Water and Wastewater Engineering

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19-10 WATER AND WASTEWATER ENGINEERING

Estimation of Wastewater Flow Rates

The required wastewater flow rates at the beginning of the service life and at the design life are

the average daily flow rate, peak hour flow rate, and the peak infiltration allowance. These were

discussed in detail in Chapter 18.

Pipe Material Selection

The principal sewer material for pipes with small or medium diameters is polyvinyl chloride

(PVC). For larger pipe diameters, ductile iron pipe (DIP), high density polyethylene (HDPE)

pipe, or reinforced concrete pipe (RCP) may be specified. Truss pipes are becoming more com-

mon for larger pipe diameters.

Vitrified Clay Pipe (

VCP). This classic pipe material has demonstrated its durability in use in

the United States for over a century. It has a high resistance to corrosion and abrasion. Its major

disadvantage is its high mass per unit length that makes it more difficult to handle and increases

installation costs. It is rarely installed today.

This pipe is made of clay or shale that has been ground, wet, molded, dried, and fired in a

kiln. Near the end of the burning process, sodium chloride is added to the kiln. It vaporizes to

form a hard waterproof glaze by reacting with the pipe surface. The firing of the cla

y produces a

vitrification of the clay that makes it very hard and dense (Steel and McGhee, 1979).

The pipe is manufactured with integral bell and spigot ends fitted with polymeric rings. It is

available in diameters from 75 mm through 1,050 mm and lengths up to 3 m (ASCE, 1982). Pipes

are typically joined

with push-on gasket joints.

Polyvinyl Chloride Pipe (PVC). This pipe is made by extrusion of polyvinyl chloride. It is

available in diameters from 10 mm through 1.2 m and lengths up to 6 m (ASCE, 1982). Rubber

gasket bell and spigot type joints are used to connect the pipes.

This pipe has been in use for over half a century. It i

s almost exclusively the material of

choice for pressure and vacuum sewers. Its advantages are corrosion resistance and low mass per

unit length. It is subject to attack by certain organic chemicals and excessive deflection if improp-

erly bedded. The low m

ass per unit length gives it some cost advantage in installation.

Ductile Iron Pipe (DIP). This pipe material was discussed in Chapter 17. Its primary applica-

tion for sewers is for force mains. Because wastewater is often corrosive, current practice is to

use a cement mortar lining and an asphaltic outer c oating. Epo

xy coating may be used in trunk

sewers. DIP manufacturers recommend that the pipe be encased in a loose-fitting flexible poly-

ethylene tube (0.2 mm thick) when the pipe is to be placed in corrosive soils.

High-Density Polyethylene (HDPE). This pipe material was discussed in Chapter 17. Its

primary us

e is as an alternative pressure pipe for force mains.

Reinforced Concrete Pipe (RCP). Precast RCP is manufactured by a variety of techniques

including centrifugation, vibration, packing, and tamping for consolidating the concrete in forms.

Adjustment of the wall thickness, concrete strength, and reinforcing allow for a wide variety of

strengths.

SANITARY SEWER DESIGN 19-11

The pipe is manufactured with integral bell and spigot ends. It is available in diameters from

300 mm through 5.0 m, and lengths up to 7.5 m (ASCE, 1982). These pipes are typically joined

with push-on gasket joints.

The normal service for RCP is for trunk lines and interceptor sewers. Its major limitations are

its high

mass per unit length and its susceptibility to crown corrosion.

Truss Pipe. This pipe is made of PVC or acrylonitrile butadiene styrene (ABS). It consists of

dual walls with a truss system between the walls. Sometimes the space between the walls is filled

with cement. It is more rigid than PVC pipe but shares the same ea

se of construction.

Crown Corrosion. The corrosion of the crown of a sewer is the result of biochemical reactions

in the sewage and in the condensed moisture at the crown of the sewer. With long sewer reaches

and little oxygen transfer from the air in the sewer, or when sewage sits for long periods between

pumping in force mains (e.g., in residential areas with little or no nighttim

e flow), the sewage

becomes anoxic or anaerobic. Under these c onditions, the sulfate in the sewage is reduced to

sulfide. At the usual pH level of domestic sewage, the s ulfide is converted to hydrogen sulfide.

In poorly ventilated sewers, moisture collects on the walls and the

crown. Hydrogen s ulfide

dissolves in this moisture. As such it does no harm.

Bacteria capable of oxidizing hydrogen sulfide, in particular those of the genus Thiobacillus,

are always present in sewage. At high flows, these bacteria are brought to the walls and crown

where they adhere after the high flows recede. They ox

idize the hydrogen sulfide to sulfuric acid

by the following reaction (Sawyer et al., 2003):

HS O HSO

bacteria

22 24

2

⎯→⎯⎯

←⎯⎯⎯

(19-1)

This strong acid-attacks acid soluble materials such as concrete, iron, or steel. The corroded

crown fails under the load of soil above it. These processes are summarized in Figure 19-8 .

Air

Wastewater

Anaerobic

conditions

2ⴚ

Home of H

oxidizing bacteria

Water droplets

S  2O

bacteria

2ⴚ

ⴙ

FIGURE 19-8

Formation of hydrogen sulfide in sewers and “crown” corro-

sion resulting from oxidation of hydrogen sulfide to sulfuric acid.

( Source: Sawyer et al., 2003.)

19-12 WATER AND WASTEWATER ENGINEERING

Where wastewater temperatures are high, detention times in the sewer are long, and the

sulfate concentrations in the sewage are high, this is a particularly serious problem that must be

considered in the selection of pipe materials. Because they are not s usceptible to crown corro-

sion, VCP and PVC pipe are frequently chosen for these applications. When very large

diameters

require the use of RCP pipe, it should be lined with corrosion resistant material.

Design Criteria

The design criteria discussed in this section apply to gravity flow sewers. Other criteria are used

for alternative sewer systems such as pressure and vacuum sewers.

Location. In the construction for new residential areas, the sewer is commonly placed on one

side of the roadway in the right-of-way (ROW). Connection

s to the sewer from buildings on

the opposite side of the street may be made by boring und er the street. In established communi-

ties (or where local codes require), it may be found in alley ways behind the residence or in the

street.

Sewers should be at such a depth that they c

an receive the contributed flow by gravity.

Where houses have basements, the invert of the sewer is placed a minimum of 3.0 to 3 .5 m

below grade. Where there is no basement, it is placed to provide sufficient cover to protect the

pipe from live load and dead load damage. Moser (2001) provides guidanc

e on design to prevent

live and dead load damage. Where building codes are in place, they should be consulted for the

appropriate depth. In the absence of other guidance, a rule of thumb is to use a sewer invert depth

of 1.8 to 2.4 m below grade when basements are not present.

B uilding codes may prohibit gravity

service from the basement. If sewage is to be removed

from the basement level, grinder pumps are installed.

When sewers cannot be placed at a depth sufficient to prevent freezing, for example, when

bedrock is near the surface, they must be insulated (GLUMRB, 2004). Countermarsh (1998) dis-

cusses alternative des

igns for these conditions.

Maximum sewer depth is approximately 8 to 9 m. When the depth exceeds 8 to 9 m, a lift sta-

tion is provided. In exceptional circumstances, the sewer may reach a practical construction limit

of 10 to 12 m depth before a lift station is constructed

GLUMRB (2004) specifies that gravity sewers shall be laid at least 3 m horizontally (edge

to edge) from any existing or proposed water mains. Sewers crossing water mains shall be laid

to provide a minimum vertical distance of 0.45 m between the outside of the water main and the

outside of the sewer. It i

s preferable that the water main be located above the sewer. At c ross-

ings, one full length of water pipe shall be located so both joints will be as far from the sewer as

possible. The sewer shall be designed and constructed equal to water pipe, and shall be pressure

tested at 1,035 kPa to assure water tightness.

Pipe Size. No public gravity sewer conveying raw wastewater s

hall be less than 200 mm in

diameter (GLUMRB, 2004). This size has been selected to minimize clogging when extraneous

material enters the sewer.

Some engineers design sewer pipes to flow half full at the design capacity to provide a fac-

tor of safety. This practice is favored when designing laterals or branches that have the potential

to be extended to acco

mmodate growth. It is not justified for mains, trunk lines, or interceptors

(Steel and McGhee, 1979).

SANITARY SEWER DESIGN 19-13

Slope. All sewers shall be designed and constructed to give mean velocities, when flowing full,

of not less than 0.6 m/s based on Manning’s formula using an “ n ” value of 0.013 (GLUMRB,

2004). Manning’s formula is discussed in the next section.

Slopes are commonly calculated using the depth of the invert of the pipe. Mini

mum slopes to

achieve 0.6 m/s are shown in Table 19-2 . Slopes greater than these may be desirable to maintain

self-cleansing velocities at all rates of flow, for construction, or to control sewer gases. A mean

velocity of 0.3 m/s is usually sufficient to prevent the deposition of the organic solids in waste-

water. To prevent depo

sition of mineral matter, a mean velocity of 0.75 m/s is required. Slopes

that result in mean velocities of 0.5 m/s have been used, but these require frequent cleaning (Met-

calf & Eddy, 1981).

Sewers 1.2 m and larger should be designed and constructed to give m ean velocities, when

flowing full, of not less

than 0.9 m/s based on Manning’s formula and an “ n ” value of 0.13.

Oversizing sewers to justify flatter slopes is prohibited. The use of larger pipes at flatter

slopes will reduce the velocity well below the self-cleaning velocity.

The erosive action of the material suspended in the wastewater depends on the nature of the

material and the velocity at which it is

carried along. The erosive action determines the maximum

safe velocity of the wastewater. In general, maximum mean velocities of 2.5 to 3.0 m/s at the

design depth of flow will not damage the sewer (Metcalf & Eddy, 1981).

Where velocities greater than 4.6 m/s are anticipated, special provision must be made to pro-

tect against

displacement by erosion and impact. Sewers on slopes greater than 20 percent must

be securely anchored.

The slope between manholes must be uniform.

Alignment. In general, sewers less than or equal to 600 mm in diameter m ust be laid with

straight alignment between manholes. Curvilinear alignment of sewers large than 600 mm may

be permitted if compression joints are specified. Slopes must be increased with curvilinear align-

ment to maintain a minimum velocity above 0.6 m/s. The recommended practice is to u se extra

manholes and straight alignment between manholes.

TABLE 19-2

Recommended minimum slopes for gravity flow sewers

Nominal diameter,

Minimum slope, m/m

n  0.013

Minimum slope, m/m

n  0.010

Capacity, flowing full,

200 0.0033 0.0020 0.019

250 0.0025 0.0015 0.029

300 0.0019 0.0011 0.042

350 0.0016 0.0009 0.058

375 0.0014

0.0008

0.066

400 0.0013

0.0008

0.075

450 0.0011

0.0007

0.095

500 0.0010

0.0006

0.118

600

0.0008

0.0005

0.170

Calculated using Manning’s equation with nominal pipe diameter and v  0.6 m/s.

The minimum practicable slope for construction is about 0.0008 m/m.

19-14 WATER AND WASTEWATER ENGINEERING

Changes in Pipe Size. When a smaller pipe joins a larger one, the invert of the larger sewer

should be lowered sufficiently to maintain the same energy gradient. An approximate method for

securing this result is to place the 0.8 depth point of both sewers at the same location.

In no instance should a larger pipe discharge into a smaller pipe. Even though a smaller pipe

at a steeper slope may be able to carry the larger flow, there i

s the potential for objects that will

travel freely in the larger pipe to obstruct the smaller pipe.

Manholes. Manholes are placed at the junction of two or more sewers , at changes in vertical

or horizontal alignment, at changes in sewer size, and at the end of each line. The spacing for

straight runs is shown in Table 19-3 .

Drop manholes are used when the inflow and outflow sewers differ in elevation by more then

0.6

m. They may also be used to reduce the slope when the veloc ities exceed erosive velocities

(2.5–3.0 m/s).

The manholes in small sewers are typically about 1.2 m in diameter. A minimum access

diameter of 0.6 m is provided. Although the same size m

anhole barrel is used for both small and

large manholes, the base will be larger for sewers larger than 600 mm.

Although the American Society of Civil Engineer’s manual on sewer design (ASCE, 1982)

suggests that it is unnecessary, current practice is to provide an arbitrary minimum drop of 30 mm

across the standard

manhole. Otherwise, the grade through the manhole should match the energy

grade line for larger diameters or with size change.

Hints from the Field. Experience has yielded the following useful rules of thumb. These are

not design criteria but rather practical considerations in applying the design criteria.

• In normal practice, the grou

nd slope is used as a first trial for selecting the slope. However,

there are a number of exceptions. For example:

a . If the ground is flat, select the minimum slope to achieve a velocity of 0.6 m/s with the

sewer flowing full.

b. If there is a slight upgrade for a short distance, select the minimum s

lope to achieve a

velocity of 0.6 m/s with the sewer flowing full.

c. If the ground slope yields a velocity greater than 2.5 m/s, then select a lower slope.

TABLE 19-3

Typical manhole spacing for straight runs

Pipe diameter Spacing

375 mm or less 120 m or less

450 to 750 mm 150 m

450 to 750 mm 180 m with adequate cleaning equipment

825 to 1,200 mm 180 m

1,200 mm or greater 460 m

The actual spacing is highly dependent on local conditions and client preference.

SANITARY SEWER DESIGN 19-15

• A drop manhole may be used to minimize excessive velocity in a steep sewer. Alternatively,

consider designing the sewer as a gravity force main or pressure pipe with energy dissipa-

tion at the downstream end of the pipe (Orsatti, 1996).

• Because lift stations are expensive to build, operate, and maintain, avoid them to the

maxi-

mum extent possible by considering alternative routing.

• Because drop manholes are expensive to bu ild and often become plugged or fail structur-

ally, avoid them to the m aximum extent possible by considering the alternative of extra

excavation.

• In the upper reaches of the c

ollection system (e.g., in residential subdivisions), there

will seldom be enough flow at average or, perhaps, even at maximum discharge rates to

achieve a minimum velocity of 0.6 m/s in a 200 mm diameter sewer at minimum slope.

This means that a regular maintenance program that includes c

leaning the sewer will be

required.

Summary. The design criteria for sewers are summarized in Table 19-4 .

Design Equations

Two equations serve in the design of sewers: the Hazen-Williams equation introduced in

Chapter 3 (Equation 3-2) and Manning’s equation. Of these two, the Manning equation has found

wide application in sewer design because it applies both to sewers flowing full and those flowing

partially full—the latter being the condition most frequently encountered. Three us

eful forms of

Manning’s equation are:

RS

2 3 12//

(19-2)



()()( )

()

1 333

6 3448.

(19-3)

RSA

2 3 12//

(19-4)

where v  velocity, m/s

n  coefficient of roughness, unitless

R  hydraulic radius, m



cross-sectional area of flow m

wetted peri

mmeter m,

S  slope of energy grade line, m/m

D  diameter of pipe, m

Q  flow rate, m

/ s

A  cross-sectional area of flow, m

The selection of an appropriate value for n i s typically based on the pipe material and its con-

dition. There is a large body of work that has been devoted to the determination of n for various

surfaces. Selected values from the literature are shown in Table 19-5 on page 9-17.

19-16 WATER AND WASTEWATER ENGINEERING

A s noted above, GLUMRB (2004) specifies that n  0.013 for the calcu lation of minim u m

velocity. Designers typically use n  0.013 for PVC because, once the pipe is in use, this n i s

more realistic.

For a pipe flowing full, the hydraulic radius is defined as



()()



(19-5)

TABLE 19-4

Typical design criteria for gravity sewers

Parameter Recommendation Comment

Pipe material PVC For main sewers

Pipe diameter 200 mm minimum Nominal diameter

Size to carry peak flow rate See Table 19-2

Slope To achieve 0.6 m/sFlowing full, Manning’s

n  0.013, see Table 19-2

Maximum velocity 2.5 to 3.0 m/s

Depth to invert 3.0 to 3.5 m minimum When basem

ents are present or

0.6 m below the lowest basement

1.8 to 2.4 m When no basements are present

8 to 9 m maximum

Depth to crown

H  D/6 minimum

For dead load; granular soil

For live load see Moser (2001)

Location with respect to

water mains

3.0 m horizontally

0.45 m vertically

When crossing

Edge to e

dge

Outside to outside; water above

sewer

Full length to joint for water main

or encased sewer

Alignment Straight between manholes 600 mm or less

Manholes Place at:

junction of two sewers

change in vertical alignment

change in horizontal alignment

change in pipe size

at end of each line

Spacing for straight runs See Table 19-3

Effluent elev.  Influent elev. 

0.03 m

To acc

ount for entrance and exit

losses

Drop manholes When inflow and outflow inverts

differ by more than 0.6 m

Manhole diameter 1.2 m minimum Access  0.6 m

SANITARY SEWER DESIGN 19-17

where D  diameter of pipe in meters. Similarly, for a pipe flowing half-full, R  D /4. For

pipes flowing full or half-full, other useful forms of Manning’s equation are:

DS

0 397

2 3 12

()

(19-6)

DS

0 312

8 3 12

()

(19-7)



10 3

16 3

.( )( )

()

(19-8)

For other fractional flows, the estimate of R i s quite complex. Camp (1946) developed

a graphical solution procedure. Camp’s graph (adapted in Figure 19-9 ) depicts the nonlinear

behavior of several variables in the Manning equation. To begin with the obvious, the curves

are based on the value of the variable when the pipe is flowing full. In common notation, a capi-

tal letter is used for the valu

e of the variable when the pipe is flowing full, that is, D, V, and Q.

A lower case letter is for the value of the variable when the pipe is flowing partially full, that

is, d, v, q.

Research on asbestos-cement, DIP, VCP, and RCP pipe has revealed that Manning’s n varies

with depth as shown on the left-hand side of Figure 19-9 (Bloodgood and Bell, 1961; Pomeroy,

1967). The functional form is as follows (ASCE, 1982):

nk D



()( )//213 6

(19-9)

where n  Manning’s n

k  effective absolute roughness, or height of wall roughness, m

m  proportionality factor

D  diameter of pipe, m

The value of m is slightly less than 1/3 for turbulent flow.

TABLE 19-5

Typical values of n that are used with the Manning equation

Condition

Pipe material GoodFair Deteriorated

DIP (lined) 0.011

0.013

0.015

HDPE

0.010

0.011 0.013

PVC

0.010

0.011 0.013

RCP 0.013

0.015

0.018

VCP

0.013

0.015

0.017

Values commonly used in design.

19-18 WATER AND WASTEWATER ENGINEERING

In contrast to the pipes noted above, PVC pipe with flows from 0.2 D to full do not indicate a

significant variation in n with depth. Data for discharge and velocity approximate the curves that

are obtained using an assumption of a constant n (Neale and Price, 1964).

The shape of the velocity ratio curve is primarily a function of the change in the hydraulic

radi

us with depth of flow. From simple geometric relationships, for pipes flowing partially full,

this can be shown to be:



180

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝

⎜

⎞

⎠

⎟

sin 



(19-10)

where R  hydraulic radius,m

D  pipe diameter, m

  angle subtended by the segment of the pipe diameter occupied by the wastewater

(see inset in Figure 19-9 ), degrees

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3

Hydraulic elements

and

V full

Q full

Ratio of depth to diameter d/D

Sewage

level

Particulate matte in sewage

n variable with depth

n constant

Values of

n full

1.1 1.2

1.3

Mannings’ n

Discharge

Velocity

␪

FIGURE 19-9

H ydraulic properties of circular sewers. (Adapted from Camp, 1946.)

SANITARY SEWER DESIGN 19-19

Note that cos 1/2 q  1  2 d / D. The shape of the discharge ratio curve is a function of the change

in both the area and hydraulic radius as a function of depth. The behavior of the area as a function

of depth is described by Equation 19-11:



4 360



sin 

⎛

⎝

⎜

⎞

⎠

⎟

(19-11)

E xample 19-1 illustrates the use of Figure 19-9 in estimating the velocity of sewage flowing

in a partially full sewer.

Example 19-1. Determine the velocity of 0.0081 m

/ s flow in a 200 mm diam eter sewer at a

slope of 0.0040 m/m. The pipe is new VCP.

Solution.

a . Estimate the velocity and flow rate for the sewer flowing full. The hydraulic radius for

flowing full is D /4 and n  0.013 for new VCP ( Table 19-5 ). Using Equation 19-6:

full

mm/m

0 397

0013

0 200 0 0040

2 3 12

..()( )

==()( )( )30 53 0 3420 0 0632066.. . . m/s

Using Equation 19-7:

full

mm/m

0 312

0 013

0 200 0 0040

83 12

..()( )

()()()24 00 0 0137 0 0632 0 0208

.. . .m /s

b. Calculate Q / Q

full

m /s



0 0081

0 0208

039

c. Enter Figure 19-9 on the abscissa at Q / Q

full

 0.39 and draw a vertical line to the dis-

charge curve labeled “ n variable with depth.” From the intersection point, draw a hori-

zontal line to the velocity curve labeled “ n variable with depth.” From this intersection

point, draw a vertical line down to the abscissa and read v / v

full

 0.78.

d. The velocity flowing partially full is then

vv 078 078 066 051....m/sm/s

full

()( )

Comments:

1 . The specification of new VCP pipe was used to select n  0.013 and to select the “ n

variable with depth” curves. For plastic pipe, the value for n i s 0.010, and the curves

selected would be “ n constant.”

2. The use of the graphs for estimating v / v

full

i mplies that calculated velocities should have

no more than two significant figures. In all likelihood, given the inherent errors in esti-

mating flow rates and Manning’s n, a s well as the use of the nominal pipe diameter, one

might argue that velocities should be limited to one significant figure.