01 Sewage Generation, Collection and Conveyance

Environmental Engineering- II
By
Akash Padole
Department of Civil Engineering
Sewage Generation, Collection and
Conveyance

1.1 Introduction:
Need for sewerage system, Domestic sewage,
Industrial waste and Storm Water, Conservancy and
water carriage system, Systems of sewerage and
their layouts: Separate, Combined and partially
combined system, Merits and demerits, Patterns of
sewerage layout, Quantity of sewage
Contents
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1.2 House drainage and Environmental sanitation
Plumbing: basic principles, Plumbing regulations,
preliminary data for design, Preparation and
submission of plans, Systems of Plumbing, anti-
siphonic and vent pipes.
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1.3 Conveyance of sewage
Sewer: Shapes and materials of sewers, open
drains,
Design of sewers: sewer size, Determination of
velocity of flow using empirical formulae, limiting
velocities.
Laying and testing of sewers, Sewer joints, Sewer
appurtenances, Ventilation of sewers.
Construction and Maintenance of sewers.
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Need for sewerage system
• From every town or cities, the different types of
waste water originated from bathroom, kitchen,
lavatory basins, house washing, various industrial
processes, semi liquid wastes of human and animal
excreta and dry refuse of house and street
sweepings, etc. and it become imperative to have a
proper sewerage system for it disposal.
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• If sewage system is not implemented, it allows the
sewage either to flow into nearby stream or river or
dump it near residential area, then it creates the
various environmental problems mentioned below:
1. STINKS
• The contents causes stinks in the sewage undergoes
the process of decomposition and smells very fast
causing air pollution.
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2.CREATE HARMFUL BACTERIA
• The human waste consist of coliform bacteria called
E-coli and other bacteria which are dangerous and
harmful to human health.
Bacteria growth on sewer
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3.INCREASE IN BOD
• The bacteria in sewage waste water consume
dissolved oxygen (D.O) and due to this, it increases
B.O.D which kills the fish and harm to other aquatic
life of the lake , pond or river.
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4.CREATE ENVIRONMENTAL HAZARD
• Industrial waste water consists of solids and
chemicals i.e. Nitrogen or phosphates which allow to
develop algae, weed causing harm to aqua
environment.
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• Hence, we should follow the good sanitation or
sewage system to keep environment clean and safe.
• In this way, it is most essential and necessary to keep
the town or city more healthy, clean, eco-friendly by
good sanitation like collection, treatment and
disposal of all waste waters in proper manner.
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• In addition to this, the disease bacteria will bread up
in stagnate water which will badly affect public
health
Breeding of mosquitoes on open sewer
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• Potable water may also get polluted and therefore
total insanitary condition will be set up in the towns
or will be developed in the cities and causing the
impossible environment or unhealthy environment to
live the public in cities or towns.
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Domestic sewage, Industrial waste
and Storm Water
• Mixture of water and liquid waste originate due to
different activities in the community is referred as
waste water or Sewage.
• Sewage are generally of three types:
– Domestic Sewage
– Industrial Sewage
– Storm water/ Drainage
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• Mixture of water or liquid waste originated due to
domestic activities is called as Domestic sewage and
that originated due to industrial activities is called as
Industrial Sewage.
• Liquid waste originated after heavy rain storm is
termed as Drainage or Storm water
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Method for the disposal of waste
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1. Conservancy or Dry System
• This method was used in old time and even now, this
method is practiced in un-sewered areas.
• In such method, accumulated night soil or human
excreta in latrines or privies, etc. can be removed
manually and transported in vehicles to the points of
ultimate disposal.
Privy
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• The waste water from the kitchen, bathrooms, etc.
called sullage and storm water are collected
separately in gutters and carried into neighboring
drainage channels, rivers, stream, nala, etc. for its
disposal.
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Advantages of Conservancy System
• It’s initial cost is less.
• The total quantity of sewage carried to the
treatment unit is small.
• There is no chance of siltation or deposition in the
sewer line because the storm water flows water in
the open drains.
• Pumping of sewage is not required during the
flooding period which makes the reduction in cost
and hence system can run at the lowest cost.
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Disadvantages of Conservancy System
• There is manual removal of human-excreta ,it is more
unhygienic.
• Decomposition of accumulated night soil in privies
create nuisance due to bad smell.
• Insanitation is due to by transportation of night soil
through streets and insanitation caused by carrying
sullage through open drain or open gutter in the
streets
• Risk of epidemics caused by improper or careless
disposal of the night soil
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• System depends on the manual labour at various
stages.
• If these labour goes on strike then there is great
problem of deposition of night soil or over flooded.
The areas having this system loose their aesthetic
appearance.
• Latrines should be constructed away from the living
areas due to foul smell.
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2. Water Carriage System
• The night soil gets mixed with sufficient quantity of
waste water in this methods.
• The mixed night soil with waste water called sewage
which is collected in a system of pipes and carried it
further for subsequent treatment and get disposed
off in a harmless way without making any nuisance
to the public.
• In short ,it is safely disposed off .
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Water Carriage System
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• There is a dilution of solid matter in water to form
sewage in which there is 99.9% weight of water and
only 0.1% of solid matter.
• Due to this proportion, sewage easily flows in
accordance with the laws of hydraulics as applied to
the flow of water .
• This method is much advantageous and universally
adopted in actual practice .
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Advantages of Water-carriage System
• It ensures hygienic conditions and better health .
• Risk of epidemics is reduced to minimal value due to
treatment given to the sewage before its final
disposal.
• There is more quantity of water used in sewers.
Hence more sewage can be carried with self-cleaning
velocity even in lesser gradient.
• No labours or worker are needed to carry out this
type of unhealthy works .
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• This method is totally hygienic method due to
collection and transportation of all type of waste
material through water only.
• Streets or roads can be kept clean because the
sewage is collected and conveyed though the close-
sewers which are laid underground.
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Disadvantages of Water-carriage System
• There is high initial cost on pipes and other sewer
appurtenance required.
• Need regular maintenance and hence its running
cost becomes more .
• During monsoon, there is load on treatment unit
because storm water is added in sewers .
• During summer , more water is required in sewers for
the smooth running. Due to this there is addition of
costs.
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System of Sewerage
• On the basis of system to convey sewage, the
sewerage systems can be further classified as:
– Combined system
– Separate system
– Partially Separate system
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1. Combined system
• When both sanitary sewage and storm water are
carried in a single sewer, it is called a combined
sewer system.
• When the flow in a combined sewer consist of only
domestic sewage and industrial waste without storm
water, flow is called ‘DRY WEATHER FLOW’
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Advantages:
• Both domestic sewage and storm water are carried in
a single sewer, so construction cost is less.
• The strength of domestic sewage is reduced because
of dilution of storm water.
• The sewers are of large size, and therefore the
chances of their chocking are rare. It is easy to clean
them.
• In town with narrow street, this systems is preferred.
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Disadvantages
• Initial cost is high because of large dimensions of
sewers.
• Because of large size of sewer, their handling and
transportation is difficult.
• Due to the inclusion of storm water, the load on the
treatment plant increase.
• During heavy rain the sewer may be overflow and
may thus create unhygienic conditions.
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2. Separate system
• In this system two separate sets of sewer are
installed, one for collection and conveyance of
sanitary sewage and other for storm water.
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• As storm water is carried separately, it is not foul in
nature and normally disposed off to natural water
coarse without any treatment.
• Where as the sanitary sewage is taken to treatment
plant separately and after requires treatment it is
disposed off.
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Advantages:
• Size of sewer requires less.
• Since the sanitary sewage and storm water flows in a
separate pipes, the quantity of sewage to be treated is
less.
• As the sewer are smaller in section, they can be easily
ventilated.
• During disposal if the sewage is to be pumped, the
separate system is cheaper.
• Rain water can be discharge in to steams without any
treatment.
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Disadvantages:
• Since the sewer are smaller size, it is difficult to clean
them.
• They are likely to get chocked.
• Initial cost is high, when two separate set are used.
• Maintenance cost of sewer is also high.
• Generally self cleaning velocity is not available due to
small quantity of sewage therefore, flushing is
required at various point
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3. Partially separate system
• In this system, only one set of underground sewer is
laid.
• These sewer admit the sanitary sewage as well as
the early washings from the drainage area due to
rains.
• When the storm water exceed its specified limit, the
excess is diverted in to natural water courses.
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Advantages:
• The sewer are of reasonable size. Their cleaning is
therefore not difficult.
• It combines the advantages of both the separate as
well as the combined systems.
• The work of house plumbing is reduced, because the
rain water from roof, sullage from baths and kitchen,
etc. can be taken in the pipe carrying the discharge
from the water closest.
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Disadvantages:
• During dry weather flow deposition take place in
sewer.
• As initial storm water is included the cost of pumping
and size of disposal units increase.
• The storm water increase the load on treatment
units.
• There are possibilities of over flow, requiring the
construction of storm water overflow
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Patterns of Sewerage Layout
1. Perpendicular Pattern
2. Interceptor Pattern
3. Radial Pattern
4. Fan Pattern
5. Zone Pattern
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Quantity of Sewage
➢ Estimating Sewage Discharge:
• The sewage discharge which has to be passed
through a sewer must be estimated as correctly as
possible; otherwise the sewer may either prove to be
inadequate, resulting in their overflow or may prove
to be too much of size resulting in unnecessary
wasteful investments.
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• The Quantity of Sewage is (Domestic sewage +
Industrial sewage) that is likely to enter the
municipal sewers under design, should be equal to
the quantity of the water supplied to the
contributing area, from the water works
• Certain additions and subtraction take place to
estimate the quantity of sewage.
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Additions due to unaccounted private water supplies:-
• The accounted water supplied to the public through
the public distribution system is not necessarily the
only water consumed by the public.
• Some private wells and tube wells may be used for
public and domestic needs and similarly certain
industries may utilise their own surface water.
• This extra quantity of water used by the town is
generally small unless the large industrial private
water uses this quantity is estimated by actual field
observation.
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ADDITION DUE TO INFILTRATION:-
• Whenever the sewers are laid below the ground
water table, certain amount of ground water
generally seeps into them due to faulty leaky joints or
cracks in the pipe.
• The quantity of the ground water entering into sewer
mainly depends upon the height of the water table.
• Since this factor cannot be computed precisely hence
an allowance of 11,000 to 2,25,000 liters per day
kilometer length of sewer pipe is generally made in
high water table areas.
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SUBTRACTION DUE TO WATER LOSSES:-
• The water lost due to leakage in the distribution
system and house connection of the water supply
scheme does not reach the consumer and hence
does not appears as the sewage.
• Subtraction due to water not entering into the
sewerage system:- Certain amount of water maybe
used by public and industry for such amount of uses
sewage is not produced at all for e.g., the water used
for boiler and steam generation.
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➢ NET QUANTITY OF SEWAGE PRODUCED:-
• The net quantity of sewage produced will be equal to
the accounted quantity of the water supplied from
the water works plus addition due to factors of
private water supplies and due to infiltration and
subtraction due to factors water losses and water
not entering into the sewer.
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• This net value may vary between 70 to 130 percent of
the accounted water supplied from the water-works.
• In India however this value is generally taken as
equal to 75 to 80% of the accounted water supplied
from the water table
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Drainage Systems
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• The occupants of a house/commercial or industrial
unit use water supplied from a distribution system in
Kitchen, Bathroom, Toilet, Sink, Wash basin etc. for
their daily needs.
• The water used in such units needs to be properly
disposed to municipal sewers for it’s safe disposal,
therefore it is necessary to construct a system of
conveyance of waste water.
• This system is known as House drainage system
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Functions of Drainage system -
• Maintain healthy environment in the building
• To dispose the waste as quickly as possible
• To avoid entry of foul gases from the sewer or septic
tank
• To collect and remove waste matters systematically
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Waste water -
• Water when used for different purpose like domestic
commercial, industrial etc., receives physical,
chemical, and biological impurities in it.
• Sewage: The wastewater coming from W.C. and
containing human excreta is known as sewage.
• Sullage: The Wastewater coming from bathroom and
kitchens which does not contain faecal matter is
known as sullage.
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Pipes in Drainage system –
• Stack - A general term used for any vertical line of soil,
waste or vent piping.
• Soil Pipe: It is pipe carrying sewage (Black water) from
W.C.
• Waste Pipe: It is a pipe carrying sullage (Grey water)
from bathrooms, kitchens, sinks, wash basins, etc.
• Vent pipe: Pipe installed for the purpose of ventilation
or to carry foul smell
• Rain Water Pipe: Pipe carrying rain water
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• Pipes in Drainage system –
• Anti siphonage Pipe: Maintains proper ventilation
and does not allow the siphonic action to take place,
it helps in maintaining the water seal of traps
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Traps in Drainage system –
• Trap: It is a device which uses a bent path to allow
waste water to pass through and at the same time
prevents sewer gases from entering buildings.
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❑ Good Traps should have the following Qualities:
✓ Should provide enough water seal with large surface
area.
✓ It’s surface should be smooth so that the flow is not
obstructed which enables self cleansing.
✓ An assess door should be provided for cleaning the
trap.
✓ It should be made of non- adsorbent material.
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• Traps in Drainage system –
1. S-Trap –
• Oldest
• Not in use
2. P-Trap –
• Sinks
3. Q-Trap –
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4. Floor trap (Nahni trap) –
• This trap is generally used to admit sullage from the
floors of bathrooms, kitchen etc. in to the sullage
pipe.
• This is provided with cast iron or stainless steel or
galvanized gratings (Jallis) at its top so that the entry
of larger matter is prevented there by chances of
blockage are reduced.
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5. Gully trap –
• A Gully trap or gully is provided at a junction of a roof
drain and other drain coming from kitchen or
bathroom.
• A gully trap is provided outside the building before
connecting to external sewerage line.
• It also prevents entry of bugs,
cockroaches and other insects from
sewer line
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6. Grease trap –
• Used where large quantity of oily waste is generated
• Prevents blockages
• Can overwhelm the septic tank
• If not removed then caused the blockage in sewer
line
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• It’s a masonry or cast iron chamber with a bent pipe
or Tee –pipe at the outlet end.
• Because of the sudden increase in the area of flow at
the entry the velocity of flow is reduced, resulting in
the separation of oily and greasy matter from the
wastewater
• This greasy matter, floating on the top can be
removed later.
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System of Plumbing for House Drainage
• There are four plumbing systems for house drainage
✓ Single Stack System
✓ One pipe system
✓ Single Stack Partially Ventilated System
✓ Two Pipe system
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1. Single Stack System
• From the figure it is clear that only single pipe acts as
soil pipe waste pipe and ventilation pipe.
• This is poorly ventilated system
• It is simple system and easy to construct.
• Risk of water seal breaking in the trap is high
because of induced siponage.
• Waste or air of the waste pipe may be forced up due
to back pressure.
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2. One Pipe System
• A Separate vent pipe is provided in this system.
• It is clear from the study of sketch that in comparison
to single stack system:
– This system is costly and difficult to construct
– Ventilation is provided to sullage pipe and soil
pipe.
– Arrangement of pipe work is difficult.
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3. Single Stack Partially Ventilated System
• Following are the features of this system:
– Only water closet traps are ventilated.
– Traps of baths, washbasins and kitchen are not
joined with vent pipe.
– This is economical system
– It is required to maintain water seal of 75 mm or
more.
– It has simple arrangement of pipe.
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4. Two Pipe System
• Following are the features of this system:
– Water closets, bath traps, kitchen traps and wash
basin traps all are connected to vent pipes.
– Separate soil pipe and waste pipes are provided.
– Two vent pipes are provided.
– There are four stakes in this system
– It is efficient system but costlier than other
systems.
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House Drainage Plan
Important points to be considered while preparing
house drainage plan are mentioned below.
• Drainage layout plan should be as simple as possible.
Pipes should be laid in straight line.
• Both vertical and horizontal pipes shall be laid in
straight lines as far as possible.
• Where there is change in the direction of pipe,
inspection chamber or manhole should be provided.
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• Sewers should not be laid under a building to avoid
the risk of decrease in bearing capacity in case of
continuous leakage of sewage from joints.
• The leaked sewage percolates in the soil and increase
moisture content of soil below the foundation.
• Increased moisture content decreases bearing
capacity which is not desirable.
• Pipes should be laid at proper gradient and proper
size.
• The usual size of house sewer are 100 mm, 150 mm,
230 mm, and 300 mm.
• They should be laid with such a gradient that there is
no deposit of solid matter.
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Need for Regulations
• Rapid development in the Indian construction
industry required the plumbing sector to keep pace
with modern plumbing technology.
• For this, existing Codes, Standards and practices
required improvements from time to time to match
international standards.
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• Codes of practice attempt to minimize risk by
technical standards of design, materials,
workmanship and maintenance for plumbing
systems.
• The main aims of the codes are:
– To ensure that planners, administrators and plumbers
develop the required competency to ensure that the
codes are applied and upheld.
– Those standards are set to ensure that the plumbing
assemblies, materials and technologies are safe and
effective.
– To ensure that plumbing installations meet these
standards.
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I.S. Codes related to plumbing
✓ IS 1200 (Part 19) : Method of measurement of Water supply,
plumbing and drains
✓ IS5329 – 1983 : Code of practice for sanitary pipe work above
ground for buildings
✓ IS 1172 - 1983 : Code of basic requirements for water supply,
drainage & sanitation (revised).
✓ IS 1742 – 1983 : Code of practice for building drainage.
(Second revision)
✓ IS 2064 – 1973 : Code of practice for selection, installation and
maintenance of sanitary appliances
✓ IS 2065 - 1983 : Code of practice for water supply to buildings
✓ IS 4515 : Specification for unplasticized PVC pipe fittings
✓ IS 4985 - 1988 : Specification for unplasticized PVC pipes for
portable water supplies (second revision)
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➢ Preliminary data for design :
• Site plan giving complete details of units & fitting
along with clearly demarcated description.
• Drainage plan
• Cover depth of sewer below ground
• Location other services
• Damage to building & structures
• Drainage into the public sewer
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➢ Design considerations :
• Aim: The main aim for designing the drainage
system of a housing colony is to provide a system of self
cleansing conduits for the safe disposal of the sewage
waste
• Fast & effective removal of the waste without any
leakage
• Non-escape of foul gases to the building & residents with
proper provision of their escape
• Prevention of any type of corrosion, erosion internal or
external in case of joints on the construction material
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➢ Layout:
• The layout should be simple & preference should be
given to alternative layout to ensure that most
economical is adopted.
• Plumbing system is decided in accordance with the
requirement.
• For laying of the sewer pipes they should be
preferably be laid in straight line; abrupt changes in
direction should be avoided & no bends & junction
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➢ Drainage pipes:
• Soil pipes: A soil pipes conveying to a drain shall be
circular & shall have a minimum diameter of 100mm
• Waste pipes: every pipes in a building for carrying off
the waste from every bath, wash basin to a drain
shall be of 32-50 mm diameter
• Ventilating pipes: They should be so carried to such a
height & in such a position as to afford by means of
the open end of such pipes for vent shaft a safe
outlet for foul air
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➢ Design:
• The maximum flow in a building drain depend on the
maximum no. of simultaneously discharge
appliances.
• Gradient & pipes sizes: Gradient should be sufficient
to prevent the temporary accumulation & blockage
of the drain.
• Normally the sewer shall be designed for discharge
the peak simultaneous flow, flowing half with a
minimum self cleaning velocity of 0.75m/sec.
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Anti-siphonic pipes
• An extra pipe connected to the outlets of toilets
seats of all the floors, the other end of which is
exposed to atmosphere is called Anti-siphonage/
Anti-siphonic Pipe.
• These are provided to maintain water seal so that
foul gases of the sewer line do not find entry in to the
toilet/ bathroom.
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Siphonic Effect
• Siphon effect takes place when the fluid flows to the
highest portion of the bend pipe due to the pressure
differences and comes down from there due to the
presence of gravity.
• Vacuum is created inside the pipe
either by simply sucking it or using a
a pump.
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• When waste water is suddenly discharge from
sanitary fixture on the upper floor it moves down
rapidly through waste pipes.
• Due to this movement it sucks some air from the
lateral pipe connecting to the waste pipes with the
fixture at lower floor.
• This sucked air causes siphonic action resulting in
flow of water from the trap of the fixture to the
waste pipe and thus breaking the water seal.
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• To over come this difficulties a separate pipe of
smaller diameter is attach to the trap which
connected with the Vent pipe.
• This pipe is known as ANTI-SIPHONAGE pipe.
• It supplies air to the lateral pipe of the lower fixture
at the time of suction.
• It acts as a vent pipe connection of the lower fixture.
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Vent Pipes
• Plumbing vents are pipes
that extend from the waste
pipes to the outside of the
building, often going through
the roof.
• The VENT PIPES allow the
sewer gases to escape to the
outside , rather than being
released inside the house.
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• Vent Pipes supply fresh air to each plumbing fixture
in the house, which helps the system move water
through the drainage pipes each time a toilet is
flushed or a sink is drained.
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Conveyance of sewage
• Sewers normally contains 99.9% water and 0.1%
solid matters, so design of sewer is similar to water
mains except the following differences:-
– Sewers are designed to achieve non-silting or self-
cleansing velocity to avoid clogging problems due
to solid deposition from sewage.
– Unlike pressurized flow in water supply pipes,
sewers in most of the cases are considered as
open channel flow and are laid at continuous
downward gradient.
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Self-cleansing Velocity/
Minimum Velocity/
Non-silting Velocity
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• It is the velocity that do not permit solids to settle
down and make them remain in suspension.
• It should develop at least once in a day so as not to
allow any deposition in the sewer leading to
blockage.
• Design velocity of flow should not be less than self-
cleansing velocity.
• Normally avg. velocity of 0.9 m/s is develop in the
sewer.
• Vs = 0.45 m/s for inorganic sand of 1 mm size particle
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Non-scouring Velocity/
Maximum Velocity
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Non-scouring Velocity for different sewer materials
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Requirements of Sewer Materials
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Types of Sewer Material
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Hydraulic Formula for design of Sewer
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Shape of Sewers
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Egg Shaped Sewer
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Disadvantages of egg shaped sewer over the circular sewer:
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Laying of sewers
1. Detailed map preparation.
2. Centre line marking.
3. Unloading.
4. Storing.
5. Cutting.
6. Trenches.
7. Laying
8. Back filling and tamping.
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2. Centre line marking 4. Storing.
5. Cutting
1. Detailed map preparation 3. Unloading
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7. Laying 8. Back filling and tamping
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Testing of Sewers
❑Test for leakage or Water Test:
• Sewers are tested to ensure no leakage in their
joints.
• They are tested between manhole to manhole under
a test pressure of 1.5m of water head.
• Lower end is plugged and the water is allowed to
flow through the upper end.
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• Depth of water in the manhole is maintained to the
testing head of about 1.5m.
• The sewer line is watched by moving along the
trench and the joints which leak or sweat, are
repaired.
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❑Test for Straightness of Alignment or Obstruction:-
• It is tested by placing a mirror at one end and the
lamp at the other end.
• If the pipe line is straight full circle of light will be
observed .
• If the pipe is not straight it would be apparent and
the mirror will also indicate any obstruction in pipe.
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• Obstruction in the pipe is tested by inserting a
smooth ball of dia. 13mm less than the internal dia.
of sewer at the upper end of the sewer .
• If no obstruction is their the ball rolled down to the
lower end.
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Sewer Joints
• The water supply system made by pipes of different
lengths which are connected to form a one
continuous length of pipe line. Such connections are
called as pipe joints.
• The design of these joints mainly depends on the
condition of the pipe, internal water pressure and
the condition of the support.
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Types of Pipe Joint
▪ Spigot and socket joint
▪ Flanged joint
▪ Expansion joint
▪ Mechanical joint
▪ Flexible joint
▪ Screwed joint
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Spigot and Socket Joint
• Bell and spigot joint
• Used for cast iron pipes
• Enlarged end – socket
• Normal end - spigot
• A few strands of jute is
wrapped around the
spigot.
• Remaining space b/w
socket and spigot is finally
filled with molten lead.
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Flanged joint
• Rigid and strong
• Expensive and mostly used for indoor works –
pumping stations, filter plants.
• They are fixed by means of nuts and bolts.
• It can be disjointed if needed .
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Expansion joint
• Used at places where the pipes expand or contract
due to change in atmospheric temperature and thus
setting of thermal stresses in pipes.
• Socket end is cast flanged.
• Spigot end is plain
• On expansion, socket end
moves forward, the gap just
closed.
• On contraction, the socket
end moves backward
creating gap.
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▪ Mechanical joint ▪ Flexible joint
▪ Screwed joint
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Sewer Appurtenances
• Sewer appurtenances are the various essential
accessories or devices for the proper maintenances &
efficient operation of sewerage system.
• Necessity of sewer appurtenances:
– To inspect the sewer.
– To help in cleaning the sewers
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Manhole
• It permits inspection and cleaning as well as repair of
sewers, and removal of any obstructions; and also
they allow sampling and flow measurement.
• Most manholes are circular in shape, with the inside
diameter of 4 ft (1.25 m) has been widely adopted.
• These are provided at every bend, junction, change
of gradient or change of diameter of the sewer.
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• It consists of working chamber, an access shaft & a
strong cover on the top.
• The lower portion of a deep manhole is known as a
working chamber and it provides a working space to
carry out cleaning and inspection of sewer line.
• Access shaft provides access to the working chamber.
• The manhole cover of RCC is provide to cover the
opening.
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Drop Manhole
• When it is uneconomical or impracticable to arrange
the connection with 0.6 m of invert of the sewer and
manhole, then vertical shaft is constructed outside
the manhole chamber through which the sewage of
branch sewer is allowed to enter the manhole.
• Such a manholes which drop the level of invert of the
incoming sewer by providing a vertical shaft are
called Drop Manhole.
• The main purpose of drop manhole is to avoid the
splashing of sewage on the man work.
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Lamp holes
• A lamp holes are the
opening or holes are
constructed in a sewer
for the purpose of
inspection of sewer and
detecting the presence
of any obstruction in
the sewer line.
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• It is a vertical stoneware or concrete pipes which are
connected to the sewer line.
• The pipes are surrounded by concrete to make them
stable.
• At the ground level, the manhole cover with frame is
provided to take up the load of traffic.
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Street Inlets
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Inverted Siphon
• A sewer line has sometimes to be laid across a
stream, a highway cut, or any other similar
obstruction, either below or above the ground level.
• When the profile laid below the ground, that portion
of the sewer is known as inverted siphon (or, a
depressed sewer).
• This section of sewer lies below the hydraulic grade.
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Sewer Outlet
• Storm water and treated water may be discharged
into small or large water bodies via sewer outlet.
• Outlets to small streams are similar to the culverts,
consisting of a simple concrete head wall and apron
to prevent erosion.
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• Sewers discharging into larger bodies of water are
usually extended beyond the banks into fairly deep
water where dispersion and diffusion will aid in the
mixing sewage with the surrounding water.
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Ventilation of sewers
• Sewer ventilation is essential due to the following
reasons:
– Sewage flowing in the sewers contain organic and
inorganic matter which on purification or
decomposition produces gases like carbon
monoxide, carbon dioxide, methane, nitrogen and
ammonia etc.
– These gases like methane are very foul explosive
and may cause fatal accidents to the maintenance
gang.
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• If the sewer is not properly ventilated the man hole
cover may be blown off.
• These gases being light move upwards and cause air
pollution when they escape into the atmosphere.
• They also interfere with the natural flow of sewer.
• Hence sewer ventilators are provided along the
sewer lines at an interval of 80 to 150 m or so.
• Ventilator shaft may be of cast iron, steel or R.C.C.
pipes of internal diameter ranging from 15cm to
25cm.
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Maintenance of sewers
• For efficient and proper working of sewer lines, their
proper maintenance is essential.
• Maintenance of sewers mainly consists of removal of
stoppages, cleaning of sewers and other sewer
appurtenances and repair work.
• Maintenance of these sewers is costly which are laid
on flat slopes as they are more prone to heavily
clogging and the self cleansing velocity is not
developed.
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• Sewers which are laid to be clogged by the entrance
of tree roots through faulty joints are costly to
maintain.
• Cleaning of clogged sewers due to silt, grease and
oily materials is one of the major problem of
maintenance of sewer line and is costly.
• The clogging mainly occurs in small sized sewers,
where a man cannot enter into them to clean.
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01 Sewage Generation, Collection and Conveyance

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01 Sewage Generation, Collection and Conveyance