Water and Wastewater Engineering
Подождите немного. Документ загружается.
HEADWORKS AND PRELIMINARY TREATMENT 20-51
Time Flow, m
3
/s BOD
5
, mg/L Time Flow, m
3
/s BOD
5
, mg/L
0000 0.0875 110 1200 0.135 160
0100 0.0700 81 1300 0.129 150
0200 0.052553 1400 0.123 140
0300 0.0414 35 1500 0.111 135
0400 0.0334 32 1600 0.103 130
0500 0.0318 42 1700 0.104 120
0600 0.0382 66 1800 0.105 125
0700 0.0653 92 1900 0.116 150
0800 0.113 125 2000 0.127 200
0900 0.131 140 2100 0.128 215
1000 0.135 150 2200 0.121 170
1100 0.137155 2300 0.110 130
20-20. Using a spreadsheet program you have written, compute and plot the unequalized
and the equalized hourly hydraulic loadings to the Cynusoidal City ( Problem 20-18 )
for the maximum day. Assume that the ratio of the maximum day flow rate to the
average day flow rate is 1.8. Determine the following ratios for h
ydraulic loading:
peak to average, minimum to average, peak to minimum.
20-21. Using a spreadsheet program you have written, compute and plot the unequalized and
the equalized hourly hydraulic loadings to the Metuchen WWTP ( Problem 20-19 ) for
the maximum day. Assume that the ratio of the maximu
m day flow rate to the aver-
age day flow rate is 2.2. Determine the following ratios for hydraulic loading: peak to
average, minimum to average, peak to minimum.
20-22. Using a spreadsheet program you have written, compute and plot the unequalized
and the equalized hourly BOD mass loadings to the Cynusoidal City WWTP
( Problem 20-18 ). Determine the following ratios
for BOD mass loading: peak to
average, minimum to average, peak to minimum.
20-23. Using a spreadsheet program you have written, compute and plot the unequalized and
the equalized hourly BOD mass loadings to the Metuchen WWTP ( Problem 20-19 ).
Determine the following ratios for BOD mass loading: peak to average, minimum to
average, peak to minimum.
20-24. Design an aerator system for Cynusoidal City’s equalization basin ( Problem 20-18 ).
Assume that the aerators in Table 20-13 are available and that Figure 20-18 applies. To
complete the design, specify the aerator by the unit’s power and provide a sketch of the
plan view of the basin with the location of the aerators and the dimensions of the basin.
20-25. Design an aerator syste
m for Metuchen’s equalization basin ( Problem 20-19 ). Assume
that the aerators in Table 20-13 are available and that Figure 20-18 applies. To com-
plete the design, specify the aerator by the unit’s power and provide a sketch of the
plan view of the basin with the location of the aerators and the dimensions of the basin.
20-52 WATER AND WASTEWATER ENGINEERING
20-11 DISCUSSION QUESTIONS
20-1. A Partial flume may be used to measure wastewater flow. True or false?
20-2. Using a sketch, show how the 10D rule is to be implemented in designing a bar rack.
20-3. A 100 mm diameter line has been installed to carry grit from an aerated grit chamber
to the grit washer. It often plugs and must be dismantled for cleaning. Suggest a
design that would minimize this difficulty.
20-4. Mixing must be provided in equalization tanks to prevent solids from settling. True
or false?
20-12 REFERENCES
Albrecht, A. E. (1967) “Aerated Grit Operation, Design and Chamber,” Water and Sewage Works,
vol. 114, no. 9, pp. 331–335.
ASTM (1991) Standard Test Method for Open Channel Flow Measurement of Water with the Parshall
Flume, American Society for Testing and Materials, ASTM D1941-91.
Burbano, M., W. McConnell, C. Knatz, D. Bisson, and B. Tarbuck (2009) “Affordable Modifications,”
Water Environment & Technology, vol. 21, no. 10, pp. 40–45.
Camp, T. R. (1942) “Grit Chamber Design,” Sewage Works Journal, vol. 14, pp. 368–381.
Daukss, P. (2006) “Design Considerations for Screening,” presented at Michigan Water Environment
Association Process Seminar, Grand Rapids, Michigan, November, 2.
E utek (2008) Eutek Systems, Hillsboro, Oregon.
Fair, G. M. and J. C. Geyer (1954) Water Supply and Waste-Water Disposal, John Wiley & Sons, New
York, p. 611.
Forstner, G. (2007) “Screen Selection Simplified,” Water Environment & Technology, October,
pp. 60–64.
Garbus, R. O. (2006) “Types of Pumps,” in G. M. Jones, (ed.), Pump Station Design, Elsevier, New York,
pp. 11.
37–11.39.
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. 60-1–60-9.
I mhoff, K. and K. R. Imhoff (1979) Taschbuch der stadtentwasserung, R. Oldenbourg Verlag, Munich.
ISO (1992) Measurement of Liquid Flow in Open Channels—Parshall and Sanitary Flumes, International
Organization of Standards, ISO 9826:1992(E).
Keller, J., C. Wallis-Lage, and I. Crabtree (2006) “The Fine Art of Screening,” Water Environment &
Technology, December, pp. 52–55.
LMNO (2008) Parshall Flume Calculations, Equations, Graphs Based on ISO 9826 Methodology, at
http://www.lmnoeng.com/Flume/parshall.htm .
Makie, A. and K. Oyler (2007) “The Industry’s Finest,” Water Environment & Technology, July,
pp. 69–74.
Metcalf & Eddy (1930) Sewerage and Sewage Disposal, McGraw-Hill, New York, p. 507.
Metcalf & Eddy (1972) Wastewater Engineering: Collection, Treatment, Disposal, McGraw-Hill, New
York, pp. 97–98.
Metcalf & Eddy (2003) Wastewater Eng
ineering: Collection, Treatment, Disposal, 4th ed., McGraw-Hill,
New York, pp. 315–344.
Morales, L. and D. Reinhart (1984) “Full-Scale Evaluation of Aerated Grit Chambers,” Journal of Water
Pollution Control Federation, vol. 56, no. 4, pp. 337–343.
HEADWORKS AND PRELIMINARY TREATMENT 20-53
Parshall, R. L. (1926a) Bulletin 336, Colorado Experiment Station, Bureau of Engineering, U.S.
Department of Agriculture, Fort Collins, Colorado.
Parshall, R. L. (1926b) “The Improved Venturi Flume,” Transactions of American Society of Civil
Engineers, vol. 89, p. 841.
Parshall, R. L. (1941) Farmer’s Bulletin 1683, U.S. Department of Agriculture, Washington, D.C.
Pugh, L. (2008) Personal communication.
Sawicki, J. M. (2004) “Aerated Grit Chambers Hydrauli
c Design Equation,” Journal of Environmental
Engineering, vol. 130, no. 9, pp. 1,050–1,058.
Spangler, J. (2006) “The Grit of the Nitty Gritty,” presented at Michigan Water Environment Association
Process Seminar, Grand Rapids, Michigan, November, 2.
Steel, E. W. and T. J. McGhee (1979) Water Supply and Sewerage, 5th ed., McGraw-Hill, New York,
pp. 456–467.
Stevens (1998) Stevens Water Resources Data Book, 6th ed., Stevens Water Monitoring Systems, Inc.,
Beaverton, Oregon.
U.S. EPA (1979) Process Design Manual for Sludge Treatment and Disposal, U.S. Environmental
Protection Agency
Pub. No. 625/1-79-011, Washington, D.C.
WEF (1998) Design of Municipal Wastewater Treatment Plants, 4th ed., Water Environment Federation
Manual of Practice 8, Alexandria, Virginia, pp. 9-1–9-58, 10-1–10-70.
Wilson, G., G. Tchobanoglous, and J. Griffiths (2007a) “The Nitty Gritty: Grit Sampling and Analysis,”
Water Environment & Technology, July, pp. 64–68.
Wilson, G., G. Tchobanoglous, and J. Griffiths (2007b) “The Nitty Gritty: Designing a Grit Removal
System,” Water Environment & Technology, Septe
mber, pp. 115–116.
Wilson, G., G. Tchobanoglous, and J. Griffiths (2007c) “The Nitty Gritty: Grit Washing System Design,”
Water Environment & Technology, October, pp. 81–84.
WPCF (1971) Aeration in Wastewater Treatment, Water Pollution Control Federation Manual of Practice
No. 5, Washington, D.C., p. 3.
21-1
21-6 CHAPTER REVIEW
21-7 PROBLEMS
21-8 REFERENCES
PRIMARY TREATMENT
21-1 INTRODUCTION
21-2 SEDIMENTATION THEORY
21-3 SEDIMENTATION PRACTICE
21-4 SEDIMENTATION BASIN DESIGN
21-5 OTHER PRIMARY TREATMENT
ALTERNATIVES
21
CHAPTER
21-2 WATER AND WASTEWATER ENGINEERING
21-1 INTRODUCTION
Primary treatment is the first process in the wastewater treatment plant to remove a significant
fraction of organic particulate matter (suspended solids). These suspended solids contribute to
biochemical oxygen demand (BOD
5
) of the wastewater. Thus, removing suspended solids also
reduces BOD
5
. The process is important because the reduction of suspended solid s and BOD
5
lowers the oxygen demand, decreases the rate of energy consumption, and reduces operational
problems with downstream biological treatment processes. Primary treatment also serves the
i mportant function of removing scum and inert particulate matter that was not removed in the grit
chamber. The sc
um consists of grease, oil, plastic, leaves, rags, hair, and other floatable material.
The principal form of primary treatment is sedimentation. Consequently, this process is often
referred to as primary sedimentation. It is the oldest and most widely used unit operation in waste-
water treatment. Other mo
difications and alternatives that have seen increasing use are enhanced
sedimentation, fine screens, and ballasted flocculation/sedimentation.
The major focus of this chapter is on the design of primary clarifiers. The use of plate and
tube settlers, enhanced sedimentation, fine screens, and ballas
ted flocculation/sedimentation will
also be discussed.
21-2 SEDIMENTATION THEORY
At its most fundamental level, the controlling parameter in primary sedimentation is the settling
velocity of the particle. Although all “types” of settling probably occur in a primary clarifier, the
behavior of settling particles is dominated by the phenomenon commonly described as Type II
(flocculant) sedi
mentation. This phenomenon is described in Chapter 10.
Nonideal Behavior
A s noted in Chapter 10, Camp (1936) demonstrated that the removal efficiency of discrete particles
of uniform size, density, and shape may be established by design of the overflow rate. Wastewater
solids seldom fit the description of discrete particles. Because they are floccu
lating, the detention
time and depth of the tank become important variables both in theory and in practice.
Detention Time. As a general rule, the rate of flocculation of particles by fluid motion ( ortho-
kinetic flocculation) may be described as first order with respect to the concentration of particles,
the velocity gradient of the fluid motion, and the flo
c volume fraction (O’Melia, 1972):
dN
dt
GN4
0
(21-1)
where dN / dt change in the number of particles per unit volume with time
collision efficiency
G velocity gradient, s
1
volume of colloidal particles per unit volume, dimensionless
N
0
initial number of particles per unit volume, m
3
Thus, up to a point in time, a longer detention time will increase flocculation and improve the
efficiency of the settling tank as the particles flocculate and settle more rapidly. Because the
PRIMARY TREATMENT 21-3
number of particles decreases with time, and because the collisions break up some particles,
there is a point in time where formation and breakup are about equal (Parker et al., 1972).
Thus, there is a time beyond which further detention adds little to the efficiency of removal.
Because the settled material (commonly referred to as
sludge or raw sludge ) is biologi-
cally active, at some point it will become anaerobic after it settles. The gases released from the
anaerobic decomposition will tend to lift the settled material back into the flow stream and, thus,
lower the efficiency. There is an optimum detention time between the detention time required to
achieve good flocculation removal and the e
xcess detention time that causes gas production to
lift the sludge. Of course, removal of the sludge in a timely manner increases the breadth of the
optimum window of time.
Depth of Tank. With the recognition that the detention time is, in addition to overflow rate,
theoretically (and practically) important in the design, then the depth of the tank also beco
mes of
theoretical significance. To capture a settling particle, the particle must strike the sludge layer at
the bottom of the tank. If the particle cannot settle through the depth of the wastewater to reach
the bottom of the tank in the allotted detention time, it will not be captured. Thus, if the tank
is too deep, the efficiency will be impaired . On the other hand, from a practical point of view,
there must be some spac
e provided to store the settled material and to accommodate the slud ge
removal equipment.
Velocity of Flow. Although there is an inherent assumption of quiescent conditions in the clari-
fier, there is a velocity of flow. This velocity must be low enough to avoid scouring of the settled
material back into the flow stream. The scour velocity
introduced in Chapter 20 (Equation 20-9)
sets the upper bound for the velocity through the tank. The critical scour velocity may be esti-
mated by Camp’s (1942) equation, repeated here for convenience:
v
gd
f
s
scour
/
12
8
1 000
()()()
(),
⎡
⎣
⎢
⎤
⎦
⎥
(21-2)
where dimensionless constant
s
density of particle, kg/m
3
density of water, kg/m
3
g acceleration due to gravity 9.81 m/s
2
d particle diameter, m
f dimensionless Darcy-Weisbach friction factor
The dimensionless constant () ranges from 0.04 for unigranular material to 0.06 for sticky,
interlocking material. The Darcy-Weisbach friction factor is taken to be in the range 0.02 to 0.03.
21-3 SEDIMENTATION PRACTICE
A Design Philosophy
Historically, the design goal of primary treatment has been framed in the context of an arbitrary
percentage removal of total suspended particles without a justification for the selection of the
percentage removal or a means of assessing whether or not the goal has been achieved. A cu r-
rently evolving philosophy is that the primary clarifier should be designe
d on the basis of the
oxidative capacity of the downstream biological processes. Primary clarifiers can remove more
21-4 WATER AND WASTEWATER ENGINEERING
BOD and solids for less operational cost than any other treatment process in use today (Wahlberg,
2006). Thus, it makes both economic sense and design sense to remove, to the maximum extent
possible, the settleable solids and settleable BOD by primary settling.
From an operational perspective, und
er most s ituations the design should minimize the
conditions that promote biological activity in the primary clarifier. An exception to this approach
occurs when biological phosphorus removal is to occur downstream. In this case, the primary
clarifier may be used to generate volatile fatty ac
ids to promote biological phosphorus removal
(Wahlberg, 2006). Biological phosphorus removal is discussed in Chapter 23.
Alternatives
Circular and rectangular tanks are the most common configurations. Square tanks with circular
sludge collection mechanisms have been used. These have generally proven unsatisfactory
because of sludge build up in the corners, and fouling of the more c omplex
sludge collection
mechanism. Stacked rectangular tanks have been used where space is highly restricted. They
have a much higher construction cost and require more complex structural design. Plate settlers
have become an important design alternative in pri
mary sedimentation.
Of the alternatives, rectangular and circular tanks with and without plate settlers are favored
for primary sedimentation. The following discussion will focus on these preferred alternatives.
Circular Tanks
In circular tanks the theoretical flow pattern is radial. The wastewater is introduced either in the
center or around the periphery ( Figure 21-1 ). The center-feed type is more commonly used for
primary treatment. The wastewater is carried to the center of the tank by either a pipe suspended
from a bridge or one that is encased in concrete below the tank floor. At the center of the tank, flow
enters a circular feedwell that is designed to distribute the wastewater flow equally in all directions.
S mall circular tanks ( 9 m diameter) have sludge removal equipment supported on beams
spanning the tank. Larger tanks have a central pier that supports the equipment. Access for service
is provided by a bridge-walkway. The bottom of the tank is sloped to form an inverted cone. The
sludge is scraped to a hopper located near the
center of the tank.
To provide redundancy, a minimum of two tanks is provided. Tanks are typically arranged
in pairs with a flow-splitting box between them ( Figure 21-2 ). Concrete is commonly used for
construction of tanks for municipal systems.
Circular tanks are favored because they require le
ss maintenance, the drive bearings are not
under wastewater, and the construction cost is generally lower than that for rectangular tanks.
The disadvantages of circular tanks are that they require a larger footprint because they cannot be
built with a common wall, and that they require more yard piping and pumping facilities.
Rectangular Tanks
Although transverse and vertical flow tanks have been used, the typical primary settling tank is
designed for longitudinal flow. The flow enters and exits through the narrow ends. Wastewater
is carried to the tank in a covered channel. It enters the tank through one or more inlet ports.
A baffle is provided immediately d ownstream of the inlet to dissipate the inlet port veloc ity
and dis tribute the flow and solids equally across the cross-sectional area of the tank.
T ypically chain-and-flight scrapers ( Figure 21-3 ) are used to remove sludge. The chain-and-
flight system differs from that used in water treatment plant settling tanks in that the “return” of
the flights is plac
ed at the top of the clarifier to remove scum.
PRIMARY TREATMENT 21-5
Bridge
Handrailing
Supports
Drive unit
Turntable
Maximum water
surface
Surface skimmer
Top of tank
Swinging skimmer blade
Scum baffle
Effluent weir
12
Adjustable
squeegees
scraper
blades
Influent pipe
(a)
(b)
Sludge draw-off pipe
Scum
pit
Scum pipe
Pler cap with
outlet port
s
Center pier and
Influent riser pipe
Influent
baffle
Drive cage
Skimmer
supports
Effluent
launder
Trussed rake arm
1
1
Scum trough
Handrailing
Alternate weir location
(center take-off)
Bridge
Drive unit
Turntable
Maximum water
surface
Surface skimmer
Top of tank
Alternate weir location
(rim take off)
Effluent pipe
12
Adjustable
squeegees
1
2
Scraper
blades
Sludge draw-off pipe
Influent
downcorner
Effluent weir
trough
Center pier
Drive cage
Effluent
pipe
Trussed rake arm
FIGURE 21-1
Circular primary setting tanks: ( a ) center-feed, ( b ) peripheral feed. ( Source: Metcalf & Eddy, 1991.)
Influent
Primary
clarifier
Primary
clarifier
Flow
splitting
box
FIGURE 21-2
Flow splitting box for two circular
primary clarifiers.
21-6 WATER AND WASTEWATER ENGINEERING
Recess for
drive chain
Flow
Take up
Influent
Sludge
50 mm
Drive sprockets with shear
pin hubs and clutches
Water level
1.7 m
Sludge
hopper
60 (min)
50 150 mm Flights spaced
approximately 3 m on centers
Pivoting flight
Effluen
t
Scum baffle
Adjustable
weirs
Travel
Average
water
depth
0.6 m (min.)
75 mm
30 mm
2.3 m
0.5 m
0.5 m
To suit
Slotted pipe for scum
0.5 m
FIGURE 21-3
Rectangular primary settling tank. ( Source: Davis and Cornwell, 2008.)
Sludge is carried to a hopper at one end of the tank. In larger tanks, this hopper will be a
transverse trough with a cross collector that carries the sludge to one side of the tank where the
sludge take-off pipe and pump are located. Because the tanks are typically built in multiple units
for redundancy, a common sludge pump ma
y serve several clarifiers.
Rectangular tanks are favored when space is a constraint because they may be construc ted
with a common wall and piping arrangements are more economical than for circular tanks. They
have had the disadvantage that drive bearings are under water. The use of nonmetallic equip
ment
in the tanks has significantly reduced the maintenance needs of rectangular tanks.
21-4 SEDIMENTATION BASIN DESIGN
General
The general design criteria for both circular and rectangular tanks are presented here. The special
considerations that apply only to one or the other type are discussed separately in later sections
that deal with these types specifically.
Redundancy. Multiple units capable of independent operation are required for all plants where
design average flow
s exceed 380 m
3
/ d (GLUMRB, 2004).
Hydraulic Load. The historic hydraulic design approach was to use the design average flow
rate. As noted in Chapter 18, the peak flows may be a factor of 2 or 3 times the design average
flow and in extreme cases, especially in very small communities or those with combined sewers,
it may be a
s high as 10 to 15 times the design average. Young et al. (1978) suggest that the peak
four-hour flow rate is an appropriate basis for design.
If the design philosophy is to maximize the efficiency of the prim ary c larifier in order to
minimize the load on the downstream biological processes, then the hydraulic design should
address the peak flow. This may be accomplished by equalization (discussed in Chapter 20),
PRIMARY TREATMENT 21-7
or by sizing of the primary clarifier for the peak flow, or a combination of these techniques. An
equalization basin provides a means of damping the peak flow, but it does not eliminate it.
An alternative approach is to design for less extreme peak-to-average flow rates in the
primary c larifier and to design the biological treatment unit to hand le the increased BOD load
during peak flow events. This approa
ch is advocated when low BOD loads predominate and a
less efficient primary clarifier provides the organic matter required to support the microorganism
mass. This condition frequently occurs in small plants at start-up.
Recycle streams from waste activated sludge, thic
kening supernatant, digester supernatant,
dewatering operations, and backwashing must be considered in the hydraulic load. The potential
for surges from these sources to disrupt the performance of the clarifier is high (WEF, 1998). The
use of the primary clarifier as a thickener for waste activated sludge is particularly tro
ublesome
and should be avoided except in extenuating circumstances. However, it is recommended that the
piping system be designed so that the primary tank can be used when the waste activated sludge
thickener is out of service. Surges from other recyc
le streams should be minimized or returned
during low-flow periods.
Overflow Rates. Recommended overflow rates range from 30 to 50 m/d (m
3
/ d · m
2
) at average
design flow without waste activated sludge recycle (Metcalf & Eddy, 2003; U.S. EPA, 1975). Rec-
ommended peak hour overflow rates range from 60 to 120 m/d (Metcalf & Eddy, 2003; Reardon,
2006). GLUMRB (2004) recommends an overflow rate of 40 m/d at the average design flow and
60 to 80 m/d at the peak hourly flow.
Where waste activated sludge must be returned
to the primary clarifier, the recommended
overflow rates at average design flow range from 24 to 32 m/d. The peak hour overflow rates
range from 40 to 70 m/d (Metcalf & Eddy, 2003). GLUMRB recommends a peak hour overflow
rate less than 50 m/d when waste activated sludge is returned to the primary clarifier.
Hydraulic Detention Time. Typical theoretical hydraulic
detention times range from 1.5 to
2.5 hours with a typical value of 2.0 hours. While not explicitly stated, it is assumed that these
detention times are at the average design flow rate. Ac tual detention times may be considerably
less than this. For example, reported dye tests on a tank with a theoretical d
etention time of
202 minutes had an actual detention time of 74 minutes (Daukss and Lunn, 2007).
From data presented by Wahlberg (2006), it appears that the fraction of solids removed by
settling reaches a maximum in about 30 minutes at average design flow rates. This coincide
s with a
30 minute flocculation time to achieve the minimum supernatant concentration (Parker et al., 2000).
Low-flow periods at plant start u p may result in substantially longer detention times with
resultant septic conditions. Detention times of more than 1.5 hours without continuous sludge
withdrawal may result in re
solubilization of organic matter. This will reduce BOD removal
efficiency and potentially result in odor problems (WEF, 1998). Multiple tanks allow more flex-
ibility during the start up of a new plant.
Velocity. In practice, the linear flow-through velocity has been limited to 0.020 to 0.025 m/s to
prevent scour and resuspension of settled solids (WEF, 1998).
Weirs and Weir Loading Rates. The most common type of weir plate is one made with 90-degree
v-notches at 150 or 300 mm intervals. This design is selected in preference to a flat plate or a square
notch weir that are subject to unbalanced flow if they are not perfectly level and/or they are subject to
wind effects (Tekippe, 2006).