Water and Wastewater Engineering

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20-1

20-8 ALTERNATIVE PRELIMINARY PROCESS

ARRANGEMENTS

20-9 CHAPTER REVIEW

20-10 PROBLEMS

20-11 DISCUSSION QUESTIONS

20-12 REFERENCES

HEADWORKS AND PRELIMINARY TREATMENT

20-1 INTRODUCTION

20-2 PUMP STATION

20-3 FLOW MEASUREMENT

20-4 BAR RACKS AND SCREENS

20-5 COARSE SOLIDS REDUCTION

20-6 GRIT REMOVAL

20-7 FLOW EQUALIZATION

CHAPTER

20-2 WATER AND WASTEWATER ENGINEERING

20-1 INTRODUCTION

Headworks refers to the unit operations that are placed at the upstream end of the wastewater

treatment plant (WWTP). These include the pumping station, flow measurement, and a group of

unit operations commonly referred to as preliminary treatment.

T ypically WWTPs are designed so that flow through the plant is by gravity. The wastewater

frequently is

carried to the WWTP by gravity and, consequently, it is at a substantial depth below

grade. Thus, a pumping station is required to raise the sewage to an appropriate level to facilitate

gravity flow through the plant. Flow measurement is an essential component of the operation and

management of the WWTP.

Preliminary treatment typically serves three important functions: re

moval of untreatable

solid materials; protection of subsequent treatment units; and improvement of the performance of

subsequent treatment units. Preliminary treatment unit operations inclu de: screens, shredders or

grinders, grit removal, and flow equalization.

20-2 PUMP STATION

The design of the pump station (or lift station) at the WWTP is to a great extent similar to those

placed in the collection system. The major differences are that the building components are incor-

porated into the WWTP facility and that an alternative to the nonclog centrifugal pump may be

appropriate. The alternative is a screw pump (often called an Archimedes screw ).

S crew pumps ( F

igure 20-1 ) are high volume, nonclog, atmospheric head devices that can

pump a variety of solids and debris in raw wastewater without screening. There are two general

types: the open screw that rotates in a trough and the enclosed screw, in which both the screw

and the enclosing cylinder rotate. A major advantage of these pumps is variable pu

mping at

constant speed, because the output, up to the design capacity, is controlled by the sump level and

equals the influent flow rate. Operators like screw pumps because the good ones, when properly

installed, are nearly trouble free (Garbus, 2006).

The overall efficiency of the pumping system may be as high a

s 80 percent at design flow.

At approximately 30 percent of design capacity, the efficiency of the open screw drops to about

Delivery point

Water level

Slope angle

One flight

Lift

“H”

Fill point

Two flight

Three flight

FIGURE 20-1

S crew pump arrangement with diagram of multiple flights.

HEADWORKS AND PRELIMINARY TREATMENT 20-3

60 percent because of friction and backflow (called slippage ) of fluid between the flights and

the trough. There is no slippage with the enclosed screw (Garbus, 2006). Major disadvantages

of screw pumps are the large area ( footprint ) of the pumping station because of the angle of the

slope of the screw and limited head (about 10 m) that can be achieved.

Design Considerations. For redundancy, the pump station must have adequate capacity to

handle the peak hydraulic flow rate with the largest pumping unit out of service. Centrifugal

pump selection is made based on the system head curve. Screw pump selection is made from

man

ufacturers’ d ata such as that shown in Table 20-1 . The static lift height is determined based

on the difference in elevation between the low flow into the plant wet well and the required

elevation to overcome headlosses as wastewater flows through the plant (i.e., the hydraulic

grade line).

E xample 20-1 illustrates the procedure for selecting an appropriate screw pump.

Example 20-1. Design a

screw pumping system for Waterloo’s WWTP that has the following

characteristics:

Interceptor sewer

Minimum sewage elevation  514.75 m

M a ximum sewage elevation  515.00 m

D i scharge elevation to stilling well  519.17 m

Average ﬂ ow rate at design capacity  37,000 m

/ d

Screw

diameter, m

Maximum

rpm

Maximum capacity at

30 slope, m

Maximum height at

30 slope, m

1-flight 2-flight 3-flight 1-flight 2-flight 3-flight

0.30 110 344252 2.4 2.2 2.1

0.41 91 66 83 103 2.9 2.7 2.5

0.51 79 112 140 175 3.4 3.0 3.0

0.61 70 168 210 262 4.0 3.7 3.7

0.76 60 288 360 451 4.2 3.9 3.7

0.91 53 434 542 678 4.8 4.4 4.2

1.07 48 621 776 970 5.3 5.0 4.6

1.22 44 881 1,101 1,376 4.7 4.3 4.1

1.37 41 1,132 1,415 1,769 5.6 5.2 4.9

1.52 38 1,486 1,8582,322 5.2 4.7 4.4

1.68 35

1,774 2,216 2,771 5.9 5.55.1

1.83332,230 2,788 3,484 5.6 5.1 4.7

2.0331 2,791 3,488 4,360 5.1 4.6 4.3

2.1330 3,219 4,023 5,029 5.8 5.3 4.9

TABLE 20-1

Typical screw pump selection chart

For academic use. Use actual manufacturers’ data for design.

20-4 WATER AND WASTEWATER ENGINEERING

Solution:

a . Calculate the maximum height the screw pump must lift the sewage. This is the dis-

charge elevation minus the minimum sewage elevation.

519 17 514 75 442...mmm

b. Calculate the minimum flow rate at start-up and the peak hour flow rate at design capacity.

Using Figure 18-1 from Chapter 18, find the ratio of the minimu

m hour on the minimum

day to the average flow to be 0.28 and the ratio of the peak hour flow rate at design capac-

ity to the average flow to be 2.8. The flow rates are then

Minimum flow rate m /d 10 36()( )028 37 000

., ,00 or 10 400 m /d

Peak hourflowrate

28 3()(77 000

,,,m /d 103 600 or 104 000 m /d) 

c. Convert the flow rates to m

/h to be compatible with the selection charts.

Minimum flow rate

m /d

h/d

10 400

433 3

.rr m /h

Average flow rate

m /d

433

37 000



hh/d

or m /h

Peak hourflowr







1 541 67 1 5 40

,. ,

aate

m /d

h/d

or m

104 000

4 333 3 4 330

,. , //h

d. Review the choices from Table 20-1 . Based on the maximum required lift height (4.42 m),

the screw diameters for 1 and 2 flights  0.91 m and  1.07 m for 3 flights will work.

e. The selection process is iterative. Begin by assuming that the smallest acceptable screw

pump is one that will pump at 30% of capacit

y at a reduced efficiency of 60%.

433

0 30

1443

m /h

,

From Table 20-1 , the standard size screw pump that will pump at this rate is a 1-flight

screw that is 1.52 m in diameter rated at 1,486 m

/h.

f. Estimate the number of screw pumps required to meet the maximum flow rate.

4 330

1 486

291 3

m /h

m /h per pump

or pum pps

g. At the average flow rate, two pumps operating at

1 540

770

, m /h

m /h

770

1 486

0 52 52

m /h

or % capacity

.

will work.

HEADWORKS AND PRELIMINARY TREATMENT 20-5

h. To meet the redundancy requirement to have adequate capacity for the peak hour flow rate

with one pump out of service, a total of four pumps rated at 1,486 m

/h will be required.

Comments:

1 . Other combinations will also work. Note that 2 and 3 flights with 1.32 m diameter will

not work.

2. Consultation with the manufacturer may reveal that alternate slopes are available that

may be a better fit.

3. Other manufacturers may have pumps that are a better fit.

20-3 FLOW MEASUREMENT

The two most common flow measuring devic es used for wastewater are the Parshall flume and

the magnetic flow meter.

Parshall Flume

The Parshall flume ( Figure 20-2 ) is an empirically rated measuring device. It was developed by

R. L. Parshall (1926a, 1926b, 1941) under the auspices of the U.S. Department of Agriculture.

The dimensions of the flume are fixed so that there is a flow transition from subcritical to super-

critical flow. The transition is cause

d by designing the flume to narrow to the throat dimension

( W ) while dropping the channel bottom. At critical flow depth, energy is minimized and there is

a direct relationship between water depth and velocity. This allows the flume, like a weir, to be

used as a measuring device. The weir crest throat width ( W ) is used to set the other d

imensions

of the flume. The Parshall flume dimensions are now specified by standard setting organizations

such as ISO (1992) and ASTM (1991). Weir crest widths vary from 25 mm to 15 m to measure

flows from 1m

/h to more than 300,000 m

/h. A list of dimensions for a limited range of flow

measuring capacities is shown in Table 20-2 .

When the flume is operating under free flow conditions, a hydraulic jump is visible at the

throat. Under these cond itions, the downstream discharge is not submerged at H

, and a depth

reading at the upstream point H

can be used to gage the flow. If the downstream discharge at H

FIGURE 20-2

Parshall flume with identification of measurement locations noted

in Table 20-2 .

2/3

20-6 WATER AND WASTEWATER ENGINEERING

is submerged, then a correction factor must be applied. Stevens (1998) provides tables for con-

verting the gage reading to flow rate and corrections for submergence.

LMNO (2008) has developed an equation and graphs that may be used for free flow conditions:



(20-1)

where Q  ﬂ ow rate, m

/ s

 depth of water at point “a,” m

C and n  coefﬁ cients to be obtained from Figures 20-3 A and 20-3B

This equation is valid for H

 2 m and 0.152  W  15.24 m.

The Parshall flume is limited to measurement of gravity flow rate. However, the flume is

favored as a flow measuring device because it will pass a wide variety of solids such as rags,

sand, and large objects that potentially will foul other flow measuring devices. Another rea

son

the Parshall flume is favored is that the flow rate can be determined manually by measuring the

water depth. This provides an independent method for calibrating the flume, and it provides

Minimum

flow rate,

Maximum

flow rate,

5 300 0.15 0.61 0.61 0.40 0.40 0.30 0.61 76 114 51

10 520 0.23 0.88 0.86 0.380.570.30 0.46 76 114 51

40 1,6300.301.371.34 0.61 0.84 0.61 0.91 76 229 51

50 2,450 0.46 1.45 1.42 0.76 1.03 0.61 0.91 76 229 51

70 3,360 0.61 1.521.50 0.91 1.21 0.61 0.91 76 229 51

100 5,100 0.91 1.68 1.64 1.22 1.57 0.61 0.91 76 229 51

130 6,900 1.22 1.83 1.79 1.52 1.94 0.61 0.91 76 229 51

TABLE 20-2

Parshall flume dimensions

A dapted from Parshall in nominal SI equivalent to U.S. customary units. For actual design see Parshall, 1926a and 1926b.

0.01

1.46

1.48

1.5

1.52

1.54

1.56

1.58

1.6

1.62

1.64

0.1 1 10 100 1,000

Throat width, m

FIGURE 20-3A

Parshall flume coefficient n.

HEADWORKS AND PRELIMINARY TREATMENT 20-7

redundancy when the electronics fail. A given size of Parshall flume is capable of measuring a

wide range of flow rates. When the flow range from minimum at plant start-up to maximum at

design capacity exceeds the range of a standard flume, a removable prefabricated insert may be

used to accommodate the low flows

at start-up.

The major disadvantage of the flum e is the comparatively large footprint for the structure

and the additional space required to achieve uniform flow into and out of the flume.

E xample 20-2 illustrates the selection of a Parshall flume.

Example 20-2. Continuing with the head works design for Waterloo ( Example 20-1 ), design

a Pars

hall flume by specifying the throat width and estimating the maximum depth of the flume.

Assume free flow conditions and 0.6 m freeboard above the maximum depth.

Solution:

a . Using Table 20-2 , select a throat width of 0.91 m with a maximum range of flows from

100 m

/h to 5,100 m

/h. This range encompasses the range found in Example 20-1 :

433 m

/h to 4,330 m

/h.

b. Calculate the depth of flow at 4,330 m

/h. Us e Figures 20-3 A and 20-3B to find the

coefficients for Equation 20-1 . For a throat width of 0.91 m, n  1.57, and C  2.5,

convert the peak hour flow rate to appropriate units and solve Equation 20-1 for H

104 000

120

, m /d

86,400

m /s

s/d

.

H

⎛

⎝

⎜

⎞

⎠

⎟⎟

⎛

⎝

⎜

⎞

⎠

⎟



1157

120

2 5

0 628 63

m /s

.or0.

c. Add the freeboard to the depth of flow to yield

063 06 12... .mdepth for the flume

0.01

0.1 1 10 100

Throat width, m

FIGURE 20-3B

Parshall flume coefficient, C.

20-8 WATER AND WASTEWATER ENGINEERING

Comments:

1 . The selection of the throat width fixes the other dimensions of the flume as specified in

Table 20-2 and Figure 20-2 .

2. Although there is no redundancy requirement for the measuring device, it would be pru-

dent to provide some method for bypassing the device to allow maintenance activities to

be perfor

med.

Magnetic Flow Meter

Faraday’s law is the theoretical basis for the operation of the magnetic flow meter (also known as

a mag meter ). It may be summarized as follows:

When an electrical conductor passes through an elec tromagnetic ﬁ eld, an electromotive force or voltage

is induced in the conductor that is proportional to the velocity of the conductor. The voltage thus gener-

ated is mutually perpendicular to both the velocity of the conductor and the magnetic ﬁ eld (Metcalf &

Eddy

, 1972).

This may be expressed as:

EvBL

ducto

" ()( )( )

(20-2)

where E  voltage generated , volts

v  velocity of the conductor , m/s

B  magnetic field , Wb

conductor

 length of conductor , m

In actual operation the liquid in the pipe serves as the conductor. The electromagnetic field is

generated by placing coils around the pipe ( Figure 20-4 ). The induced voltage is measured by

electrodes plac ed on either side of the pipe. If the pipe is a conductor, the ele

ctrodes do not

have to penetrate the wall of the pipe. The output signal must be calibrated to provide a flow

reading.

The mag meter has the advantage that it can be loc ated on a force main from a centrifu-

gal pump. It can pass objects that can travel in the pipe. The major disadvantage is the de-

pendence on the conductivity of the wastewater, which may vary widely and thus affect the

calibration.

Location

To measure the flow accurately, both the Parshall flume and the mag meter require uniform

flow upstream and downstream of the measuring point. For the Parshall flum e a straight uni-

form channel should be maintained for a reasonable dis tance. An approach equal to at least 10

times the throat width ( W ) is d esirable. Down

stream changes in direction must be sufficiently

distant so that the resulting backwater curve does not reach the flume (Steel and McGhee,

1979). The requirements for the mag meter are illustrated in Figure 20-4 b.

HEADWORKS AND PRELIMINARY TREATMENT 20-9

5 pipe

diameters

Mag

meter

Bypass line

Flow

(b)

(a)

Isolating

valve

(normally full open)

Flow

10 pipe

diameters

Isolating valve

(normally full open)

Bypass valve

(normally closed)

Insulating

liner

Electrode

assembly

Steel meter

body

Magnet coils

Potting compound

FIGURE 20-4

( a ) Schematic of magnetic flowmeter and ( b ) correct installa-

tion locations. (Schematic courtesy of Fisher & Porter.)

20-4 BAR RACKS AND SCREENS

The nomenclature of rac ks and screens is typic ally based on their purpose and the size of the

openings. Table 20-3 provides a summary of the types. The size range of the openings as a means

of differentiation is not well defined.

20-10 WATER AND WASTEWATER ENGINEERING

Bar Rack Cleaning Mechanisms

Although manually cleaned racks are often provided in bypass lines, mechanically cleaned

screens are typically used in the United States. These fall into four categories: chain-driven,

reciprocating rake, catenary, and continuous belt. These are illustrated in Figure 20-5 . Ad

van-

tages and disadvantages of each type are presented in Table 20-4 on page 20-12.

Chain-Driven. In general, front-cleaned, front-return chain-driven screens are more efficient in

retaining captured solids, but they are less rugged and are susceptible to jamming. They are seldom

used for plants receiving combined sewage. Back-c

leaned screens are more rugged but are more

susceptible to carryover of solids to the downstream side. Both types suffer from the disadvantage

of submerged sprockets that require frequent operator attention and are difficult to maintain.

Reciprocating Rake. The reciprocating rake or climber screens i mitate the motion of a person

raking the screen. They

are front cleaned, front return. Thus, they are effective in capture and

discharge of screenings without carryover. They have the advantage that all parts requiring main-

tenance are above the water line. The disadvantages are that a single rake may limit capacity to

handle high loadings of screenings, and they have a requirement for a high overhead clearance to

accommo

date the raking mechanism.

Catenary Screen. The catenary screen is a front-cleaned, front-return chain-driven screen. In

contrast to the chain-driven screens, it has no submerged sprockets. It can handle heavy objects.

It has a large footprint.

Continuous Belt. The continuous belt can handle both fine and coarse solids. It has no su

merged sprocket.

Type Typical opening Typical use

Trash racks 40–150 mm To prevent logs, stumps, and large heavy debris from

entering treatment processes. Principally used in combined

sewers ahead of pumping units. In WWTPs, frequently

followed by coarse screens.

Bar rac

ks or coarse screens 6–75 mm To remove large solids, rags, and debris. Typically used

in WWTP.

Fine screens 1.5–6 mm To remove small solids. Typically follows a

coarse screen.

Very fine screens 0.25–1.5 mm To reduce suspended

solids to near primary treatment

level. Typically follow a coarse screen and/or fine screen.

May be used when downstream processes do not include

primary treatment.

Microscreens 1m–0.3 mm Used in conjunction with very fine screens for effluent

polishing.

TABLE 20-3

Nomenclature of racks and screens

Sources: Daukss, 2006, WEF, 1998.