S. Sarkar

Pradeep Kumar

Sewers and Sewer Networks Design, Construction and Maintenance

RAW WATER

TREATED WATER

WASTEWATER

TEATED WASTEWATER

WASTEWATER TREATMENT PLANT

WATER TREATMENT PLANT

Classification of Sewers

Domestic or Industrial Sewers

Storm Sewers

Combined Sewers

They are designed to carry wastewater generated from domestic establishments or small- and medium- sized industrial establishments in a municipal area but not storm-water

They are designed to carry off only stormwater and groundwater but excludes sewage from domestic and/ or industrial source

They are designed to carry off stormwater, domestic and industrial wastewater

Advantages and Disadvantages of Combined Sewers

It is initially economical to set up a combined sewer rather than separately installing domestic sewers and stormwater sewers

During dry season lack of stormwater causes a low flow rate. Low flow rate gives rise to low velocity of flow. At low velocities, due to less turbulence, the deposition of sewage solids are more. Result is siltation and consequent foul odor generation due to degradation of the settled solids.

In contrast, during wet or rainy seasons, the flow rate is very high. Therefore, pumping costs are more, causing high operation and maintenance cost.

Pumps that are designed to operate at high flow rate to tackle the wet season flow, runs in low flow condition in dry season which is an inefficient operation that consumes more power than usual.

Due to the above reasons, combined sewers are not generally recommended by the manual of sewerage and sewage treatment, Government of India

Estimation of Quantity of Sanitary Sewage

The sewers are designed to carry:

Spent water from a community Some groundwater Fraction of the stormwater Industrial wastewater for small establishments

The sanitary sewers are designed to carry the wastewater from the above sources to a sewage/wastewater treatment plants

Carrying capacity of the sewers depends on: 1. Present and 2. Future quantities of flow rate expected.

Thus, it is important to estimate the design flow rate for the sewers to be constructed.

Estimation of Sewage Flowrate

Two Parameters: 1. The contributing population, and 2. Per capita (per person) flowrate of

sewage

Both of these quantities depend on the design period

Design period: The length of the time up to which the capacity of a sewer will be adequate is called a design period.

Normally design period for a sewerage system is considered as 30 years

But, mechanical rotating equipment such as pumps are designed for 15 years

Forecasting the Population

Prospective population of the project area (may be a city, town or a metropolitan area)

Methods: Demographic population projection Arithmetical increase method Incremental Increase method Geometrical Increase method Growth rate Graphical method Logistic method Method of density

Where is the forecast found for design purposes?

Normally for a city, population growth forecasts are found from the master-plan prepared by town planning or other relevant authorities.

What to do when masterplan or planners documents are unavailable?

Floor-Space Index Based Calculation

1. From the city-plan find out the % of the total area available for residential development

2. Actual total floor area = Area for residential development X Floor Space Index (FSI)

3. Find out floor area required for one person or assume it depending on the available data from the city. Normally it is 9 sqm/ person.

4. Find out the density of population per hectare

5. Multiply the density with the total area of the city to find out the total population

This total population can be used for estimating the quantity of total sewage flow.

Example: Finding out population density based on Floor Space Index method

A well-planned city has following areas earmarked for its development in the planning stage: Roads- 20%; Gardens- 15%; Schools 5%; markets and Commerical places 2%; Hospital and medical facilities 2% and rest is residential area. The Floor Space Index (FSI) for the city is fixed at 2. If the floor area is 9 sqm/ person, find out the projected population density of the city in numbers/ hecatare.

Residential Area (%) = 100 (20+15+5+2+2) = 56

Actual Floor Area = Area of the land X FSI

Population that can reside in the area= Actual Floor area / Area required by a person

= 0.56X2 /9

Population density (numbers / hectare) = 0.56X2X10000/9 = 1244

numbers / sqm

Per Capita Sewage Flow rate

Ideally the entire amount of water used by a community should appear as the total flow in a sanitary sewer

Water is lost due to:

Evaporation Loss Seepage into ground Leakges

The dry weather flowrate is slightly less than the per capita water consumption

For very dry and arid regions, Average sewage flowrate 40% of water consumption rate

In well-paved and well-developed areas, Average sewage flowrate 90% of water consumption rate

Conservative estimate is 80% of water consumption rate

Design water consumption in India = 130 LPCD (litre per capita per day)

Design minimum wastewater flow in India = 100 LPCD

Variations in Flow and Peak Factor

Time of the day

Flo

w r

ate

Water consumption varies from hour to hour. Along with daily variations, there also are seasonal variations.

For design purpose, sewers are always designed to carry maximum or peak flow rates, rather than designing it for average flowrate.

Peak Factor (PF) = Maximum wastewater flow rate

Average flow rate of wastewater

Population Peak factor

< 20,000 3.0

20,000 50,000 2.5

50,000 7,50,000 2.25

> 7,50,000 2.0

4 8 12 16 20 24

Average

In addition, commercial and industrial contributions are to be considered into the total flow rate.

Groundwater Infiltration into Sewer lines

The sewers have joints. Some groundwater runoff may also seep into the sanitary sewers.

The extent of groundwater infiltration into the sewers depend on the workmanship and the level of the groundwater table with respect to the sewers.

Usually, for a sanitary sewer below the groundwater table the following values are taken,

Minimum Maximum

Liters/ha.d 5000 50000

Liters per day/ manhole

250 500

Maximum sanitary flow rate = Average domestic flow rate X PF + infiltration flow rate

PEAK FLOW RATE or MAXIMUM FLOW RATE

Area with Sufficient Urbanization Area with little or no urbanization

More paved surface, higher imperviousness, less absorption by soil

High volume of water on the surface, High runoff, needs quick evacuation to avoid flooding/ inundation

RAINFALL

How to evacuate this increased runoff? BUILD EFFICIENT STORM SEWER SYSTEM

Finding Out of Runoff

Runoff quantity depends on:

Rainfall Characteristics (Intensity, Duration and space-time distributions)

Characteristics of the watershed surface (nature, permeability, slope, and landscaping)

Time of concentration (time required for flow to reach the sewer)

The design should be adequate to carry from a basin or watershed the maximum runoff caused by the design rainfall.

Storm sewers are designed for a rainfall with particular frequency or return period. The design rainfall is fixed after economic considerations involving the Intensity-duration and frequency (IDF) curves in an area.

Case I

tnttt

tttt

tttt

nn

0

022

011

2

Rainfall duration is t

Time Runoff

t0= 0 Q0 = Q(t=0) =0

t1= t Q1=A1IC1

t2=2t Q2=A2IC2

tn=nt Qn=AnICn

Rainfall over a watershed draining at a single discharge point

I = Intensity of the rainfall A = Area C= Run-off coefficient

Case II Rainfall duration is 2t

Time Runoff

t0= 0 Q0 = Q(t=0)= 0

t1= t Q1=A1IC1

t2=2t Q2=A1IC1+A2IC2

t3=3t Q3=A2IC2+A3IC3

tn=nt Qn= An-1ICn-1+AnICn

tn=(n+1)t Qn+1 =AnICn

tn+2 =(n+2) t Qn+2 =0

Case III Rainfall duration is nt

Time Runoff

t0= 0 Q0 = 0

t1= t Q1=A1IC1

tj=jt

t3=nt

tn=(n+1)t

tn=(2n-1)t Q2n-1=A1IC1

tn+2 =2nt Q2n =0

j

k

kkj ICAQ1

n

k

kkn ICAQ1

n

k

kkn ICAQ2

1

A Few Observations

If the duration of the rainfall is tn and tn is the time necessary for the water droplet to reach to the basin outlet from the hydraulically most distant place in the basin, the entire surface area of the basin contributes to the flow rate or the runoff observed from the basin.

If the duration of the rainfall is longer than tn, the runoff value remains equal to the same as the case above, from the time tn until the end of the rainfall duration.

If the duration of the rainfall is shorter than tn, the maximum runoff occurs at the end of the rainfall and is smaller than the runoff obtained for a precipitation of duration tn.

The maximum runoff flow is always reached at the latest by the end of the rainfall.

The maximum runoff due to a precipitation of uniform intensity I falling all over the drainage basin, and of duration tn (the longest time for water to travel to the outfall from the basin), is thus given by

n

k

kk

n

k

kkn CAIICAQ11

Rational Equation

Q = 10 CIA

Q = Run-off in cum/hr C= coefficient of run-off I= Intensity of design rainfall, mm/hr A = Area of drainage basin in hectares

n

k

kk

n

k

kkn CAIICAQ11

In familiar terms, the above equation is thus given by,

AICQ

Values of C Absolutely impervious basin.1.0 Paved Areas0.9 Lawn and Gardens.0.15 Water-bound macadem roads0.45

The period of time after which the entire basin area starts contributing to the run-off is called the time of concentration. Varies from 3 to 30 minutes

Maximum run-off is obtained from a rain having a duration equal to the time of concentration. SEWER

OUTFALL

DRAINAGE BASIN

tC

Time of Concentration (tc)

The duration of such a rainfall is called critical rainfall duration and the intensity of such rainfall is known as critical rainfall intensity.

fec ttt te

tf

SEWER OUTFALL

te= time of entry

tf= time of flow

Sub-basin

Time of entry is the longest time required for a water droplet in an urban sub-basin to travel to a street inlet.

Kirpichs model:

385.0

77.00195.0

s

FLte

L= maximum distance travelled by the water on the surface

s= average slope of the route travelled by water

F = friction factor

Surface type F

Rural watershed (flat ground) 1.0

Grass surface 2.0

Concrete or Asphalt surface 0.4

Concrete channel 0.2

Time of flow is the time required for water to travel to a sewage outfall from the street inlet in the urban sub-basin. It is always computed considering that the pipe is running full.

21

321sR

nv

v

Lt f

Typical Rainfall Intensity-Duration-Frequency Curves R

ain

fall

Inte

nsi

ty, m

m/h

r

Duration, minutes

The curves can vary from place to place and the shape of the curve follows different patterns.

kt

aI

kt

aI

n

n

x

tb

CNI

)(

I ( rainfall intensity) and T (duration) are variables; other terms are constants that can be found out from fitting the curve with the field data obtained.

How to find out the design maximum run-off of a basin?

1. Decide on the frequency of rainfall on which the design will be based on. Lets assume it is twice in a year (that means we shall allow flooding to occur on average twice in a year).

2. From the contour map of the area find out the time of concentration of the basin (say 15 minutes)

3. Find out the rainfall intensity corresponding to the time of concentration. (TOC = duration of rainfall )

4. Apply Rational Formula to find out the maximum or design runoff

/1.2 ha

/2.4 ha

/1.8 ha

/120 m

/180 m

Find out the maximum design runoff at the discharge point Assume: C = 0.3 (Entire area), 5-year frequency, vel. In sewers = 0.6 m/s

200

25

175

75

100

125

150

50 R

ain

fall

Inte

nsi

ty, m

m/h

r

Flow time in sewer from MH 1 MH 2 = (120 m)/ (0.6 m/s) (60 s/ min) = 3.3 min Flow time in sewer from MH 2 MH 3 = (180 m)/ (0.6 m/s) (60 s/ min) = 5.0 min Time of concentration from remote points of 3 separate areas to MH 3: Area 1: 5.0 + 3.3 + 5.0 = 13.3 min Area 2: 5.0 + 3.3 = 8.3 min Area 3: 8.0 min (inlet time only) Max. time conc. = Duration of rainfall = 13.3 min I = 110 mm/hr. for 5-year frequency Sum of CA values = 0.3 (1.2 + 2.4 + 1.8) = 1.62 Q = 10 x 110 x 1.62 = 1782 m3/hr.

HYDRAULIC DESIGN OF SEWERS

Design of sewers are done assuming steady-state conditions. Steady-state means that the discharge or flow-rate at a point remains time-invariant.

Objectives:

1. Carry the peak flow rate for which the sewer is designed

2. Transport suspended solids in such a manner that the siltation in a sewer is kept to a minimum

This is directly connected with the maximum achievable velocity in the sewers. We do not want the sewage pipe materials to get worn out. The wastewater manual recommends a maximum velocity of 3 m/s.

This condition gives us an idea about the minimum velocity that has to be maintained inside a sewer during a low flow period.

Sewers versus Treated Water Conduits

SEWER WATER CONDUITS

1. They are never designed to run full; there is always an empty space provided at the top.

1. They are always designed to run full.

Reasons: a) Biodegradation causes generation of gases like methane, hydrogen sulfide, ammonia etc. which can get dissolved if running under pressure. b) At same slopes, the velocity and carrying capacity is more when it runs partially full.

2. It is unpressurised. It maintains a gravity flow; It is laid in gradients or slopes.

2. It is pressurized. Normally, we do not worry about the slope of the water mains or lines when we lay them.

Minimum Velocity in a Sewer

The velocity should be such that:

A) It will not allow the particles to settle inside the sewer

B) Even if there is a deposition, it will promote scouring of the particles so that it can self-cleanse itself

The generation of Self-cleansing velocity should occur within the sewer for at least once in a day.

W

W cos

W sin

Drag Force RSw

If the block (Particle) has a unit length and unit width and thickness is dp , then

From the force balance, when the particle is on the verge of slipping down the plane,

sinW

Volume

WeightSubmergedsubmrged

)])(g*d*nV)-V[(1

wnVVV

])[1( wdn

buyoancy)g*d*V(1

s V

]1[)1( sw Sn

psub dW *1*1*

sin]1)[1( psww dSnRS

R= Hydraulic mean radius S= Slope of the channel

ps dSR

kS ]1[ sin)1( nk

pss dSkR

Rn

SRn

v )1(111

21

32

21

32

pss dSkR

nv )1(

16

1

Where,

SELF-CLEANSING VELOCITY

Self-Cleansing Velocity

pSS DSkRn

V )1(1

6

1

n = roughness coefficient R = Hydraulic Mean Radius =

P

A

A= Area of the channel P= Wetted perimeter of the channel

Ss = Specific gravity of the particle

k = Dimensionless constant, 0.04 for granular particles, 0.8 for organic matters

DP = Diameter of the particle for which the sewer will be designed, this is the maximum particle size the sewer can safely carry

Sewers are always designed to attain the self cleansing velocities

JAPAN

D

d

/2 /2

]2

cos22

[DD

d ]2

cos1[2

1

D

d

2

4DA

360.

4

2 Da 2

cos2

*2

sin2

*2

1*2

DD

]2

sin

360[

4

2

Da

]2

sin

360[

A

a

D

d

/2 /2

4

4

2

D

D

D

P

AR

]2

sin3601[

4

D

p

ar

DP

360*

Dp

360

360*

D

D

P

p

]2

sin3601[

R

r

D

d

/2 /2

2/13/21 SRn

V

2/13/21 srn

v

3/23/2

3/2

3/2

2

sin3601

R

r

R

r

V

v

3/2

2

sin3601

2

sin

360*

.

.

V

v

A

a

VA

va

Q

q

D

d

/2 /2

]2

cos1[2

1

D

d

]2

sin3601[

R

r

3/2

2

sin3601

V

v

3/2

2

sin3601

2

sin

360

Q

q

In all the above expressions, is the only variable, all other parameters are constant. Thus at different values of , the above proportional elements can be easily calculated

d/D a/A v/V q/Q

1.00 1.00 1.00 1.00

0.9 0.949 1.124 1.066

0.8 0.858 1.140 0.988

0.7 0.748 1.120 0.838

0.5 0.5 1.000 0.500

0.4 0.373 0.902 0.337

Capital Letters denote the situation when the sewers run full

Maximum velocity is achieved when the sewers are designed to run at 80% of the full depth.

Designing Sewer Systems

Sewers are designed taking consideration of 30 years.

Population in the initial years of the design period are low compared to the design population at the end of design period

Peak flow rate in the initial years is low compared to the designed peak flow rate (ultimate peak flow)

Sizing should be such that it will attain the self-cleansing velocity at the average design flow rate or at least at the maximum flow rate at the beginning of the design period.

s

1000

2/13/21 srn

v

]2

sin3601[

4

D

p

ar

3/2

2

sin3601

V

vVelocity at partially full flow

Velocity at full flow

For Partially-full flow v is not influenced by the diameter of the pipe, rather is much influenced by the slope of the channel

After finding the minimum slope required, the pipe size is decided on the basis of ultimate design peak flow rate and the permissible depth of flow. Adoption of the above slopes would ensure minimum flow velocity of 0.6 m/s

Minimum size for a public sewer is 150 mm diameter

Minimum size for a public sewer in hilly terrain is 100 mm diameter

FROM THE SEWAGE TREATMENT MANUAL, GOI

Gravity Sewer: Minimum Pipe Slope for Attaining Vmin= 0. 6 m/s

Diameter (mm)

Discharge (lps)

Slope (m/m)

n= 0.013 n= 0.015

200 19 0.0033 0.0044

250 30 0.0025 0.0033

300 40 0.0019 0.0026

400 75 0.0013 0.0017

450 95 0.0011 0.0015

500 115 0.001 0.0013

600 170 0.0008 0.0010

700 230 0.0006* 0.0008

900 380 0.0004* 0.0006*

A slope below 0.0008 becomes practically difficult for construction purposes

Sewers with flat slopes may be required to avoid excessive excavation where surface slopes are flat or change in the elevation is small.

The slope and size of the sewer should be such that the velocity of flow shall increase progressively or shall remain steady throughout the length of the sewer.

Sewers shall have slope steeper than or equal to the ground slope, otherwise the minimum ground cover may not be maintained through out the length of the sewer.

What will be the diameter of the sewer designed with the following conditions: a) Population to be served: Present = 50,000; Design= 100,000; b) Water consumption: Present = 130 lpcd; Design = 180 lpcd c) 80 % of supplied water appears as wastewater d) Self-cleansing velocity to maintained in the sewer = 0.6 m/s; e) Maximum velocity in the sewer 3 m/s; f) Minimum size of the sewer = 150 mm; g) Peak factor = 2.5 h) n=0.015 i) Average Ground Slope = 1 in 5000

d/D a/A v/V q/Q

1.00 1.00 1.00 1.00

0.9 0.949 1.124 1.066

0.8 0.858 1.140 0.988

0.7 0.748 1.120 0.838

0.5 0.5 1.000 0.500

0.4 0.373 0.902 0.337

Slope to be provided = s=0.8 in 1000 = 0.8/1000 = 0.0008

We want the sewer to run 80% full at its ultimate peak flowrate. From the chart q/Q = 0.988 when d/D =0.8

Q = A.V

STEP 1. Find out the average flowrate and maximum flow rate at present and after the design period

STEP 2. Find out the optimum slope to be provided

STEP 3. Find out the size based on the ultimate peak flowrate.

2/13/21 sRn

V 2

4

DA

4

4

2

D

D

D

P

AR

2/1

3/22

*4

1*

4. s

D

n

DVAQ

Time Average flowrate Peak factor Peak flowrate

Present 50,000* 130*0.8 L/d=0.06 cum/s 2.5 0.15 cum/s

Design 100,000* 180*0.8 L/d= 0.167 cum/s 2.25 0.375 cum/s

Q = 0.375/0.988 = 0.380

Q=0.380 m3/s S= 0.0008 n =0.015

380.0)0008.0(*4

*015.0

1*

4

2/1

3/22

DDD = 850 mm

2/1

3/22

*4

*1

*4

. sD

n

DVAQ

(OK)

m/s 671.0)0008.0(4/850.0015.0

11 2/13/22/13/2 sRn

V

> 0.6 m/s

cum/s 381.0671.0*4

)85.0(.

2

VAQ

Velocity is maximum when the depth of flow d = 0.8 D

At d/D = 0.8, v/V = 1.140

For a circular channel running under gravity,

Hence, vmax = 1.140*0.671 m/s = 0.765m/s < 3 m/s (Maxm. Velocity allowable)

O.K. At the average flowrate q, q/Q =(0.167/0.381)=0.44

From the proportionality table, extrapolating, v/V = 0.97

Hence, v = 0.94* 0.671 m/s = 0.65 m/s >0.6 m/s

O.K. At the peak present flowrate q1, q1/Q =(0.15/0.381)=0.39

v/V = 0.93 Hence, v = 0.93* 0.671 m/s = 0.62 m/s >0.6 m/s OK

Sewerage System

Dr. Sudipta Sarkar

Preliminary Requirements

It is meant for the transport stormwater and wastewater from the generation point to the treatment plant. So it should be laid as deep as possible so that all wastewater or storm water flow can be collected and transported.

Erosion and corrosion resistant. Should be structurally strong enough to resist impact loads or overburden and live loads

Size and slope to be designed to carry the peak load as well as to carry average flow in such a manner that the deposition shall be minimized.

Maintenance should be easy, economical and safe for the workers.

Aims of the design are: a) make the system operational and b) Economical to build and c) make the system durable through out its entire design life

Layout of Sewer Lines

Steps followed for making the layout:

Selection of an outlet or disposal points

Fixing limits to the drainage area or zone boundaries

Finalizing the location of Trunk and Main sewers

Finalizing the location of Pumping stations wherever necessary

Trunk sewer is the sewer in the network with the largest diameter that extends farthest from the sewage outfall

All other sewers are considered as branches

Whenever two sewers meet at a point, the incoming one with larger diameter is called the main sewer.

Trunk Sewer Outfall

Nomenclature System Followed in Sewer Systems

Trunk Sewer

3 2

4

L.3.1

R.3.1

R.3.2

L1.R.3.1.1

L1.R.3.1.2

L2.R.3.1.1 L2.R.3.1.2

Outfall

Network

manhole

Most common location of laying sanitary sewer is along the center of the streets

House

House

The individual domestic connections can be from either side of the streets

For very wide streets the sewers are laid on each side of the streets in the curb or under the sidewalk

House

House

Street

Street

Sewer

Sewer

Sewer

To avoid any contamination sewer lines are never laid near to the water mains. If it is unavoidable, the sewers are encased in concrete

Slope of the sewers generally follow the natural slope of the ground or the street

Design Approach

1. On a map of the area locate all the sewer lines and measure the contributory area to each of the sewer lines or points.

2. Also, draw the longitudinal section or profiles of the sewer lines. Mark on the profile view the critical points such as basements of the low lying houses, levels of existing sewers, disposal points, etc.

3. Design all the branch sewers, main sewers and trunk sewers, starting from the farthest point in the network and based on the following considerations: a) A self cleansing velocity is maintained at present peak flow b) The sewer should run 0.8 full at the design ultimate peak flow c) Minimum velocity of 0.6 m/s is obtained d) Maximum velocity should not be beyond 3 m/s

Example of a Profile of a Sewer Line

A view inside a sewer in London

Sewer Appurtenances

These are devices necessary (except pipes and conduits) for proper functioning of the sanitary, storm and combined sewers

The appurtenances include: 1. Manhole 2. Drop Manhole 3. Lampholes 4. Gully-traps 5. Intercepting chambers 6. Flushing tanks 7. Street Inlets 8. Siphons 9. Grease traps 10. Side-flow weirs 11. Leaping weirs 12. Venturi flumes 13. Outfall structures

MANHOLES

Manholes are RCC or masonry chambers, constructed at suitable intervals along the sewer lines, for providing access to the inside of the sewers.

Helps in: a) Joining the sewer pipes b) Inspection and cleaning of pipes c) Maintenance d) Ventilation if manholes are perforated

Water main

Electric cable

Gutter Curb manhole

Sewer

Manholes are provided at every transition points such as bends, junction, change of gradient, or change in diameter

Between two adjacent manholes, the sewer line runs straight with constant slope or gradient

Types of Manholes

Manholes with Depth less than 0.9 m

A. Shallow manholes

Suitable for branch sewers or places at no heavy traffic

It is also called an inspection chamber

Manholes with Depth 1.5 m

B. Normal or Medium manholes

Heavy cover is provided at the top

May be either square or rectangular (1m X 1m and 1.2m X 1m

900X 800 mm

Manholes with Depth> 1.5 m

C. Deep Manholes

Heavy cover is provided at the top

May be either square or rectangular or circular

Size in the upper portion is reduced by offset

Access shaft: Minimum size is 0.75 X 0.6 m

Working chamber: Provides working space for inspection and cleaning operations, Minimum size 1,2 m X 0.9 m or 1.2 m dia; minimum height is 1.8 m

Benching: concreted portion sloping towards semicircular or U -shaped bottom part of the main sewer, the slope facilitates the entry of sewage into the main sewer

Steps or ladders: for accessing

DROP MANHOLE

It is used when a branch sewer joins a main sewer at a height more than 600 mm above the main sewer or the drop is more than 600 mm.

Advantages: 1) Steep gradients in the branch sewer can be avoided ; 2) The sewage from the branch sewers may fall on the person working; This is avoided.

Plug Inspection Arm

FLUSHING MANHOLE

Provided where it is not possible to gain enough flow so as to maintain a self-cleansing velocity.

Often such condition is prevalent at the beginning of the branch sewers.

Generally provided at the head of the sewers where enough storage is provided to generate a high velocity to flush out the obstructions

Automatic Flushing Tanks

Curb Inlet

Gratings

Different Types of Street Inlets

GUTTER TYPE

CURB TYPE INLETS

COMBINATION MULTIPLE TYPE INLETS

CATCH BASINS

SEWER

A Type of Street Inlet

The basin helps in settling the grit, sand, debris, etc. before the storm water enters the sewer line

Hood prevents the escape of the foul gases into the sewer line and network

Oil and Grease Trap

Generally located near the sources which can generate oil and grease- contaminated wastewater. Restaurants, garages, automobile repair workshops

Oil and grease in the sewer system can : a) sticks to the inner surface of sewers and reduces the sewer capacity; b)entraps suspended matter, further reducing the capacity; c) adversely affect the performance of wastewater treatment plants

REGULATOR OR OVERFLOW DEVICES OR STORM-RELIEF WORKS

The regulators are provided to avoid overloading of sewers, pumping stations, treatment plant or disposal arrangements by diverting excess flow to relief sewers or overflow stream.

The overloading is caused by excess flow coming in a pipeline due to heavy rainfall or excess stormwater. As they are not expected to carry huge pollutant load, the excess stormwater can be safely disposed of to natural streams without any treatment.

Three types of Regulator devices: a) Leaping Weir b) Side-flow or Overflow weir c) Siphon spillway

Leaping Weir

Arrangement consists of an opening at the invert of a storm drain through which the normal storm flow is taken into an intercepting sewer and excess flow leaps over the combined sewer to flow to a neighboring stream

INCOMING FLOW

Intercepting Sewer

Overflow or Side-flow Weir

Excess water is allowed to overflow the combined sewer in the manhole, from where it is taken to another channel that leads to stormwater drain or manhole.

The weir length has to be sufficiently long for effective regulation

Siphon Spillway

Air Line

Receiving Stream Sewer

Spillway

Different Cross-sectional Shapes of Sewers

Most widely used cross-sectional shape is a circular-section sewer. The reasons behind the preferences are: a) A circular section provides the maximum area of flow for a given

perimeter, therefore higher value of hydraulic mean radius.

P

AR

2/13/21 sRn

V

It is the most efficient section, among all possible variations

b) It uses the minimum amount of materials for is manufacture, therefore it is economical to use such a section

c) Manufacture is easy and convenient

d) Structurally more stable (without any corners, hence load is evenly distributed all around

e) Chances of deposition is less

d/D a/A v/V q/Q

1.00 1.00 1.00 1.00

0.9 0.949 1.124 1.066

0.8 0.858 1.140 0.988

0.7 0.748 1.120 0.838

0.5 0.5 1.000 0.500

0.4 0.373 0.902 0.337

0.3 0.252 0.776 0.196

0.2 0.143 0.615 0.088

Advantages of a circular sewer diminishes when the sewer is not running at least half-full

Lesser the discharge, poorer is the performance

OVOID OR EGG-SHAPED SEWER

At low discharges 2- 15% higher velocities are available for these type of sections compared to Hydraulically Equivalent Circular Sections

Standard Oval Shaped Sewers

New Type Oval Shaped Sewers

Hydraulically Equivalent Section: Two sewers of different shape (i.e. different sections) are said to be of hydraulically equivalent when they carry the same discharge when running full at the same slope.

d/D v/V

Ovoid circular

0.25 0.7 0.698

0.20 0.62 0.61

0.10 0.44 0.4

0.05 0.29 0.25

Design of Ovoid-Shaped Sewers

1. Calculate the approximate diameter of a hydraulically equivalent circular sewer that would carry the same discharge at the same slope as the ovoid-shaped sewer.

2. Top horizontal diameter of the Ovoid-sewer = 0.84 X Diam. of the circular sewer

3. Find out the other dimensions from the following figures, according to the type of sewer to be designed

Horse-Shoe Type of Sections

Open-Drain Sections

P

AR

2/13/21 sRn

V

VAQ *

Design a gravity flow trunk sanitary sewer for the area . The trunk sewer is to be laid along Peach Avenue starting at 4th Street and ending at 11th Street. Assume that the that the following design criteria have been developed based on an analysis of local conditions and codes: 1. For design period use the saturation

period. 2. For population densities use the data given

in the table.

3. For residential WW flows use the data given in the table. 4. For commercial and industrial flows (average): a. Commercial 20 m3 /ha . d b. Industrial - 30 m3 /ha . d

5. For institutional flows (average): College - 400 m3 / d (5330 students x 75 L/ student . d)/ (1000 L/ m3 ) 6. For infiltration allowance: a. For residential areas, obtain the peak infiltration values from the fig. (b):

b. For commercial, industrial, and institutional areas also obtain the peak infiltration values from the fig. (b). However, to take into account that the total length of sewers in these areas will generally be < that in residential areas, use only 50% of the actual area to compute the infiltration allowance.

7. For infiltration allowance Assume steady flow 8. Peaking Factors: a. Residential Use the curve, fig. (c) b. Commercial 1.8 c. Industrial 2.1 d. Institutional (school) 4.0

Pe

akin

g Fa

cto

r

9. Hyd. Design Eq. Manning Eq. , n = 0.0013, Use Fig. 6 -10 (Nomogram) 10. Min. pipe size As per local Bldg. Code, 200 mm 11. Min. velocity 0.75 m/s 12. Min cover As per local Bldg. Code, 200 mm, 2.0 m

Solution: 1. Lay out the trunk sewer. Draw a line to represent the proposed sewer [Fig. (a)]. 2. Locate the no. of MHs: (a) Change in direction (b) Change in slope (c) Pipe junctions (d) Upper end of sewers (e) Intervals: 90 120 m or less (As per Code) Identify each MH with a no. In Fig. (a), only MHs at major junctions numbered. In an actual design, intermediate MHs to be located and numbered.

a. Column 1 5, Identify lines, Summarize data b. Column 6 13, Obtain cumulative peak domestic flows

3. Prepare design tables. Comments:

Table 1

c. Column 14 18, Obtain cumulative peak commercial flows d. Column 19 23, Obtain cumulative peak industrial flows

Table 2

e. Column 24 26, Obtain cumulative peak institutional flows f. Column 27 28, Obtain cumulative average and peak flows g. Column 29 32, Obtain infiltration allowance

h. Column 33 Total Cumulative Peak Design Flow Columns 28 + 32

Table 3

i. Columns 35 38 , Sewer Design, Mannings Eq., n = 0.013 , v > 0.75 m/s j. Columns 39 42, Layout Data Column s 39/40 Ground surface elevations obtained by interpolation from Fig. (a) Column s 41/42 Sewer invert elevations (By Trial and Error from Work Sheet)

Table 4

0.1

21

m

3/s

0.0018 m/m

0.3

30

m

3/s

0.0009 m/m

Line 2-3: q/Q=0.313/0.330 =0.95 d/D=0.86 v/V=1.04

WORK SHEET

(1) Plot ground surface elevations, working backwards (2) Sketch invert and crown (3) Line 1: Locate the invert of the upper end of the pipe Upper Invert Elevation=Ground surface depth of cover pipe wall thickness pipe dia. = 20.00 m - 2.00 m - 0.05 m - 0.45 m =17.5 m Lower Invert Elevation= Upper Invert Elevation-(Slope of sewer)x(Length of sewer) 17.5 m - (0.0018 m/m) x (707 m) =16.23 m Check: Depth of Cover Adequate/ Not adequate ? =19.00 m (16.23 m + 0.45 m + 0.05 m) = 2.27 m OK If Depth of Cover Not adequate / too shallow Two alternatives: (1) Repeat with a lower invert elevation, or (2) A steeper slope

Depth of cover

Wall thickness

Ground surface

Inside top Crown

Inside bottom Invert Bottom

Some Other Important Considerations

(1) When a MH is located at a sewer junction: Outlet sewer invert elevation is fixed by the invert level of the lowest inlet sewer (2) If the pipe size increases: The crowns of the two pipes must be matched at the MH To avoid the backing up of WW in to the smaller pipe. An example: Increase in size from 450 mm 750 mm at MH 2

450 mm dia. 750 mm dia.

16.23 m 16.23 m +0.45 m -0.75 m =15.93 m

15.93 m -(0.0009 m/m)x(707 m) =15.29 m

Sewer junction

Example of a Profile of a Sewer Line

Small Bore Sewer System

They are designed to carry only the liquid part of the domestic sewage generated for off-site treatment or disposal

Septic Tank or interceptor tank

Sewer

Solids are separated at a septic tank or at the aqua-privies before the sewage reaches the sewers

The advantages: a) The sewer can have less velocity and flowrate

as it receives only settled wastewater b) Economic as it requires less cost of

excavation, material and treatment c) Upgradation from on-site treatment system to

conventional treatment system is easily done d) Maintenance of strict sewer gradients is not

required as there is no self-cleansing velocity requirement

Minimum diameter of the sewer pipes is recommended to be 100 mm

Small Bore Sewer System

The small bore sewer system outfall can be any of the following:

a)The conventional sewer system b) Waste stabilization ponds c) Any other low cost treatment systems followed by fish ponds or land-based disposal with precautions

Limitations:

a) Interceptor tank requires periodical cleaning and disposal of solids b) Any illegal connection without any interceptor tank shall ruin the

system. So, strict vigilance is required.

Shallow Sewer System

These are modification of surface drain with covers and consist of a network of pipework laid in the areas away from the places where heavy sewage loads are expected.

Pipes are laid in flat gradients following the natural slope of the ground. The minimum depth is 0.4 m

System contains: a) House connections b) Inspection

chambers c) Laterals d) Street-collector

sewers e) Pumping stations

The laterals are minimum diameter 100 mm

The street collectors have a minimum diameter of 150 mm

Shallow Sewer System

Suitability of the system:

1. High density habitats such as slums or squatter settlements ( with population density more than 170 per hectre)

2. Ground-condition is adverse and on-site disposal is not possible 3. Sewage has to be disposed of and minimum water consumption is 25 lpcd.

Limitations: a) It is suitable when suitable ground slope is available b) Unless flushed out at peak flowrates, there is a possibility of solids

deposition if there is not enough ground slope available c) May require frequent cleaning