note of SANITARY ENGINEERING Chapter-4-1.pptx

Sanitary Engineering: Er. Santosh
Poudel
Chapter-4
Design and Construction of Sewer
Generally Sewage contains 99.9 percent water and
0.1 percent solid particles. The design of sewer is
similar to that of water supply mains but here in
sewer lines clogging may occur So, they should be
designed to achieve no silting and self cleansing
velocity.

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 Self Cleansing
Velocity:
It is the minimum
Velocity at which the
solid particles will
remain in suspension
without settling at the
bottom of the sewer.
It depends on nature
of solid particles and
diameter. It is
generally 0.6-0.9 m/s
for separate sewer
and 0.75 for
combined sewer
system.

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 Non Scouring Velocity:
It is the maximum velocity at which no scouring or abrasion takes place. For
safer Design, it is basically taken so as not exceed 3 m/s.

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 Hydraulic Formula for Sewers:

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 Circular Sewer
Advantages of circular Sewer
• Easily manufactured
• Least perimeter of cross-section so least construction material is required
• Uniform curvature so least chance of deposition of materials.
• Provides maximum hydraulic mean depth when running partially full or
half full.
 Hydraulic Elements of Circular Sewer:

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2θ
2θ
2θ

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Material of sewer: ( Requirement of a sewer material)
Sewage consists of various chemical and non-chemical constituents coming out of
home or factories. So while choosing appropriate material, its workability, strength
for internal and external pressure, corrosive nature, should be studied. Following
things should be considered in general:
1. It should be easily cast able and moldable .
2. It should be economical.
3. It should be resistive to chemical reaction/corrosion.
4. Should be able to sustain internal pressure , external ad abrasion.
5. It should be such that water tight joints could be easily constructed.
6. It should be easily transportable and easy handling.
7. Shape and size should be uniform
8. It should be lightweight and durable
9. It should be flexible in design, easy for repair in maintenance and environment
friendly.
10.It should be impact resistant.

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Types of sewer pipes based on materials:
1. Cast iron: These pipes are used in places to withstand high internal pressure and
external load. They are naturally resistant to various soil but not
suitable to carry peaty or sulphate rich sewage.
2. Plain or reinforced cement concrete: it can be precast or cast-in-situ. It can
withstand both external and internal pressure. It is difficult to make joints but
can resist erosion and abrasion.
3. Vitrified clay or stoneware: These are constructed with clay burnt in kiln and are
hydraulically efficient. They are light weight and brittle.
4. Asbestos Cement Pipe: They are manufactured from cement and asbestos fiber.
They are light weight and transportable. They can be easily drilled, trapped and
joined. They are mostly suitable for domestic sanitary fittings.

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Sewer appurtunenaces:
These are structures/appliances constructed at various suitable location in a sewer system.
They help in efficient operation and maintaince.
1. Manholes:

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2.Drop Manhole

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3.Lamp hole

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4.Street Inlets:
Street Inlets:

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Catch Basin:

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Flushing Tank:

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Sand Grease and Oil Traps:

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Inverted Siphons:

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Sewer Outfall:

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Ventilation Shaft

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Methods of Ventilation:
Ventilation in sewer lines can be provided with the help of following
different method
• Providing ventilation shaft or column
• Laying sewer at proper gradient helps achieve self cleansing and
sewage does not get chance to accumulate for decomposition to
leave gasses
• Running sewers 2/3 full leaves room for gas accumulation.
• Providing man holes with gratings which gas to escape

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Numericals:
1. Discharge देको भए ठिकै छ नत्र अघिल्लो पालि निकाले जस्तै Sanitary र Storm Discharge निकाल्ने
र जोड्ने
2. Discharge निकालेपछि, Mannings formula प्रयोग गरि पाइप को Diameter निकाल्न सकिन्छ
3. यो फर्मुला मा R लाइ Diameter को terms मै राखेर Q=AV गरि area लाइ Diameter कै terms
राखेपछि Diameter निस्किन्छ .
4. एदी अरु कुनै Value नदिएर निकाल भन्यो भने बाकि दिएको Value हरु लाइ माथिकै फर्मुला मा राखेर
निकाल्न सकिन्छ.
5. अब एदी, Half FULL भन्यो भने वा 0.7 times अथवा 0.6 times वा येस्तई कुनै भन्यो भने चाहि Self
cleansing achieve हुन्छ कि नाइँ चेक गर्नु पर्ने हुन्छ. त्यो भनेको θ को मान निकाल्ने कुनै फर्मुला प्रयोग
गरि d/D को ratio को फर्मुला वा डिस्चार्ज ratio q/Q को फर्मुला प्रयोग गरि.

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• माथि को q/Q वाला फर्मुला प्रयोग गर्दा q को value PF ले multiply नगरेको sanitary discharge
को value राख्ने र Q= Total discharge = sanitary +strom discharge राख्ने ratio मा र θ निकाल्ने
तेसपछी v/V को ratio प्रयोग गरि v को value निकाल्ने जसमा V को ठाउँमा manning's formula
देखि आको value राख्ने तर maximum velocity भनेर दियो भने V को ठाउमा तेही देको value राख्ने
• अब यदी एस्तो खालको particle छ भन्यो भने तेस्को settling velocity कति छ निकाल्ने र त्यो
velocity भन्दा v को value धेरै आउनु पर्यो . नत्र partice को कुनै value नदेको खण्डमा v को value
0.6 – 3.2 को बीच मा आउनु पर्यो .

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For θ,
0.6*2-1=-Cos θ/2
So, θ=203.07 Degree

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v
v

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A main combined sewer is to be designed to serve an area of 12 sq KM with a population
density of 250 person/hectare. The average rate of sewage flow is 250 l/capita/day. The
Maximum velocity of flow is 100% in excess of average together with rainfall equivalent of
15mm in 24 hours all of which are runoff. Determine the capacity of sewer taking maximum
velocity of flow as 3m/s. Determine the size of circular sewer. Check self cleansing durng dry
flow.
Solution:

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Design diameter of outfall sewer in a separate system running 0.7 times
full depth at maximum Discharge for a town of population of 24000. The
rate of water supply is 135lpcd and peak factor is 2.75. The sewer is to be
laid at a slope of 1 in 250. 80% of supplied water is to be converted to
sanitary sewage
80%

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Calculate the diameter of combined circular sewer for the following data: rate of water supply
140 lpcd. Popn density = 160 person/hectare. Area= 6000 ha Intensity of rainfall = 0.75mm/hr.
Inverted slope of sewer = 1 in 1100. Impervious factor= 0.85 and manning's coeff. of rugosity =
0.014. The sewage should run half full during peak flow. Check self cleansing.

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If you don’t have any
questions, please do
solve all numerical
questions related to
this chapter, in the
question bank.

note of SANITARY ENGINEERING Chapter-4-1.pptx

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