Water and Wastewater Engineering

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21-18 WATER AND WASTEWATER ENGINEERING

Solution:

a . From Example 21-2 , the flow rate for the clarifier is 7,100 m

/ d . The diameter of the

tank is calculated as follows:

Surfacearea

m /d

m /dm

 

3 2

7 100

177

177 5

177 5 4

tank

()

)( )



⎛

⎝

⎜

⎞

⎠

⎟

15 03 15.or m





This is a standard diameter for sludge removal equipment.

b. Assuming a detention time of 20 min in the feedwell, and a depth equal to 50% of the

tank depth, the feedwell dimensions are estimated as follows:

()7 100

1 440

98 6

,.m /d

min

min/d,

⎛

⎝

⎜

⎞

⎠

⎟

Depth of feedwell mm

0 5043 215()( ).. .

rrfaceareaoffeedwell



98 6

215

45 86

feedwell

45 86

()

(



764

)( )





⎛

⎝

⎜

⎞

⎠

⎟

This is slightly more than half the diameter of the tank and appears to be quite large.

c. Check the flow velocity.

The area of the cylinder through which the wastewater must flow is

ADh

cylinder

mm m  



()( )764 43 215516... .mm

Note that h  side water depth minus the depth of the feedwell.

The velocity through this area is

v 

7 100

51 6 86 400

0 002

m /d

m s/d

m/s

()( )

This is substantially below the criteria of 0.020 m/s. Therefore, increase the depth of the

feedwell to reduce the diameter.

PRIMARY TREATMENT 21-19

d. After another iteration, the feedwell dimensions selected are 6.2 m diameter and 3.2 m

depth. The resulting velocity is estimated at 0.004 m/s.

e. Using 10 s detention and a depth of one-half that of the feedwell (1.6 m), the EDI

diameter is estimated as

()7 100

86 400

0 822

.m /d

s/d

⎛

⎝

⎜

⎞

⎠

⎟

SSurfaceareaofEDI

m

0 822

0 51



(()

()()

EDI

0 51

0 514







⎛

⎝

⎜

⎞

⎠

⎟

.. .809 0 8or m

f. The bottom of the sludge hopper should not exceed 0.6 m in width, and the angle of

the side wall must be greater than 60  . The shape of the hopper is the frustum of a right

pyramid. The volume is given by:

WW WW

⎛

⎝

⎞

⎠

{[()()]}

top bottom top bottom

The volume required is 1 m

. Assume W

bottom

 0.6 m. There are two unknowns: the

height, h, and W

top

. Using the Solver * tool in a spreadsheet with W

bottom

 0.6 m, the

dimensions were found to be:

Bottom  0.6 m

Top  1.19 or 1.2 m

Height  1.139 m or 1.2 m

Angle of side wall  75 

g. Place baffles as shown in Figure 21-11 .

h. The weir loading rate is the flow rate divided by the perimeter of the tank. At the design

flow rate:

Weir loading

m /d

or

7 100

15066 15

()

m /dm

This is within the customary range of weir loading rates.

* Solver is a “tool” in Excel

. Other spreadsheets may have a different name for this program.

21-20 WATER AND WASTEWATER ENGINEERING

Comments:

1 . These dimensions are only preliminary estimates. The dimensions must conform to the

manufacturer’s standard dimensions to minimize special-order costs.

2. The placement of flocculator paddles in the feedwell may allow s

ome reduction in volume.

It will increase the efficiency of flocculation.

Rectangular Tanks

Dimensions. The length of the tank is seldom greater than 110 m and is typically in the range

of 30 to 60 m. In very small tanks, a minimum flow length of 3 m is recommended. Because of

wind currents, the difficulty in moving sludge to the hoppers, as well as the mechanical stress on

the chain and flight

system, long lengths are limited to very large plants.

Widths range from 3 to 24 m. The width increments are determined by the manufactured

widths of the chain-in-flight sludge scrapers. If widths greater than 6 m are required, multiple

bays in one tank may be used. This will permit width

s up to 24 m (Metcalf & Eddy, 2003).

The floor slope is typically 1 percent toward the sludge hoppers. Side water depth is gener-

ally measured at the effluent end wall. The range of depths is from 2 to 5 m with a typical value

of 4.3 m (Metcalf & Eddy, 2003). It is good practice to provide a depth of 4 m below the effluent

weirs (Pettit, 2006). Co

mmon practice is to provide a freeboard of 0.5 to 0.7 m.

Flow Distribution. Because rectangular tanks are typically constructed side-by-side to take

ad vantage of common walls, the distribution of wastewater is by a s ingle channel that runs

perpendicular to the flow through the tanks. The channel is covered with re

movable grates that

allow access for cleaning. The design velocity of the c hannel should be a minimum of 0.3 m/s to

prevent deposition of organic matter and a minimum of 0.75 m/s to prevent deposition of mineral

matter at 50 percent of the design flow (Metcalf & Eddy, 1991; WEF, 1998). The channel design

must provide an allowance for scum

and a method for transferring scum to the primary tank. The

scum from the primary tank and the channels is collected together for disposal.

One of three alternatives is generally used to provide equal flow distribution: inlet weirs,

submerged orifices, or inlet gates. Weir inlets that discharge directly into the tank surface

should

be avoided to minimize odors and to prevent undue turbulence at the head end of the tank.

S ubmerged orifices ( Figure 21-13 ) are acceptable. They may cause an undesirable backwater

curve during peak flows. Storm inlet gates may have to be provided for this possibility. When flows

FIGURE 21-13

Distribution channel with submerged orifice.

⌬h

Settling tan

Distribution

channel

PRIMARY TREATMENT 21-21

are too low, the orifice may not be sufficiently restrictive to balance the flows. In this instance, the

operator must recognize the need to take one or more units off-line. Orifices are plugged when a

unit is taken out of service for maintenance. This will, of course, unbalance the flows.

S ubmerged gates seem to provide the most flexibility. The po

sition of the gates can be

adjusted for any extended flow condition. If gates are used, it is essential that automated

actuators be provided. Each gate must have an associated accurate flow meter with a feedback

signal to modulate the valve position (Tekippe, 2006; Wahlberg, 2006).

The design of the distribution channel is a complex

open channel flow problem. Benefield

et al. (1984) provide a detailed calculation procedure and a Fortran program for the design.

Inlet Configuration. There is no consensus on the design of the inlet. Recommendations

include placing the inlet port lower in the tank, limiting inlet port velocities to a range of 0.075

to 0.150 m/s, using inlet diffusers, placing an inlet baffle ( target or finger baffle ) in the flow path

( Figure 21-14 ), and

placing slotted or perforated baffles across the full width of the settling tank

( Figure 21-15 ). There is agreement on one point: in no case should the design permit a waterfall

into the tank (Pettit, 2006).

The typical inlet configuration includes multiple ports placed and sized to uniformly distribute

the flow over the width of the clarifier. For a 6 m wi

de tank, there are typically three or four inlet

ports. Maximum horizontal spacing is less than 3 m and typically about 2 m.

Recess for

drive chain

Flow

Take up

Influent

channel

Sludge

Water level

Sludge

hopper

60º (min)

Travel

Finger baffle

Recess for

drive chain

Flow

Take up

Influent

channel

Sludge

Water level

Sludge

hopper

60º (min)

Travel

Target baffle

(a)

(b)

FIGURE 21-14

E xamples of ( a ) target and ( b ) finger baffles.

Slotted baffle

(b) Plan(a) Profile

Influent

channel

Influent channel

FIGURE 21-15

Slotted baffles in a rectangular primary tank.

21-22 WATER AND WASTEWATER ENGINEERING

An orifice inlet that exits the bottom of the inlet channel allows for a discharge pipe exit that

is below the liquid surface. The range of location for the discharge is from a depth of approxi-

mately 2 m to midtank depth (Petit, 2006).

The function of target or finger baffles is to dissipate the jetting energy of the influent. The

target baffle is solid. The finger baffle is perforated. Baffles are typically placed 0.6 to 0.9 m from

the inlet and submerged 0.5 to 0.6 m, depending on the tank depth. The top of the baffle must be

far enough below the wastewater surface to allow scum to pass over the top (WEF, 1998).

Two offset, slotted baffles with the slots offs et have proven effective in distributing flow

evenly across the tank. The baffles should extend to at least one-half the tank depth (U.S. EPA,

1974). Kawamura (1981) recommends the ins

tallation of three perforated baffles across the full

width of the tank. Individual slots should be not less than 50 mm wide (or in diameter, if perfo-

rations are used) to minim ize plugging with rags or plastic bags. The diameter of perforations

should not be greater than 100 mm to avoid persistent jets . A porosity factor of 0.05 (5 percent

open area) is typi

cal (Krebs et al., 1992). This baffle system provides the additional advantage of

providing some flocculation.

Weir Configuration. Unwanted hydraulic patterns that are produc ed by the bottom density

current can be s trong in the region next to the end of the tank. When the flow reaches the end

wall, there is a strong upward current. This is particularly of con

cern if the sludge hoppers are

placed at the end of the tank.

Although a number of alternative arrangements of the effluent launders have been used, the

orientation does not appear to be critical (Kawamura and Lang, 1986; Pettit, 2006). What does

appear to be important is that to avoid the strong upflow when the tank current reaches the end

wall, the launder cannot be placed near the end wall. Comparable results can be obtained with

either long parallel flow launders or short parallel flow launders if they are placed away from the

end of the tank. From an economic point of view, short or transverse launders are less expensive

to build. This appears to be the most typical arrangement (WEF, 1998).

To avoid loss of solids due to end wall upflow, placement of the laund

er at a distance from

the outlet wall equal to the wastewater depth is recommended (ATV, 1988).

Sludge Removal. The two classes of sludge removal systems are the chain-and-flight and the

traveling bridge. In the United States, the chain-and-flight systems are more common. Although

the traveling bridge may be equipped with either a scraper or a suction s

ystem, the suction mech-

anism is not used in primary clarifiers. The traveling bridge is noted for its higher construction

and maintenance costs (Pettit, 2006).

With the exception that the return flights move across the surface of the tank ( Figure 21-3 ),

chain-and-flight systems are very similar to those

discussed in Chapter 10, and the reader is

referred there for more details. The flight speed ranges from 0.3 to 1.2 m/min with a typical speed

of 0.9 m/min. In general they are operated periodically rather than continuously.

Although the sludge hoppers may be placed at the influent end, effluent end, or in the middle

of the tank, they are often pla

ced at the influent end in primary settling tanks. This places the

hoppers near where the bulk of the solids settle. It also alleviates the scour that lifts the solids

up into the effluent weirs at the end wall. The disadvantage of locating the hopper at the head

end of the tank is that the flight scraper induces a volumetric flow rate at the bottom of the tank

that is countercurrent to the den

sity flow current in the fluid above the sludge blanket. This may

resuspend solids.

PRIMARY TREATMENT 21-23

A single sludge hopper with a cross-collector is preferred over multiple hoppers (Metcalf &

Eddy, 2003).

Sludge transport, treatment and disposal is discussed in Chapter 27.

Scum Removal. The typical scum removal system utilizes the return of the chain-and-flight

scraper at the surface of the tank to move the scum

toward the effluent ( Figure 21-3 ). The scum is

trapped by a scum baffle and/or, more commonly, a slotted pipe. The slotted pipe is periodically

turned to allow the scum to be carried into the slot. The scum baffle should extend 0.3 to 0.6 m

below the surface and 30 mm above the su

rface. It should be not less than 0.6 m upstream from

the outlet weir (Pettit, 2006).

S cum transport, treatment, and disposal are discussed in Chapter 27.

Primary Sedimentation Tank Design Criteria

T ypical design criteria are summarized in Table 21-1 . Some design criteria are quite rigid, while

others only provide guidance. For example, because of manufacturing constraints, the length of a

chain-and-flight collection sets a firm maximum on the length of the settling basin. Although the

maximu

m width is 6 m, multiple units may be mounted in parallel to achieve widths up to 24 m.

Parameter Range of values Typical/comment

General

Overflow rate (average flow) 30 to 50 m

/d · m

40 m

/d · m

Overflow rate (peak flow) 60 to 120 m

/d · m

100 m

/d · m

Detention time (average flow) 1.5 to 2.5 h 2.0 h

Flow velocity 0.020 to 0.025 m/s

Weir loading rate 125 to 500 m

/d · m 250 m

/d · m

Sludge hoppers 1.7 vertical to 1 horizontal Minimum; bottom width  0.6 m

Geotechnical Consider potential for flotation

when tank is empty

Circular tanks

Dimensions

Diameter 3 to 100 m 12 to 45 m

Standard 9 to 45 m In 1.5 m increments

Side water depth 3 to 5 m 4.3 m

Floor slope 1 vertical to 12 horizontal

Splitter box

Inlet velocity 

0.3 m/s At peak flow

Inlet configuration

Detention time 20 minutesFeedwell

Submergence 30 to 75% of depth Size to prevent scour

EDI detention time 8 to 10 s

TABLE 21-1

Typical design criteria for primary sedimentation basins

(continued)

21-24 WATER AND WASTEWATER ENGINEERING

Parameter Range of values Typical/comment

Baffles

Effluent Below weir

Horizontal Below feedwell

Rectangular tanks

Dimensions

Length 30 to 110 m 30 to 60 m

Width 3 to 24 m 6 m max per flight

Depth 2 to 5 m 4.3 m

Floor slope 1%

Distribution channel

Velocity 0.3 to 0.75 m/s

Flow distribution Prefer orifices or gates

Inlet configuration

Ports

3 to 4 per tank at  3 m 2 m

Energy dissipation Target or finger baffle

Baffles

Distance 0.6 to 0.9 m from inlet

Submergence0.5 to 0.6 m

Porosity Individual openings  5 cm

and  10 cm

5% open area

Sources: GLUMRB, 2004; Krebs et al., 1992; Metcalf & Eddy, 2003; Pettit, 2006; Tekippe, 2006; Wahlberg et al., 1994;

Wahlberg, 2006; WEF, 1998.

TABLE 21-1 (continued)

Typical design criteria for primary sedimentation basins

A n e xample of the design of horizontal flow basin is presented in Chapter 10. Of course, the cri-

teria for primary sedimentation basins are different and these must be accounted for in the design.

Operation and Maintenance

Provisions should be made for taking tanks out of service and dewatering. This includes recogni-

tion of the following:

• Dewatering a tank may result in untoward groundwater pressure that will have to be

relieved.

• Taking one rectangular tank out of service that is paired with another tank requires provi-

sion for disengaging the drive and isolating the sludge pump so the rem

aining tank can

remain in service.

• Interior walls must be designed for hydrostatic pressure on one side only.

Clean weir plates prevent unbalanced flows. Some designers provide covers to prevent algae

growth. Alternatively, wiper blades are provided.

In circular tanks, the beach should be continuously flushed

to prevent scum from clogging

the lines.

PRIMARY TREATMENT 21-25

Hose bibs should be provided at each tank, scum trough, sump, and pumping station for

maintenance and clean up. Piping for treated wastewater may be used, provided it is separated

from potable water and is clearly marked as nonpotable.

21-5 OTHER PRIMARY TREATMENT ALTERNATIVES

Three modifications/alternatives may be used in primary treatment. Enhanced sedimentation and

plate settlers are modifications to standard sedimentation, and fine screens are used in lieu of

sedimentation. Each of these will be discussed in the following sections.

Enhanced Sedimentation

The simple act of promoting increased contact between particles at the inlet of the sedimentation

basin is a form of enhanced sedimentation. More commonly, enhanced sedimentation refers to the

addition of chemicals. This practice is called chemically enhanced primary treatment (CEPT). As

discusse

d in Chapter 6, the addition of chemicals, followed by gentle agitation results in coagula-

tion of particles. The resulting increase in particle size enhances the efficiency of sedimentation.

Increases of 40 to 80 percent in organic carbon removal and 60 to 90 percent in total suspended

solids removal can be achieved in shorter settling times than conventional

sedimentation.

CEPT is most effective if a complete treatment train, including rapid mix, coagulation, and

sedimentation, are provided. However, substantial improvements can be achieved by adding the

chemicals to aerated grit chambers or other upstream facilities for mixing, and us

ing the inlet

structures of a conventional primary settling tank to provide flocculation.

Alum (Al

(SO)

· 14H

O) or ferric chloride (FeCl

) added in conjunction with anionic poly-

mers are the chemicals most frequently used. Current practice is to use metal salt doses on the

order of 20 to 40 mg/L in combination with polymer doses of less than 1 mg/L. Metcalf & Eddy

(2003) recommends velocity gradients

for flocculation in the range 200 to 400 s

 1

The use of metal salts also results in precipitation of phosphorus. This may be a positive step

in meeting discharge standards. It also may be detrimental to the downstream biological processes

that require phosphorus. As an alternative, anionic polymers alone in high doses (8 mg/L) are

effective coag

ulants that do not remove phosphorus (Reardon, 2006).

CEPT may be used on an intermittent basis to achieve effective primary sedimentation during

peak hydraulic events. This approach uses less chemicals and produces less sludge to handle.

Table 21-2 summarizes the advantages an

d disadvantages of CEPT.

Plate Settlers

Theoretical and design practices for inclined plate and tube settlers were discussed in Chapter 10.

For more details the reader is referred there.

Although plate settlers have not been commonly used in municipal wastewater treatment

plants in the United States, they have been used extensively in Europe. Metcalf & Eddy (2003

)

suggests an appropriate application is in conjunction with CEPT.

The common design is a countercurrent flow pattern. The influent is fed under the plates or

tubes and flow is upwards. Solids settle to the plate and slide down the surface to the bottom of

the tank.

For primary sedimentation applications, they inc

rease the settling area by a factor of 8 to 10.

This permits a smaller footprint or increases the capacity of existing overloaded tanks. They have

been reported to produce a more dilute sludge. This may increase the cost of sludge handling.

21-26 WATER AND WASTEWATER ENGINEERING

Potential problems that must be addressed are clogging from solids, algae, or grease. Fine

screening, adequate grit removal, and enclosing the tank are some of the rem edies offered for

these problems. Maintenance requirements are expected to be higher than for standard settling

tanks. Provision of independently supported units, eas

y access to the plates, and plates that can be

independently removed are recommended (Reardon, 2006).

Fine Screens

Where high removal efficiencies are not required, for example, in biological phosphorus removal

or membrane bioreactors, fine screens may be appropriate in place of sedimentation. Fine screens

with openings from 0.25 to 1.5 mm typically only achieve removal efficiencies of 5 to 45 percent

for suspended s

olids and 5 to 50 percent for BOD

(Metcalf & Eddy, 2003). The design practices

for fine screens are discussed in Chapter 20.

TABLE 21-2

Advantages and disadvantages of CEPT

Advantages Disadvantages

Increased removal of BOD, TSS, phosphorus,

and metals

May remove too much phosphorus, thus making the

primary effluent nutrient deficient

Increases primary tank capacity by allowing higher

overflow rate

Chemical handling facilities required

Increases abilit

y to absorb shock loads/wet weather

flows

Chemical safety issues and regulatory requirements

increase

Reduces size or increases capacity of biological

process

Sludge quantities increase

Enhances biological treatment kinetics May decreas

e biological sludge settleability

Decreases carbon to nitrogen ratio thus increasing

the fraction of nitrifying microorganisms and

enhancing ammonia removal

A dapted from Reardon, 2006.

Visit the text website at www.mhprofessional.com/wwe for supplementary materials

and a gallery of additional photos.

21-6 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 why there is an optimum hydraulic detention time for a primary settling tank.

2 . Describe some of the practical aspects to be considered in setting the depth of a primary

settling tank.

3. Describe the method for establishing the limiting upper bound

of the velocity through a

primary settling tank.

PRIMARY TREATMENT 21-27

4. Discuss the proposed design philosophy for primary sedimentation practice explaining

when it does and does not make sense.

5. Sketch a flow-splitting box and describe how flow is adjusted when the clarifiers are

not identical.

6. Explain why hydraulic detention time

s greater than 1.5 hours are undesirable.

7. Draw a sketch to explain why an empty settling tank might “float.”

8. Sketch an EDI.

9. Sketch a baffle to be used at a perimeter weir on a circular settling tank.

10. Define CEPT and explain its role in improving settling tank efficiency.

W ith the aid of this text, you should be able to do the following:

11. Determine the half life of the number of particles per unit volume or the n

umber

remaining, given the collision efficiency, velocity gradient, and volume of particles

per unit volume.

12. Estimate the scour velocity for sticky organic particles given their density.

13. Given diurnal flow pattern, estimate the ratio of the peak four-hour flow to the average flow.

14. Estimate the actual overflow rate and/or detention time for a given s

ettling tank design

and a diurnal flow pattern.

15. Determine whether or not a tank will “float” given the tank dimensions and the eleva-

tions of the tank bottom and the groundwater table.

16. Estimate the thickness of the bottom of an empty tank to prevent it from floating given

the tank dimensions and the elevations of the tank bottom and the groundwater table.

17. Design a splitting box for two or three circular clarifiers given their dimens

ions.

18. Design a distribution channel for a set of rectangular settling tanks.

19. Design a circular sedimentation basin.

20. Design a rectangular sedimentation basin.

21-7 PROBLEMS

21-1. Estimate the half-life of particles in a flocculating solution based on the following

a ssumptions:

Uniform particle diameter (d)  5  m

  1.0

Velocity gradient  200 s

 1







()dN

where N  number of particles per unit volume  10,000/mL.