Water and Wastewater Engineering

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HEADWORKS AND PRELIMINARY TREATMENT 20-11

(a)

(c)

(b)

Flow

Raking tynes

Continous

chain scraper

Drive

Deck

Automatic

cleaning device

Head space

Screening

trough

Bar rack

Hydraulic

cylinder

Bar

Boom

assembly

Flow

Bar rack

(d)

Flow

Stainless-steel

screen elements

Influent cover

guard

Drive motor

Doctor blade

assembly

Guide track

Deck

Dec

Head space

FIGURE 20-5

Mechanically cleaned coarse screens: ( a ) front-cleaned, front-return chain driven, ( b ) reciprocating rake, ( c ) catenary, and

( d ) continuous belt. ( Source: Metcalf & Eddy, 2003.)

Bar Rack Design Practice

Capacity and Redundancy. Two or more mechanically cleaned screens are provided so that

one unit may be taken out of service without impairing performance. Each should have adequate

capacity to pass the peak hydraulic flow rate with the largest unit out of s ervice. The design

should provide that each unit c

an be isolated from the others.

In very small plants, a single unit may be installed with a bypass channel. The bypass chan-

nel will have a manually cleaned screen.

20-12 WATER AND WASTEWATER ENGINEERING

Type of screen Advantages Disadvantages

Chain-driven screen

Front clean/

back return

Multiple cleaning elements (short

cleaning cycle)

Unit has submerged moving parts that require

channel dewatering for maintenance

Used for heavy-duty applications Less efficient screenings removal, i.e., carryover

of re

sidual screenings to screened wastewater

channel

Front clean/

front return

Multiple cleaning elements (short

cleaning cycle)

Unit has submerged moving parts that require

channel dewatering for maintenance

Very little screenings carryover Submerged moving parts (chains, sprockets,

and s

hafts) are subject to fouling

Heavy objects may cause rake to jam

Back clean/

back return

Multiple cleaning elements (short

cleaning cycle)

Unit has submerged moving parts that

require channel dewatering for maintenance

Submerged moving parts (chains,

sprockets, and shafts) are prote

cted

by bar rack

Long rake teeth are susceptible to breakage

Some susceptibility to screenings carryover

Reciprocating rake No submerged moving parts;

maintenance and repairs can be done

above operating floor

Unaccounted for high channel water level

can submerge rake motor and cause motor

burnout

Can handle large objects (bricks,

tires

, etc.)

Requires more headroom than other screens

Effective raking of screenings and

efficient discharge of screenings

Long cycle time; raking capacity may be

limiting

Relatively low operating and

maintenance costs

Grit accumulation in front of bar may

impede rake movement

Stainless-steel constru

ction reduces

corrosion

Relatively high cost due to stainless-steel

construction

High flow capacity

Catenary Sprockets are not submerged; most

maintenance can be done above the

operating floor

Because design relies on weight of chain for

engagement of rakes with bars, chains are

very heavy and diffic

ult to handle

Required headroom is relatively low Because of the angle of inclination of the screen

(45 to 75), screen has a large footprint

Multiple cleaning elements (short

cleaning cycle)

Misalignment and warpage can occur when

rakes are jammed

Can handle large objects May emit odors because of open design

Very little screenings

carryover

Continuous belt Most maintenance can be done

above operating floor

Overhaul or replacement of the screening element

is a time-consuming and expensive operation

Unit is difficult to jam

( Source: Metcalf & Eddy, 2003.)

TABLE 20-4

Advantages and disadvantages of various types of bar screens

HEADWORKS AND PRELIMINARY TREATMENT 20-13

Location. In nearly all cases, screens should be installed ahead of the grit chambers to prevent

fouling of the grit chamber equipment.

Velocities. The approach velocity should be at least 0.4 m/s to minimize deposition of solids

in the channel. The velocity through the screen should be less than 0.9 m/s

at peak flow rates to

minimize forcing of material through the screen (GLUMRB, 2004).

One of several alternatives are employed to regulate the velocity through the screen. Place-

ment of a control structure such as a Parshall flume downstream of the screen or control of the

wet well operating levels can be used. Sizing the channel for velocity control by widening the

channel at the screen is another tec

hnique that may be employed.

Channels. Dual channels must be provided. Typically they are constructed of concrete. They

must be c apable of being isolated by the use of slide gates or recesses in the channel walls

for the insertion of stop plates or stop logs. The term stop log i s derived from the early

use of

wooden logs set in a groove to stop the water flow. Modern stop plates are made of extruded

aluminum. The channel invert s hould be 75 to 150 mm below the invert of the incoming sewer

(GLUMRB, 2004).

The channel dimensions are selected to achieve the approach velocity constraints. The floor

of the channel should be level, or shoul

d slope downward through the screen without pockets that

may trap solids. Fillets ( Figure 20-6 ) are provided to minimize the accum ulation of solids. The

channel approach to the screen should be straight and perpendicular to the screen for a distance

equal to 10 times the depth of flow (Metcalf & Eddy, 1972).

10 x depth of wastewater

Flow

(a)

(b)

(c)(d)

Stop plates

Fillets

Stop plate or

slide gate

Bar

rack

Bar

rack

Stop plate grooves

or brackets

FIGURE 20-6

Two channels with bar racks. ( a ) Top channel is isolated by stop plate or slide gate. ( b ) Slide gate.

( c ) Stop plates in channel. ( d ) Channel fillets.

20-14 WATER AND WASTEWATER ENGINEERING

The velocity of the open channel flow in the channel may be described by Manning’s equation:

v 

2 3 12

(20-3)

RSA

2 3 12//

(20-4)

where v  velocity, m/s

n  coefficient of roughness, unitless

R  hydraulic radius, m



cross-sectional area of flow m

wetted

iimeter m

S  slope of energy grade line, m/m

A  cross sectional area of ﬂ ow, m

A typical coefficient of roughness ( n ) for smooth concrete is 0.012. Because the channel slope

is horizontal or nearly so, a very small slope is used in the calculation. At first glance, the solu-

tion appears to be indeterminate because both the width of the channel and the depth of flow are

unknown. However, the width of the channel may be assumed bas e

d on either the width of the

screen or the Parshall flume inlet.

Although the continuity equation ( Q  vA ) is a satisfactory starting point for estimating the

area and, from it, the depth, the open channel flow conditions described by Manning’s equation

include the area in a nonlinear form because the area is used to compute the hydrauli

c radius. In

this case an iterative solution is required.

E xample 20-3 illustrates a channel design using Manning’s equation and a spreadsheet with

the S olver * tool.

Example 20-3. Design the channel for the bar rack for Waterloo ( Ex amples 20-1 and 20-2 ).

Assume the following:

1 . Approach velocity at average flow rate  0.4 m/s.

2. Approach velocity at peak ho

ur flow rate at design capacity 0.9 m/s.

3. Slope of channel is 0.0001 m/m.

4. Width of channel is equal to Parshall flume inlet.

5. Two channels will be provided for redundancy, but one channel must handle the flow for

the peak hydraulic flow rate.

6. Freeboard  0.6 m.

Solution:

a . Use the Solver program * in a spreads

heet to perform the iterations to solve this problem.

The spreadsheet cells are shown in Figure 20-7 . The cell locations used in the figure are

identified by the brackets [ ] in the discussion below.

* Solver is a “tool” in Excel

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

HEADWORKS AND PRELIMINARY TREATMENT 20-15

A B C D E

4 Average Q

5 n  0.012 for smooth concrete

6 Q  0.4282 m

7 S  0.0001 m/m

8 Width  1.60 m

9 Depth  0.64 m

12 Calculate velocity

14 A 1.024 m

15 P 2.879867

16 R 0.355535

17 R^0.6667 0.501849

18 S^0.5 0.01

20 v 0.42 m/s

/v 1.024 m

Solver parameters

(a)

(b)

Set target cell: $C$20

Options

Reset all

Help

Add

Guess

Change

Delete

$C$14  $C$22

$C$20 >  0.4

$B$9

Subject to the constraints:

By changing cells:

Equal to: Max. Min. Value of:

Solve

FIGURE 20-7

( a ) Spreadsheet cells and ( b ) dialog box for “solver”

parameters.

b . Begin with the average design flow rate and set the fixed parameters as follows:

[]B5 0 012n  .

[]B From Ex ample

m /d

6 20-1

37 000

86 400

Q 

ss/d

m /s 0 4282

[]B m/m7 0 0001S  .

[] ()BWidth m816W  .

c. Make a first-trial estimate of the depth by using the continuity solution.





0 4282

107

m /s

m/s

a n d

 

107

067

20-16 WATER AND WASTEWATER ENGINEERING

Enter this value in cell [B9] as the initial trial value.

d. Write an equation for the cross-sectional area of flow ( A ) using the width of the channel

and the depth of the flow in the channel, that is, B8 * B9 (see cell [B14]).

e. Set up the equation for v.



2 3 12

in cell B[]

with





()()

()2

f. Set up the equation to compute the area from:

in cell B .[]22

g. Activate the dialog box for solver and designate the target cell [B20], that is, the one

containing the computation of the velocity.

h. Set Equal to to “Min.”

i. Set By changing to the cell containing the depth of the flow in the channel [B9].

j. Add the following two constraints in the dialog box:

(1) Cell with area computed from Q / v  area compute

d from “width  depth.”

[][]CC14 22

(2) Velocity computed in step (e) above  0.4 m/s.

[]C20 0 4 .

k. Execute solver to find: depth  0.64 m and a velocity of 0.42 m/s.

l. Repeat the solution process for the peak flow to find: depth  1.4 m and the veloc-

ity  0.

53 m/s.

m. Repeat the solution process for the minimum flow to find: depth  0.265 m and the

velocity  0.28 m/s.

n. The channel depth is the depth at maximum flow plus the freeboard. The channel dimen-

sions are then:

W i dth  1.6 m

Depth  1.4 m  0.6 m  2.0 m

Comments:

1 .

The primary constraints on the approach velocity at average design flow and peak

flow were met with only one channel in service. The velocity at the minimum flow

HEADWORKS AND PRELIMINARY TREATMENT 20-17

at start-up is low. This will be an operation and maintenance issue early in the life of

the plant. The channels may have to be dewatered and cleaned more frequently than

otherwise may have been anticipated. Because there are two channels, this will be

possible without undue interruption in operation.

2. The commercially available widths of bar racks may be a more appropriate point for

selecting a starting width.

3. The arrangement of the spreadsheet cells shown in this example is presented for the con-

venience of explaining the solution. Other arrangements may be more convenient and/or

efficient.

Bar Design. Typical design ranges for bar racks are summarized in Table 20-5 .

Materials. Current practice is to use

corrosion-resistant materials such as stainless steel and

plastics for the bar rack. Though much more expensive, the stainless-steel systems are preferred

because of their better maintenance history.

Headloss. The headloss through mechanically cleaned coarse screen

s is a key design parameter.

Based on an assumption of steady, two-dimensional flow, the clean water headloss through a bar

screen can be calculated using the Bernoulli equation. The headloss created by a clean screen may

be estimated based on the flow and the effective area of the screen openings.

For clean and

partially clogged bar screens, the Bernoulli equation yields (WEF, 1998):

kv v



[( ) ( ) ]

thru approach

(20-5)

where H

 headloss, m

k  empirical discharge coefficient

thru

 velocity through the bar screen, m/s

approach

 approach velocity, m/s

g  gravitational acceleration  9.81 m/s

TABLE 20-5

Typical bar design ranges

Cleaning method

Parameter Manual Mechanical

Bar size

Width 5–15 mm 5–15 mm

Depth 25–40 mm 25–40 mm

Clear spacing between bars 25–50 mm 6–75 mm

Slope from vertical 30–45 0–30

Sources: Daukss, 2006; Metcalf & Eddy, 2003; WEF, 1998.

20-18 WATER AND WASTEWATER ENGINEERING

The discharge coefficient is typically taken to be 1.67 for a clean screen and 1.43 for a partially

clogged screen (Metcalf & Eddy, 2003). The approac h velocity is calculated for the channel

upstream of the bar rack. The velocity through the bar rack is based on the effective area of the

screen openings. It is estimated as the sum of the verti

cal projections of the screen openings

(WEF, 1998). The screen opening may be approximated by assuming a bar arrangement like the

one shown in Figure 20-8 . The approximate number of bars is:

bars

Width of channel bar spacing

Bar width





bar space

(20-6)

The number of bar spaces is N

bars

 1.

Headloss through mechanically cleaned bar racks is typically limited to 150 mm by opera-

tional controls. The controls activate the raking mechanism based on measurement of the differ-

ence in head upstream and downstream of the screen. Alternatively, the raking mec

hanism may

be operated by a time clock. A cycle length of 15 to 30 minutes is typical. For the timed cycle

system, either a high water or high d ifferential override switch should be provided that places

the rake mechanism in continuous operation when excessive loads are encountered (Met

calf &

Eddy, 2003). Increases in flow rate that indicate wet weather flow should activate the screens to

run continuously (Spangler, 2006).

E xample 20-4 illustrates the use of the headloss equation to select a bar arrangement and

estimate the headloss with partial blockage of the screen.

Example 20-4. Estimate the headlosses for a bar rack for Waterloo ( Examples

20-1 , 20-2 , and

20-3 ) with a clean bar rack and with partial blockage of the screen. Use the following assumptions:

1 . Mechanically cleaned bar rack.

2. Bar width  15 mm.

3. Bar spacing  20 mm.

4. Angle from vertical  30  .

FIGURE 20-8

E xample of bar arrangement for coarse screen.

Channel width

35 mm

25mm 25mm

10 mm

Right wall

Left wall

HEADWORKS AND PRELIMINARY TREATMENT 20-19

5. Differential headloss for activation of the cleaning rakes is 150 mm.

6. Maximum flow area blockage to initiate continuous operation of rakes is 50%.

Solution:

a . Begin by checking the velocity through one bar rack at the peak hour flow rate at design

capacity.

b. Estimate the number of bars and openings. From Example 20-3 , the channel width i

1.6 m. Using Equation 20-6 , the approximate number of bars is:

1 600 20

15 20

45 14 45

mm mm

or bars







The number of bar spaces is 45  1  46.

c. Determine the effective area of the screen openings by calculating the projected total

open area. Use the depth of flow in the channel at the peak hour flow rate at design ca-

pacity from Example 20-3 .

Area spaces mm/space m/

mm



()( )( )(46 20 10 1

.44129

mm)  .

d. From Example 20-1 , the peak hour flow rate at design capacity is 104,000 m

/ d . The

velocity through the bar rack is

thru

m /d

s/d



104 000

86 400

129

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝⎝

⎜

⎞

⎠

⎟

 093. m/s

This is slightly above the recommended velocity of 0.9 m/s.

e. Using Equation 20-5 and the approach velocity of 0.53 m/s from Example 20-3 , the headloss

through the clean bar rack is



167 093 0 53

2981

.. .

[( ) ( ) ]

(

m/s m/s

m/s

))



098

19 62

005

. m or 50 mm

f. If the bar rack is 50% clogged, the area of flow is reduced by 50% and the velocity

through the bar rack is doubled to 1.86 m/s. The headloss is then



143 186 053

2981

.. .

[( ) ( ) ]

(

m/s m/s

m/s

))



4 55

19 62

023

. m or 230 mm

This headloss is greater than that set to initiate cleaning. In addition, the velocity con-

straint is exceeded.

20-20 WATER AND WASTEWATER ENGINEERING

g. Check the velocity and headloss at the average flow rate at design capacity of 37,000

/ d. From Example 20-3 , the depth is 0.64 m and the approach velocity is 0.42 m/s.

Area spaces mm/space m/mm



()( )( )(46 20 10 0

.664

37 000

86 400

m)059 m

m /d

s/d

thru



⎛

⎝

⎜⎜

⎞

⎠

⎟

⎛

⎝

⎜

⎞

⎠

⎟

0 59

073

m/s

This meets the recommended velocity through the bars.



167 073 042

2981

.. .

[( ) ( ) ]

(

m/s m/s

m/s

))



0 595

19 62

003

. m or 30 mm

h. Two bar racks are required to meet redundancy requirements.

Comments:

1 . The 50% blockage at peak flow conditions is extreme. This condition warrants an

alarm to the operator and initiation of continuous cleaning when the headloss becomes

exc essive.

2. The clean bar rack headloss is acceptable.

3. Typi

cally, manufacturers will provide standard bar rack bar sizes and spacing as well as

headloss curves or tables.

Screenings Handling. Screenings from the rake are usually discharged directly into a hopper

or movable container. The design must provide head space above the deck for the d

ischarge as

well as the screen return and motor assembly (see Figure 20-5 ). The spacing required is particular

to the manufacturer’s model that is selected.

Finer spacing results in wetter screenings. Wetter screenings create handling and transport is-

sue

s. The design practice for treating and disposing of the screenings is discussed in Chapter 27.

Hints from the Field. Operational experience has yielded the following observations:

1. If material is left on the screen too long it will work through to the downstream side. To

avoid this, one of the following alternatives is recommended:

• T ypically, timed

screen cleaning should be on for about 5 m inutes and off for 25

minutes.

• Flow-based screen cleaning uses a system that changes the timing of cleaning to increase

the frequency of cleaning for high flows.

• Headloss across the screen plus a minimum cleaning cycle appears

to be the system that

works best to control the activation of the rakes.