sewers and sewer netwrok - design construction and maintenance
TRANSCRIPT
S. Sarkar
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 Δt
Time Runoff
t0= 0 Q0 = Q(t=0) =0
t1= Δt Q1=A1IC1
t2=2Δt Q2=A2IC2
tn=nΔt 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 2Δt
Time Runoff
t0= 0 Q0 = Q(t=0)= 0
t1= Δt Q1=A1IC1
t2=2Δt Q2=A1IC1+A2IC2
t3=3Δt Q3=A2IC2+A3IC3
tn=nΔt Qn= An-1ICn-1+AnICn
tn=(n+1)Δt Qn+1 =AnICn
tn+2 =(n+2) Δt Qn+2 =0
Case III Rainfall duration is nΔt
Time Runoff
t0= 0 Q0 = 0
t1= Δt Q1=A1IC1
tj=jΔt
tn=nΔt
tn+1=(n+1)Δt
T2n-1=(2n-1)Δt Q2n-1=A1IC1
t2n =2nΔt 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 Areas……0.9 Lawn and Gardens….0.15 Water-bound macadem roads…0.45
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.
Kirpich’s 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
321
sRn
v
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
α
W cosα
W sinα
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
α/2 α/2
]2
cos22
[DD
d ]2
cos1[2
1
D
d
2
4DA
360.
4
2 Da
2cos
2*
2sin
2*
2
1*2
DD
]2
sin
360[
4
2
Da
]2
sin
360[
A
a
D
d
α/2 α/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
α/2 α/2
2/13/21SR
nV
2/13/21sr
nv
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
α/2 α/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, α is the only variable, all other parameters are constant. Thus at different values of α, 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
2/13/21sr
nv
]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 (from the table)
We want the sewer to run 80% full at its ultimate peak flowrate so that maximum possible velocity can be attained).
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.
2/13/21sR
nV
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
From the chart q/Q = 0.988 when d/D =0.8
Q=0.380 m3/s S= 0.0008 n =0.015
380.0)0008.0(*4
*015.0
1*
4
2/1
3/22
DD
Take D = 900 mm (next available size)
2/1
3/22
*4
*1
*4
. sD
n
DVAQ
m/s 697.0)0008.0(4/900.0015.0
11 2/13/22/13/2 sRn
V
cum/s 395.0697.0*4
)85.0(.
2
VAQ
D = 849 mm
949.0395.0
375.0
Q
qAt ultimate peak flow,
77.0D
d135.1
V
v
m/s 791.0697.0*135.1 v >0.6 m/s [OK]
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*0.697 m/s = 0.794m/s < 3 m/s (Maxm. Velocity allowable)
O.K. At the ultimate average flow rate q, q/Q =(0.167/0.395)=0.42
From the proportionality chart, extrapolating, v/V = 0.93
Hence, v = 0.97* 0.697 m/s = 0.676 m/s >0.6 m/s O.K.
At the peak present flowrate q1, q1/Q =(0.15/0.395)=0.38
Hence, v = 0.93* 0.697 m/s = 0.65 m/s >0.6 m/s OK
From the proportionality chart, extrapolating, v/V = 0.97
NOTE: If the velocity at the present peak flow rate is found to be below 0.6 m/s, then a slight increase in the slope with the same diameter may help attain the minimum required velocity of 0.6 m/s
Sewerage System
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
NOMENCLATURE IN CASE OF DESIGN OF SEWER NETWORK USING COMPUTER PROGRAMME
In case of design of sewer network using computer programme, there is no restriction in the nomenclature of the sewers and manholes as required for the manual design.
It is sufficient to give node
numbers as well as pipe (link) numbers in any manner in the sewer network for design of the network for using computer software.
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
Sewer lines
Brickwork sewer line HDPE sewer pipe
RCC sewer pipes
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) mainte-nance; d) Ventilation if manholes are perforated
Water main
Electric cable
Gutter Curb manhole
Sewer
Manholes are provided at every transition points: bend junction Change in gradient Change in sewer diameter At regular intervals
Between two adjacent manholes, the sewer line runs straight with constant slope
Manholes
Brickwork HDPE
RCC precast RCC precast
TYPES OF BRICKWORK MANHOLES
Rectangular manhole (900x800 mm) SHALLOW MANHOLE: •depth less than 0.9 m •Suitable for branch sewers or places with no heavy traffic • It is also called an inspection chamber
Rectangular manhole for (1200× 900mm) NORMAL OR MEDIUM MANHOLES: •depth 0.9 m to 2.5 m •Heavy cover is provided at the top • May be either square or rectangular (900mm X900mm and 1200mm X 900 mm)
TYPES OF BRICKWORK MANHOLES
Arch type manhole for (1400 mm × 900 mm) Typical circular manhole
DEEP MANHOLES •deeper than 2.5 m •Heavy cover is provided at the top •Size in the upper portion is reduced by offset: May be either square or rectangular or circular
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
RCC AND COMBINATION MANHOLES • Advantages over brickwork manholes:
– better quality control in raw materials and workmanship
– easier fixing in the field with maximum speed and minimum disturbance to traffic
• Concerns: – The concrete corrosion of the inside by sulphide gas
and the soil side by sulphate in soil water.
• Solution: – The use of high alumina cement is advisable in
manufacture itself or sulphate resistant cement with extra lining of 25 mm thickness over inner wall with high alumina cement.
• Two types of RCC manholes can be used –
– Manholes with vertical shaft in RCC and the corbelled cone portion in brickwork
– Entire manhole in RCC and corbelled cone portion separately precast and jointed
• The entries and exits of main sewers as well as house service sewers requires careful detailing because the issue of puncturing the walls for insertions of especially house service sewers later on is impossible.
RCC AND COMBINATION MANHOLES
HDPE MANHOLES
• HDPE manholes with EN 13598-2: 2009 and ISO (ISO 9001: 2008) specifications are recent entrants. (Indian std. not yet brought out by BIS)
• Advantages:
– Speedy construction as compared to brickwork manholes as these come ready made.
• Site-specific precautions:
– To be safeguarded against uplift pressure due to high GW level and crushing under heavy traffic load.
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
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
2/13/21sR
nV
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
2/13/21sR
nV
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 MH’s: (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, Manning’s 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