Design of Sewers

Design of Sewers:

• Difference in the design of water supply pipes and sewer pipes:

1. Sewage contains solid particles either of organic or inorganic  nature. They may settle down or clog.They may also cause wearing of pipe material. Hence the sewer pipes be such  a size and laid at such a gradient so as to generate Non silting and Non scouring velocities  at different possible discharges.
2. Water pipes are generally Presser  Pipes and hence they may be carried up and down  the hills and valleys. Seer pipes are Non pressure pipes. Therefore they must be laid at a continuous downward gradient up to out fall. from where it will be lifted up treated and disposed off.

Self cleansing velocity: it is the velocity which causes both floating and heavy solid to  get transported easily.

It may be noted that, larger the size of the sewer, the higher is the velocity of flow.

Self cleansing velocity, Vself =[ 8k/f (S-1)gd]1/2

S = Specific  gravity of sediment

K = a sediment characteristic constant to be determined by experiment

d = diameter of grain

f = friction factor

• Minimum velocity to be generated in sewer to avoid silting is equal to self cleansing velocity. It is taken as 0.75 m/sec generally
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Sewer Construction And Appurtenances

SHAPES OF SEWER:

1. Circular 
2. Egg shaped

CIRCULAR SEWER:

• Preferred for separate system because discharge does not vary too much and chances of sewer running at low depth are less
• Utilises minimum quantities of materials there force economical

EGG SHAPED SEVER:

• Preferred for combined sewerage system because low discharge (D. W. F) is about 1/20 to 1/25 times the maximum discharge.
• t low discharges maintains hydraulic mean depth nearly uniform
• The hydraulic mean depth R) of the egg shaped sewers of equivalent section is same as that of the circular sewers when running full, but is higher for smaller depths of flow and therefore higher velocity.

SEWER MATERIAL:

Asbestos cement pipes:

• Cannot bear huge compressive stresses there fore used for bringing down nun water from roofs or sullage from kitchens
• Simplex joint — flexible enough and permits as much as 12° deflection.

R.C.C. Pipes:

• Strong enough to be used for branch sewers and main sewers
• Bell and spigot joint 
• Crown corrosion: easily get corroded by action of emanating gases such as H2S
• Vitrified clay lining to project from crown corrosions

Vitrified or stone ware sewers:

• Widely used for carrying sewage as house connections and lateral sewers
• Manufactured from clays and shales of specified quality 
• Offers good resistance to Sulphide corrosion

Brick sewers:

• Particularly used for Storm water drains

Cast Iron sewers:

• Structurally stronger hence used for out fall sewers, rising mains at pumping stations, for inverted syphons.

Testing of sewer pipes

• Water test — for leakage
• Mirror test — for straightness of sewer line
• Ball test — to detect any obstruction