channel equipped with mechanical aerators provides
the benefits of plug flow and completely mixed reactor in one tank ( Figu re 23-2d ). The energy
used for aeration also provides mixing. The mixed liquor completes the circuit in 5 to 15 m
inutes.
The flow in the channel dilutes the incoming wastewater by a factor of 20 to 30. As a result,
Plug flow (conventional)
Step feed (step aeration)
Complete mix
Oxidation ditch
Sequencing batch reactor
Staged activated sludge
Contact stabilization
Conventional with selector
Tapered aeration
Extended aeration
TABLE 23-1
Selected activated sludge configurations
for oxidation of biodegradable COD and nitrification
23-6 WATER AND WASTEWATER ENGINEERING
(a) Plug-flow (conventional) activated sludge
(b) Step-aeration activated sludge
Influent
Influent
Return activated sludge
Return activated sludge
(g) Contact-stabilization activated sludge
Stabilization tank
Contact tank
Influent
Effluent
(h) Activated sludge system with anoxic selector
Selector
Aeration tank
Influent
Return activated sludge
Effluent
Influent
Effluent
Sludge
Secondary
clarifier
Return activated sludge
Secondary
setting
Secondary
setting
Effluent
Effluent
(d) Oxidation ditch
Return activated sludge, Q
R
Stage 1
Influent
Effluent
Sludge
Secondary
clarifier
Air
Stage 2
Air
Stage 3
Air
Stage 4
Air
Settle
React/aeration
Decant Idle
Effluent
Fill
Influent
Air
(e) Sequencing batch reactor
(f) Staged activated sludge
(c) Complete mix activated sludge
Influent
Return activated sludge
Effluent
Secondary
setting
FIGURE 23-2
Selected activated sludge configurations for oxidation and nitrification.
( Sources: Adapted from Rittmann and McCarty, 2001 and Metcalf &
Eddy, 2003.)
SECONDARY TREATMENT BY SUSPENDED GROWTH BIOLOGICAL PROCESSES 23-7
120
100
Substrate (S, mg ⁄ L)
80
60
40
20
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
1,200
1,000
DO uptake (mg ⁄ L·d)
800
600
400
200
0
0 0.1
Relative distance from inlet
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
(a)
(b)
FIGURE 23-3
Changes in contaminant (substrate) concentration and oxygen (DO) uptake rate along
the reactor length for plug flow (PF, solid lines), step aeration (SA, small-dash lines),
and continuous-stirred tank (CSTR, large-dash lines) reactors for a typical loading
with a dilute wastewater. ( Source: Rittmann and McCarty, 2001.)
the process kinetics approach that of a completely m ixed reactor but with plug flow along the
channel. In typical designs, there is no primary treatment. Although the flow completes a circuit
in a short time, the hydraulic detention time is relatively long.
This system may be operated to achieve denitrification a
s well as BOD reduction. It is
relatively easy to operate and achieves better treatment than oxidation ponds. This process typi-
cally finds application in smaller, rural communities where space is not limited.
Sequencing Batch Reactor (SBR). The SBR is a completely mixed reactor that is operated
on a batch bas
is. It has application in small communities where space is limited and/or treatment
requirements do not permit the use of oxidation ponds. This system may be operated to achieve
denitrification as well as BOD reduction.
These systems have five steps that are carried out in a time
d sequence: (1) fill, (2) react
(aeration), (3) sedimentation, (4) decant the supernatant, and (5) idle ( Figure 23-2e ).
Staged Activated Sludge. In this system several completely mixed tanks are placed in series
( Figure 23-2f ). Although each tank is mixed, the contents do not mix a
mong them. Three or more
tanks in series approximate a plug-flow system. This system provides the advantage of the plug
flow system’s efficiency as well as that of the complete mix system’s capacity to deal with high
organic load and ability to maintain acceptable DO levels.
Contact Stabilization. Was
tewater is mixed with return activated sludge in a reactor that has
a relatively short detention time. This contact reactor takes advantage of the fac t that the most
readily biodegradable COD is oxidized or stored in a relatively short tim e and the fact that par-
ticulate COD is adsorbed on activated s lud
ge floc in the same time frame. The treated wastewater