WATER & WASTEWATER
ENGINEERING
Water Supply Engineering
Lecture No. 5
Sewerage System
Sewerage System
Wastewater Works
• Collection Works (Sewage Network)
• Treatment Works(Wastewater treatment
plant)
• Final Disposal works( of treated ww and
sludge)
Sewerage System
• The objectives of WW works(projects)
Protecting the environment with all its components
from pollution due to untreated ww.
• Protecting public health from disease spreading.
• Protecting buildings and infrastructures
• Re-use of treated ww .
Sewerage System
• Definitions
A wastewater project is generally designed and
constructed to serve the community for a
number of years in the future.
• Initial Year: is the year when the construction of
the project is completed and the initial operation
begins.
• Design or planning year: is the year when the
facility is excepted to reach its full design capacity
and further expansion may become necessary.
Sewerage System
• Data to be collected for a WW project for a certain city
o Maps and layout of the city.
o City’s topography
o To estimate ww discharges:
• Present population, population’s growth rates and trends.
• Design period
• Per capita water consumption
o Climate Conditions
• Rain intensity
• Prevailing wind direction
o Data about depth of GWT and soil type.
o Type of surfaces in the city and their percent of the total city’s
area.
Sewerage System
General Considerations in Planning and Design of
Sewerage Networks
• Sewers’ slope should follow the natural ground slope
whenever possible to decrease the pipes’ depth.
• This is why , pump stations are usually in the lowest
parts of the service area.
• Make sure that all the area is served with the network.
• Avoid U-path.
• WWTP should be down wind.
Sewerage System
• Definition of sewerage system:
It is a complete system to collect wastewater consist of
a gravity sewer network , pump station , force main.
Sewerage System
Collection
Treatment
distribution loop system
- hardy cross method
- section method
- circle method
‫‪Sewerage System‬‬
‫‪• Force Main:‬‬
‫مثل ماسورة مياه الشرب و لكن تكون مغطاه من الداخل بمواد‬
‫تحميها من تفاعالت المواد الموجودة بمياه الصرف‪.‬‬
‫‪Sewerage System‬‬
‫‪• Kinds of pipe material used in sewerage system:‬‬
‫‪1- vitrified clay‬‬
‫‪2- DCI‬‬
‫‪3- UPVC‬‬
‫‪4- P.E‬‬
‫‪5- R.C‬‬
‫نراعي عند اختيار نوع الماسورة‪:‬‬
‫‪ -1‬التكلفة‬
‫‪ -2‬نوع التربة‪ :‬يبين تقرير التربة مدي عدوانية التربة فاذا كانت التربة‬
‫عدوانية يفضل عمل الحماية الكاثودية و ال يفضل استخدام مواسير‬
‫حديدية في اماكن احتمال حدوث تيارات شاردة‪.‬‬
Sewerage System
• Commercial Diameters
Commercial Diameters for vitrified clay pipes
175, 225, 300, 375, 400, 450, 500, 600, 700,
800, 900, 1000 mm.
Commercial Diameters for uPVC pipes
150, 200, 250, 300, 350, 500, 600, 700, 800,
1000 mm.
Sewerage System
• Components of WW Flows
• Domestic ww : it includes ww from residential,
commercial, small industry, and public facilities.
• Industrial ww: ww in which industrial wastes
predominates.
• Infiltration Water: water that enters the sewer
system through leaking joints, cracks and breaks,
or porous walls.
• Rain Water( storm water): runoff resulting from
rainfall.
Sewerage System
Factors affecting Domestic WW discharges
• Community size
• Standard of living and population’s activities.
• Climate conditions .
• Quality of water.
• Price of water.
• Metered services
Sewerage System
Factors Affecting Industrial WW discharges
• Type of industry and the resulting organic
loads.
• Working hours.
• Water consumption rates.
Sewerage System
Factors affecting Infiltration Water discharge
• Sewers' diameters.
• Sewers’ pipe material.
• Sewers’ depth.
• Sewers’ length below GWT.
• Type of connections.
Sewerage System
• The flow velocity in the sewers should be such
that the suspended materials in sewage do not
get silted up; i.e. the velocity should be such as to
cause automatic self cleansing effect. The
generation of such a minimum self cleansing
velocity(0.6m/s) in the sewer, at least once a day,
is important, because if certain deposition takes
place and is not removed, it will obstruct free
flow, causing further deposition and finally
leading to the complete blocking of the sewer.
Sewerage System
• The smooth interior surface of a sewer pipe
gets scoured due to continuous abrasion
caused by the suspended solids present in
sewage. It is, therefore, necessary to limit the
maximum velocity in the sewer pipe. This is
called limiting or non-scouring velocity
( < 1.5m/s)
Sewerage System
Combined and Separate Sewers
• Combined sewers is a single pipe network that
carries a mixture of ww and rain water and
conveys it to the WWTP,
while separate sewers are two separate
networks : one intended to carry ww while the
other is for rain water. Rain water is disposed
off in a nearby water body(usually the sea in
coastal areas).
Sewerage System
• Separate sewers are used in cities where rain
intensity is high as in Alexandria, Egypt. This is
done in order to avoid increasing the amount of
ww to be treated at the WWTP . Its main
disadvantage is the high cost due to construction
of two networks.
• Combined sewers are used where the rain
intensity is low. Its main disadvantage is the
increased ww discharge to the WWTP with a rain
water flow that doesn’t require treatment. 12
Sewerage System
COMBINED SYSTEM
• the streets are narrow
•The quantities of rain
water are small
•The land is flat and
sewage is concentrated
and dilution with rain
water is being a method of
treatment
SEPARATE SYSTEM
• the streets are wide and
there is a space for two
sewers lines
•Rainwater is higher than
sewage from the
remaining sources
•Treatment of sewage is
expensive
Sewerage System
Pipes are designed partially full for the following
reasons.
• Aeration: as ww is to be treated aerobically in
WWTP .
• To meet daily, seasonally and future variations
in ww flows.
Sewerage System
• ½ full
2/3 full
¾ full
Sewerage System
Manholes
Manholes are concrete chambers (circular, square or rectangular) which
has an opening and a cover, constructed on sewers network for
maintenance and aeration purposes.
•
1.
2.
3.
4.
5.
6.
Location of manholes
At the intersections between two sewers.
At any house connection.
At the change in slope.
At change in size.
At change of direction.
In straight sewers there is max spacing between manholes to permit
inspection and cleaning of sewers