Sanitary engineering

1. CH:3 Design and Construction of Sewers
1) Shape of Sewers
 Most common section is circular for closed sewer.
 Other non-circular section may be used for open and closed sewers.
 Parabolic shape is used generally in the unlimited drains to carry storm water
Shape of sewer section should be selected in such a way that it should
 Develop self-cleansing velocity in minimum or dry weather flow
 Have sufficient free board during maximum discharge
 Easy in cleaning and maintenance
 Structurally safe and stable
Shape of sewer can be classified as:
i. Circular sewer
ii. Non-circular sewer

2. i. CIRCULAR SEWER
 Most commonly used sewers
 Material required is less
 Not suitable for combined system as there is large variation in flow.
 Generally, these sewers are up to 1.5m diameter.
MERITS
 Gives higher discharge: section has least perimeter for the given area so it ha maximum hydraulic depth hence
hydraulically efficient section.
 Less chance of deposition of organic matter as it has no corner.
 Easy to construct, transport and handle and utilize minimum material of construction.
 Can be easily manufactured.
 Stable in laying and useful for separate sewers.
DEMERITS
 Not suitable for combine system because self-cleansing velocity cannot be maintained at DWF conditions.

3. ii. Non-circular section
 Section of sewer other than circular sections.
 Chosen when circular section doesnot meet the appropriate criteria.
 There are various opens and closed non circular sewer
 Open sewer is generally used for storm water
 Closed type is normally used for sanitary sewage.
General types of Non-circular section
a. Egg shaped sewer
b. Rectangular sewer
c. Other sections sewer

13. Sewer Material
Requirements of sewer material
 It should be economical and easy in construction.
 It should be water-tight.
 It should offer resistance to the penetration of the tree roots.
 It should be capable of resisting the effects of acidic, alkaline or
gaseous actions of the sewage.
 It should be flexible and should not get damaged by settlement.
 It should non-absorbent
 It should be durable.
 It should be cheap and economical.
 It should be easy to construct.

14. Types Of Sewer Materials
i. Salt Glazed Stoneware
 made of stoneware virtified clay which manufactured from clays and shells.
 Has smooth surfaces.
 Generally used in house drainage and for laterals.
Advantages
 Are hydraulically efficient due to their smooth interior surface.
 Pipes are highly impervious cheap and easily available
 Are durable and economicl for small diameters
 Are strong in compression to other pipes
 Highly resistance to erosion due to grit and high velocity and also to sulphide and most acids.

15. Disadvantage
 Are heavy, bulky and brittle so difficult to transport, handling and laying
 Pipes are individually, small pipes so require large number of joints
 Pipes cannot be used as pressure pipe since these pipe weak in tension.
 Very weak in tension
ii. Cement Concrete Sewers
 Most widely used sewers
 Either cast or cast-in-situ
 Made of concrete and can be joined with bell and spigot joints.
 P.C.C pipes are not suitable for pressured flow.
 R.C.C pipe are generally preferred.
 P.C.C are up to 60 cm dia and used where pressure is less than 15 m of water and for dia > 60 cm, RCC should
be used.

16. Advantages
 Strong enough to withstand external and internal pressure and can be designed for any strength.
 High resistance to erosion and abrasion
 Economic
 Can be used in place, where ground water table is much higher.
 Available in different size
 Constructed in site in required shape and size as well as in factory.
 Economical for medium and large size hence heavily used in factory.
 Least thermal expansion, its maintenance cost is low and can be laid under water.
Disadvantages
 Difficult in transportation if precast are used as they are heavy to handle
 May be affected by acids and alkali and salty waters but major disadvantages is that it most liable to crown
crossion.
 Intial cost is high
 Difficult to join so liable to leak due to porosity and difficult to repair.

22. f=0.03
β=0.04-0.06

38. SEWER
APPURTENANCE
S

40. 3. To make the process of construction
of sewerage system easy and for the
efficient operation of the sewerage
system.
4. If sewerage system is not properly
maintained silt, ashes,grit, oil, fats
etc. will chock the sewer lines
therefore for propoer operation and
Maintenance various devices are
required.
5. These are constructed at suitable
intervals along the sewerage system
to help its efficient operation and
maintenance.

49. • WHERE
• A bend in sewer is necessary
• Construction of manhole is necessary.
• spacing of manhole is more than usual.
• WHY
• Helps in examining the sewer length between adjacent
manholes.
• It may be used as flushing devices, under some circumstances.
• If its top cover is kept perforated, it can be used for ventilation of
sewer. Also called fresh air inlet

53. Curb inlet
• Vertical opening in road curbs through which
storm water enter into stormwater drain.
• Opening bars are provided to prevent passage of
dry leaves, papers etc.
• Less quantity of floating matter enter in the catch
pits as compared to gutter inlet.
• It is cheap
• Does not have to bear traffic load
Gutter inlet
• Horizontal opening in the gutter which is covered
by one or more grating through which storm water
is admitted .
• Provided with cast iron gratings at their top to
prevent the floating matters entering the sewer.
• more quantity of floating matter enter in the catch
pits as compared to curb inlet.
• Expensive
• Sufficiently strong to bear traffic load.
Combined inlets:
 Curb and gutter inlets both are provided to be act as a single unit.
 The gutter inlet is normally placed right infront of the curb inlet.

54. 5. The basin is connected to sewer by a pipe having hood.
6. The hood prevents escape of foul sewer gases.

57. Hand operated flushing tank
• Flushing and cleaning operation
is carried out at suitable
intervals by manual labour.
• Carried out by operating the
sluice valve fitted at the outlet
end and inlet end of manhole
suitably.
• The flushing and cleaning
operation is carried out
automatically at regular intervals.
• Consists of u-tube encased in
compartment and overflow pipe is
provided to drain away ecxess
water.
• This tank functions automatically
by siphon action.
Automatic flushing tank

58. Automatic flushing tank